Carb Hints and Tricks

There are a few things that may not always be obvious to a new owner of one of these bikes. I will try to list a few that are carb-related. Some are very obvious, some less so. I hope that these ideas help when you are working with your carbs.

1. Before reinstalling the insulators, swab the inside groove with Dow 111 (a semi-solid silicone). If this isn't available, any rubber lube will work. This helps with the seal, and makes them so much easier to re-install, and to remove the next time.

2. If the tabs on the pilot (idle mixture) screws are intact, grind down the stopping flange on the float bowl. This will allow for full turning of the screw.

3. Broken-off mixture screws can be salvaged by hard soldering a thumbscrew to the broken-off end. If carefully done, they work like the originals.

4. By removing the top caps on the vacuum pistons, you can gain a bit of vertical room when re-installing the carbs. And keep from scratching them if you just polished them.

5. Always check that the jets are in the proper towers. The larger number goes in the brass hex fitting.

6. A possible fix for the black rubber plug in the float bowl is the end of a Bic pen. (I haven't tried this, so you are on your own.)

7. Make sure the spring tip of the float needle springs back up. If not, try cleaning with carb cleaner, or replace.

8. Replace the Phillips screws with Allen headed screws on the float bowl. Then you can remove it while the carbs are still mounted.

9. Bench sync the carbs by observing the tiny hole at the bottom of the butterfly. Adjust so both carbs are exactly the same.

10. You don't have to separate the carbs for cleaning. The inner ACV valve can be accessed with a needle-nosed Vise Grip.

11. If you do separate, place the venturi opening down on a flat surface while tightening the bracket mounting screws. This helps align the linkage so that it doesn't bind.

12. Hold the cable attachment as far rearward as possible when tightening its mounting screw. If not, the bike may not return to idle.

13. The primary jet needle is supposed to be loose when installed.

14. Lift up the vacuum slide thru the choke opening before installing the carbs. The piston should rise and lower smoothly.

15. Use a perfect fitting screwdriver on the recessed emulsion tubes. Anything less and you will strip the slot. I like a hollow ground bit type.

16. Don't put the accelerator pump cover in carb cleaner. It is a different alloy and will pit.

17. Always remove all rubber from the carbs before soaking or spraying carb cleaner. The cleaner eats up rubber.

18. Watch for the primary jet falling out when you remove the hex emulsion tube. It goes back in with the taper up, and the tube holds it in.

19. You can check fuel level in the bowls by routing a clear tube from the overflow nipple to aside the bowl and open the drain. Fuel level should be just below the bowl seam.

20. Check the overflow tube for splits when you have the bowl off. Often hard to see, they are not uncommon and will cause the bowls to leak. Fix easily by soldering.

21. A high E guitar string is the proper size to clear jets. Do NOT try to enlarge any openings with wire, only clear deposits.

22. A set of small numbered drill bits soldered inside small brass tubes makes a more accurate way to clean carb holes and jets.

23. Take pictures and more pictures if you haven't done many of these. They can be of great help when re-installing the springs, etc.

24. A tip from the old forum, I have not tried this: Recently I removed six idle jets with a #1 extractor with ease. I dripped some CLR in on the idle jet, let it sit for 5 minutes and they all came right out. (every minute or so, I'd drip a bit more in there) Even the two that would not come out with all kinds of penetrants, tons of torch heat and muscle. I have not noticed any trouble with the CLR eating away at things it shouldn't but I don't think you'd want to let that stuff sit in there too long before cleaning it out. http://s325.photobucket.com/albums/k378/LRCXed/CARB%20STUFF/#!cpZZ3QQtppZZ16

by Blue Fox

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Are you having trouble getting the choke spring back on correctly after separating the carbs? This post from LRCXed will probably help:

Question: Well, cleaning is done. Oh, and thanks for getting back to me, Larry. I've run into a bit of a snag (no pun intended) and I can't tell from the black and white pics what I am doing wrong. Here are some shots and you will probably know in half a sec what I've done wrong.

http://postimage.org/image/cskktl5ht/ http://postimage.org/image/thm0pi235/ http://postimage.org/image/lddwkrfo1/

As you can see the coil in the front looks a little... umm... loose. I'm pretty sure it shouldn't look like that. Can it be fixed? As you can see, I was able to get the rear spring mounted correctly but the front assembly seems a bit off, and yes, the two pipes that connect the carbs are seated all the way.

Thanks.

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Larry replies: What happened there is that someone forgot to unhook the spring before they took the carbs apart. To fix it you'll have to unscrew the choke butterfly, remove the shaft and spring and then reshape it slowly with a pair of needle nose pliers. It's not difficult, just take your time. When you reinstall the shaft and plate, use a little loctite on the screws and center the plate. Then wind the spring one full turn and hook it back in place. Larry

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Question: OK. So it's been a while as I was waiting for some items to come in. Also, I have been trying to fix that coiled wire to where it should be but have so far only been able to get it wrapped around one time. It's very "slippery" and I don't want to risk bending it. I was wondering: if I remove these two screws, it feels like the rod that the coil is on would slide out and it would hopefully be easier to re-do the coil. Would you recommend removing the screws and rod or recommend not doing that?

http://postimage.org/image/8e71knyw7/ http://postimage.org/image/a79y8zk2v/

Also, the separate coils are overlapping and I have been unable to correct this. Don't know what kind of effect that would have on the mechanism. Thinking it isn't a good thing. Thanks.

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Larry replies: As I mentioned in the last post, it's ok to take out the two screws and the choke shaft to straighten the spring out. Just be sure to center it when you put it all back together. One wrap around is all you'll need to make it function properly. Here are pictures of what it takes and what it should look like when it's done. Although your spring may not be so flat due to straightening it out depending on how patient you are with it.

Loose with no tension. Resized to 38% (was 1023 x 687) - Click image to enlarge

Slide it under the tab with a small hooked pic/wire. The stiffer the hook, the easier it is. Resized to 38% (was 1023 x 687) - Click image to enlarge

Hold the spring with your fingernail so it doesn't get away, and rehook the pic from the other side so you can rotate it around and up. Resized to 38% (was 1023 x 687) - Click image to enlarge

Again, hold the spring tension with your finger and reposition the pic so you can bring it around and hook it into the notch. Resized to 38% (was 1023 x 687) - Click image to enlarge

This is what yours should look like when installed correctly. Resized to 38% (was 1023 x 687) - Click image to enlarge

NOTE, you can put the spring in position BEFORE you put the choke shaft back in. Just pull it sideways and hook it onto the upright arm until you put the carbs back together. Then you just pull it off with the pic and hook it into place.

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How do I synchronize the carburetors?

Carburetor sync is matching the butterfly valves in the two carburetors so they both open the same amount. Your bike will not run at its optimum level if one cylinder is running harder than the other. The easiest way to understand how the adjustment works is to look at your carb linkage. There's a screw adjustment in between the two carbs that adjusts one butterfly in relation to the other.

http://faq.ninja250.org/images/thumb/5/58/Carb_adj_screw.jpg/150px-Carb_adj_screw.jpg%7C150x113px%7Clink=http://faq.ninja250.org/wiki/File:Carb_adj_screw.jpg%7CCarb adj screw.jpg 

Carb sync is adjusted by measuring vacuum through each carburetor and setting the butterflies so vacuum matches. There are fancy gadgets to do this, but there are also a couple quick and easy ways to do it at home. Keep reading.

Hints & Tips

• Carburetor synchronization is done with the bike warmed up.
• “You shouldn't need to, but...” Doing this procedure shouldn't take much time at all, once you know how to do it. The first couple times you try it, however, you may find yourself running out of gas in the carburetors before you have a chance to properly adjust everything. Should this happen, you can use a funnel or a turkey baster (remove the bulb from the end after it's in place) that will fit inside the fuel line, and add more fuel that way. You can use tape or wire to hang it in a good position. This is greatly preferable to reattaching your fuel tank.

Under normal conditions, you should get several minutes' worth of idle from the gas in your carbs.

• You don't want to let the bike idle a LONG time (while doing the job) and warm up above normal operating temps. It's a quick job on the 250 (only one screw), but get a 4 cylinder and you have to be quick with the adjustments.

Carb sync tool options

There are numerous tools which will help synchronize carburetors. These are all systems which measure the difference in pressure between two (or more) carburetors. All are functional and accurate. There is no advantage to buying the commercial models except that then you don't have to make it yourself. Note that the only other tool required to synchronize the carburetors on the Ninja 250 is a #2 Phillips screwdriver, ideally with a longish shaft (6" or more). You'll also need the tools necessary to remove the fuel tank. Of the commercial tools available, the most commonly used devices are:

• Carb tune sticks (old-time mercury ones seemingly outlawed due to toxicity)
• Steel-cylinder carb sticks (no mercury, can't spill)
• Twinmax electronic synchronizer
• Vacuum gauges

The "carb sticks" have the advantage that they're designed for four-cylinder motors, which is quite handy when working with other bikes with 3 or 4 cylinders. However, for the Ninja 250 (or any other twin), they're overkill. The Twinmax's main advantage is its easily modified sensitivity, but it's also dependent upon battery power and could be damaged if dropped or used in severe environments (the carb sticks suffer from these flaws as well). The Twinmax can be used with a 4 cylinder bike, but it isn't as convenient and may be impossible to use if the manufacturer specifies different pressures for each cylinder. The vacuum gauges are useful, but if mistreated will go out of adjustment, possibly without you knowing it. They are also highly dependent upon being set up correctly when manufactured. However, a system for a two-cylinder engine is quite easy and extremely cheap to build at home. One type is Ian J's simple water manometer. Others are described below. These systems are accurate, sensitive, and require no calibration, as well as being durable and inexpensive to build.

Process

Go for a 30 minute or so ride to make sure your engine is nice and warmed up. Carb sync is usually done after a valve adjustment, but you don't have to do them together. Remove your fuel tank. On the top side of each intake - in front of the carbs - is a vacuum nozzle.

http://faq.ninja250.org/images/thumb/0/00/E77b601f.jpg/150px-E77b601f.jpg%7C150x113px%7Clink=http://faq.ninja250.org/wiki/File:E77b601f.jpg%7CE77b601f.jpg 

Remove the black hose from each (plug the hoses to keep out dirt) and place one end of the tubing on each fitting. The next bit is the actual adjustment. It is more or less the same, regardless of which of the tools (described below) you build. Start up your engine for a moment. If your carbs are in sync, you will have equilibrium: the water in the two bottles will be at the same level, or the oil in the oil-in-tube or the ATF in the manometer will not flow one way more than the other. If there's more vacuum from one carb or the other, the oil or water will flow that direction. Turn the linkage adjustment screw (you'll need either a very long screwdriver or a stubby one) in between your carbs so the vacuum is equal, and you are all set. It only takes a slight amount of turning to affect the adjustment. Don't open the throttle (butterflies). This introduces other factors that affect vacuum. Assuming your vacuum is even between the carburetors at idle, you have adjusted it correctly. Directions for adjustment: Turn one way, see the result. Turn the other way, see what changes. Adjust as necessary. When they're even, you're done.

The two-bottle method

Honestly, officer, that's what it is! It's not a drug apparatus, I swear!

http://faq.ninja250.org/images/thumb/e/ea/Carb_sync_tool_1.jpg/150px-Carb_sync_tool_1.jpg%7C150x160px%7Clink=http://faq.ninja250.org/wiki/File:Carb_sync_tool_1.jpg%7CCarb sync tool 1.jpg 

Our thanks to Payne for the original writeup. This has become the preferred tool for this job in the club. Payne: "The inspiration for this project came from a post a good while back, in which someone described this design using beer-making equipment. I always found the oil in the tube thing (below) to be a pain and not terribly accurate, and I certainly did not want mercury sticks because of the toxicity and possible engine damage if it gets sucked in. And I didn't want to spend a lot of money on a tool I'd rarely use”. Parts:

• 2 empty Nantucket Nectar glass bottles (any similar GLASS bottle with a large neck should work, such as Snapple 16 oz. ones)
• 2 rubber stoppers ($2.50 each at Lowe's or other hardware places)
• 10 feet of 1/4" ID (3/8" OD) tubing ($2.00 at Lowe's; almost every hardware store has this)
• Motor oil is optional in place of water

Tools:

• Drill with 3/8" bit.
• Pliers to pull hose through once it emerges.
• Lubricant (such as cooking oil; extra-virgin olive oil optional) to help get the tubing through the stoppers.

Drill two holes in each stopper. The rubber is a little difficult to drill through; you may try smaller drill bits first, working up in size as needed. One short piece of tubing goes through each and will go down to the bottom of each bottle. A long piece goes though each just enough to go into the bottle. Put about 2" of water in each bottle, not so much that if all the water goes into one bottle it will make it to the upper tube. Some people have found clean motor oil easier to work with than H2O.

http://faq.ninja250.org/images/thumb/0/04/Carb_sync_tool_2.jpg/180px-Carb_sync_tool_2.jpg%7C180x135px%7Clink=http://faq.ninja250.org/wiki/File:Carb_sync_tool_2.jpg%7CCarb sync tool 2.jpg   http://faq.ninja250.org/images/thumb/5/56/Carb_sync_tool_3.jpg/120px-Carb_sync_tool_3.jpg%7C120x160px%7Clink=http://faq.ninja250.org/wiki/File:Carb_sync_tool_3.jpg%7CCarb sync tool 3.jpg   http://faq.ninja250.org/images/thumb/1/1a/Carb_sync_tool_4.jpg/300px-Carb_sync_tool_4.jpg%7C300x141px%7Clink=http://faq.ninja250.org/wiki/File:Carb_sync_tool_4.jpg%7CCarb sync tool 4.jpg 

What happens: if one side pulls harder, the pressure will decrease in the upper portion of that bottle, causing liquid from the other bottle to move through the transfer tube and raise the water level in that bottle. Since there can never be any liquid in the tubes connected to the carbs, there's no chance of the engine sucking in liquid. Plus, the movement is very easy to see and not jumpy, as can be the case with the oil in the tube trick. It's extremely easy to see in an instant which side is pulling harder and correct it. Quoth Payne: “In my opinion, this is THE way to do this procedure. I think for the first time my carbs are truly synced. The way my bike runs backs that up; it idles and accelerates noticeably smoother”.

Alternatives

• Baby bottles work well.

http://faq.ninja250.org/images/thumb/e/e5/Baby_bottles.jpg/170px-Baby_bottles.jpg%7C170x128px%7Clink=http://faq.ninja250.org/wiki/File:Baby_bottles.jpg%7CBaby bottles.jpg 

• So do various glass alcoholic beverage containers.

http://faq.ninja250.org/images/thumb/5/51/4um1s8w.jpg/110px-4um1s8w.jpg%7C110x147px%7Clink=http://faq.ninja250.org/wiki/File:4um1s8w.jpg%7C4um1s8w.jpg   http://faq.ninja250.org/images/thumb/7/7d/53t1kz9.jpg/110px-53t1kz9.jpg%7C110x147px%7Clink=http://faq.ninja250.org/wiki/File:53t1kz9.jpg%7C53t1kz9.jpg 

• You can get an 1 1/8" Basin/Bath Stopper from Ace Hardware. Part# 40174. Fits Snapple 16 oz. bottles well.

http://faq.ninja250.org/images/thumb/d/d6/592771.jpg/150px-592771.jpg%7C150x113px%7Clink=http://faq.ninja250.org/wiki/File:592771.jpg%7C592771.jpg 

Slack-tube manometer

One of the guys in the club who is a mechanic came up with this one. It's similar to the simple water manometer (above). Total cost was about $3.00. All you need is about six feet of clear vinyl tubing, a wooden yardstick (or similar), some nylon wire ties (zip ties), and automatic transmission fluid (ATF) for the fluid. Drill small holes near the edges of the yardstick to attach the tubing as shown.

http://faq.ninja250.org/images/thumb/7/7f/Dawgsync1.jpg/100px-Dawgsync1.jpg%7C100x101px%7Clink=http://faq.ninja250.org/wiki/File:Dawgsync1.jpg%7CDawgsync1.jpg 

Find the center of the tubing and make a loop at the bottom of the yard stick, securing the tubing up to the top. The open ends of the tubing will attach to the left and right carb. As usual, make sure the bike is warmed up before you start the adjustment process.

http://faq.ninja250.org/images/thumb/5/5b/Dawgsync2.jpg/50px-Dawgsync2.jpg%7C50x102px%7Clink=http://faq.ninja250.org/wiki/File:Dawgsync2.jpg%7CDawgsync2.jpg 

Once you start the engine, any difference in vacuum will cause the fluid to be pulled toward the cylinder with more vacuum. Simply turn the carb sync screw until the fluid is level on both sides of the tube as shown. If the carbs are really unbalanced, it is pretty easy to have some of the fluid sucked into the intake. That's another reason for using trans fluid. It won't really do anything to the engine; the oil will just burn off.

http://faq.ninja250.org/images/thumb/8/8d/Dawgsync3.jpg/100px-Dawgsync3.jpg%7C100x100px%7Clink=http://faq.ninja250.org/wiki/File:Dawgsync3.jpg%7CDawgsync3.jpg 

This tool works great and is cheap to build. You can use most any type of fluid you want, but an oil will work best. ATF was used here because the bright red color makes it easier to see. Our mechanic friend felt it was easier and cheaper to build than the bottle method, but both will get the job done.

Oil-in-tube method

This is considered old school by many, but it still works. Get a piece of clear plastic tubing about 3' long. Place some thick liquid in it, like motor oil (something that won't hurt your engine if it gets sucked in). Remove the black hose from each vacuum nozzle (plug the hoses for the duration of the job) and place one end of the clear tube on each fitting. Let the clear tube hang down so the oil gathers at the bottom of the loop.

http://faq.ninja250.org/images/thumb/7/79/Tubing_setup.jpg/150px-Tubing_setup.jpg%7C150x200px%7Clink=http://faq.ninja250.org/wiki/File:Tubing_setup.jpg%7CTubing setup.jpg 

Start up your engine for a moment, and if your carbs are in sync the oil will not flow one way more than the other. If there's more vacuum from one carb or the other, the oil will flow that direction.

If you can't find rubber stoppers for the two-bottle method

Here's another solution: Go to a place that sells home brewing supplies. They should have white rubber "gum" stoppers. These are quite soft. Therefore, you have to do them differently, so you don't just end up with some fairly mangled-looking rubber chunkage. You need a little bit of 3/16" rigid brake line. This fits nicely inside the 1/4" clear tubing. Two #6 1/2" stoppers fit in Welch's grape drink bottles. Put them in the freezer overnight. The next day, drill them with a 3/16" bit. Cut four 2" lengths of the 3/16" brake line. Put a small squirt of oil in each hole. Tap lightly with a hammer to drive the 4 tubes into the four holes of the two stoppers. Cut the vinyl tubing to length and stick it on the rigid tubing snugly. Photos: Two #6 1/2 White Rubber "Gum" Stoppers

http://faq.ninja250.org/images/thumb/e/e1/Gum_rubber_stoppers_1.jpg/150px-Gum_rubber_stoppers_1.jpg%7C150x113px%7Clink=http://faq.ninja250.org/wiki/File:Gum_rubber_stoppers_1.jpg%7CGum rubber stoppers 1.jpg 

Drilling holes into frozen stoppers with a 3/16" bit

http://faq.ninja250.org/images/thumb/8/8a/Gum_rubber_stoppers_2.jpg/150px-Gum_rubber_stoppers_2.jpg%7C150x113px%7Clink=http://faq.ninja250.org/wiki/File:Gum_rubber_stoppers_2.jpg%7CGum rubber stoppers 2.jpg 

Rigid 3/16" brake line snugly inserted

http://faq.ninja250.org/images/thumb/b/b9/Gum_rubber_stoppers_3.jpg/150px-Gum_rubber_stoppers_3.jpg%7C150x113px%7Clink=http://faq.ninja250.org/wiki/File:Gum_rubber_stoppers_3.jpg%7CGum rubber stoppers 3.jpg 

Add liquid and go.

http://faq.ninja250.org/images/thumb/5/5b/Gum_rubber_stoppers_4.jpg/150px-Gum_rubber_stoppers_4.jpg%7C150x113px%7Clink=http://faq.ninja250.org/wiki/File:Gum_rubber_stoppers_4.jpg%7CGum rubber stoppers 4.jpg 

Category: Intake

Here is a link from the OZ site on tuning your CV carbs:

CV Carb Tuning

High RPM engines

(OR "TUNE LIKE A STAR IN 4 EASY STEPS!)

Have a website? Want to provide tuning information?

Please do not plagiarize this page - it took years to develop and write and is not based on any other previous work by anybody else. If you want to link to this page on your site, please ask - we are perfectly willing to allow links as long as proper credit is provided. Thanks - Marc

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• • Follow steps in order....First, dial in:
• #1. Top end (full throttle / 7.5k to redline 1. Top end (full throttle / 7.5k to redline -

Best Main Jet must be selected before starting step 2 (needle height)!

• • • SelectBest Main Jet
  • To get the best, most even top end power (full throttle/after 7500 rpm), select the main jet that produces the hardest pull at high rpm.
    • If the bike pulls harder at high rpm when cold and less hard when fully warmed up, the main jet is too large. Install a smaller main jet and retest until you find the main jet that pulls the hardest at high rpm when fully warmed up. This must be donebefore moving on to the other tuning ranges.
      • In order to properly tune the midrange and low rpm carburetion, THE MAIN JET MUST FIRST BE PROPERLY SELECTED, determined after 10 to 15 minutes of hard use!
        • • Do not pay too much attention to the low-end richness when you are changing main jets - you still need to be using the main jets that produce the best power at high rpm. You will deal with the low-end / cruise later - after step 2.
• 2. Midrange (full throttle /5k-7k)

Step 1 (Best Main Jet) must be selected before starting step 2!

• • • Select best needle clip position
  • To get the best power at full throttle / 5k-7k rpm, adjust the needle height, after you have already selected the best main jet.
    • If the engine pulls better or is smoother at full throttle/5k-7k in a full throttle roll-on starting at <3k when cool, but soft and/or rough when at full operating temperature, it is too rich in the midrange and the needle should be lowered.
    • If the engine pulls better when fully warmed up but still not great between 5k-7k, try raising the needle to richen 5k-7k.
    • If the engine pulls equally well between 5k-7k when cooler as compared to fully warmed up, the needle height is probably properly set.
      • • • Do not pay too much attention to the low-end richness when you are changing needle clip positions - you still need to be using the clip position that produces the best full throttle / 5k-7k power in conjunction with the main jet (Step 1) that produce the best power at high rpm. You will deal with the low-end / cruise next.
• #3. Low end (full throttle / 2k-3k)3. #3. Low end (full throttle / 2k-3k) Low end (full throttle / 2k-3k)

Step 1 (Best Main Jet) and Step 2 (needle height) must be selected before starting step 3!

• • • Float height (AKA fuel level & how to..)
  • To get best low-end power, set float height (fuel level) so that the engine will accept full throttlewithout missing or stumbling, in 2nd gear from 2.5k to 3k rpm at minimum.
    • Float heights, unless otherwise specified in the installation guide, are measured from the "gasket surface" of the carb body to the highest part of the top of the float - with the float tang touching but not compressing the float valve spring.
    • If the engine has a "wet" rhythmic, soggy area at full throttle / 3k-4k rpm, that gets worse as the engine heats up, lower the fuel level by resetting the float height 1mm greater (if the original was 13mm - go to 14mm). This will lower the fuel level, making full throttle / 2k-3k rpm leaner.
    • If the engine is "dry" and flat between 2k to 3k rpm, raise the fuel level.
    • Example: change float height from 15mm to 14mm to richen up that area.
    • REMEMBER, since the main jet WILL affect low speed operation, the MAIN JET has to be within 1 or 2 sizes of correct before final float setting.
      • Warning: If the engine is left with the fuel level too high,, the engine may foul plugs on the street and will be "soft" and boggy at part throttle operation. Adjust Floats to raise / lower the Fuel Level.
        • Base settings are usually given if a particular application has a history of fuel level criticalness. The Fuel level height in the float bowl affects full throttle / low rpm and also richness or leanness at cruise / low rpm.
      • Reference: a bike that runs cleanly at small throttle openings when cold, but starts to show signs of richness as it heats up to full operating temperature, will usually be leaned out enough to be correct if the fuel level is LOWERED 1mm. Check out and RESET all: Suzuki (all), Yamaha (all) and Kawasaki (if low speed problems occur). Needless to say, FUEL LEVEL IS EXTREMELY IMPORTANT!!!
    • If there are low-end richness problems, even after lowering the fuel level much more than 1.5mm from our initial settings, check for needle wear and needle jet (part of the emulsion tube). See Worn Needle and Worn Needle Jet diagram. It is VERY common for the brass needle jets (in the top of the "emulsion tube") in 36mm, 38mm and 40mm Mikuni CV carbs to wear out in as little as 5,000 miles. Check them for "oblong" wear - the needle jet orifice starts out round! Factory Pro produces stock replacement needle jets / emulsion tubes for 36mm and 38mm Mikuni carbs. Click here
• 4. Idle and low rpm cruise
  • • Fuel Screw setting (AKA mixture screws)
      • There is usually a machined brass or aluminum cap over the fuel screws on all but newer Hondas. It's about the diameter of a pencil. Cap removal details. Newer Honda carbs have no caps, but use a special "D" shaped driver, usually supplied in the carb recal kit. We do have them available separately, too. 800 869-0497 to order -
    • Set for smoothest idle and 2nd gear, 4k rpm, steady state cruise operation. Set mixture screws at recommended settings, as a starting point. For smoothest idle, 2nd gear 4000 rpm steady state cruise , and 1/8 throttle high rpm operation. (pj tuning information)
    • Pilot fuel mixture screw settings, float level (but you "fixed" the fuel level in Step 3- which you have already done!) AND pilot jet size are the primary sources of mixture delivery during 4000 rpm steady state cruise operation.
      • If lean surging is encountered, richen mixture screws (turn out) in 1/2 turn increments. Alternative pilot jets are supplied when normally required.
      • Pilot fuel mixture screw settings, float level and pilot jet size also affect high-rpm, 0 to 1/8 throttle maneuvers. Too lean, will cause surging problems when the engine is operated at high rpm at small throttle openings! Opening the mixture screws and/or increasing pilot jet size will usually cure the problem.
        • NOTE: A richproblem gets worse as the engine heats up.
          • If the throttle is lightly "blipped" at idle, and the rpm drops below the set idle speed, then rises up to the set idle speed, the low speed mixture screws are probably set too rich: try 1/2 turn in, to lean the idle mixture.
        • NOTE: A leanproblem gets better as the engine heats up.
          • If the throttle is lightly "blipped" at idle, and the rpm "hangs up" before dropping to the set idle speed, and there are no intake leaks and the idle speed is set at less than 1000 rpm, the mixture screws are probably too lean: try 1/2 turn out, to richen mixture. Be sure there are no intake leaks and the idle speed is set at less than 1000 rpm!
• Carb Kit Design is a combination of science, art, intuition and and at times, a fair dose of wizardry. There is no dyno that "tells" one how to assemble or modify the carb to deliver proper power and response.
• Perfect Carb Kit TUNING requires patience and perseverance and "reasonable" feel to feel the changes - of which - most motorcycle riders have a good ability to do.

When a dyno "operator" says he/she has to ride the bike after dyno tuning to do the final tune for cruise smoothness - that's what they are doing. Avoid any dyno operator who says that they don't have to do that!!!

The only dyno that I know of that will duplicate and visually display the engine smoothness is the EC997 dyno (yes, I know, we make it) - that's one reason why, if you can, you'd like to use one for tuning - a smoother engine IS getting the best mixture. Other dynos claim to "tune to an "A/F Ratio" - probably the biggest marketing scheme in the dyno industry at this time - and they never can equal the quality of tune as designed -

These tuning kits have been thoroughly tested to ensure easy, trouble-free, optimized performance.

• Please note: If you have installed the kit and gone through the optional screw settings, clip positions and main jets, and still have a persistent flat spot / problem, we ask you to call us. Unique engine / exhaust / filter / altitude / temperature combinations may require individualized setups. We are here to help. The information gained in your solution will be installed in our computerized reference database. PLEASE CALL!
• We ask that upon completion of installation and tuning, you call us with specifications of your installation, (pipe brand, filters, advancer, altitude, humidity, temperature and final carb settings) to be entered in our TUNING DATABASE. The database allows us to include the "most used" jet sizes and setup specifications in every kit.
• Use (415) 491-5920, (800) 869-0497 or fax (415) 492-8803.

Thanks!

Marc W. Salvisberg

---

Authorized links to Factory Pro CV Carb Tuning Instructions: http://www.sport-touring.net July 20, 2002

[Input? Questions? Click here to Email Factory]

Here is more information on the CV type carbs. This is not CX/GL specific, but may contain some information you can use.CV Carb Tuning Lower rpm engines http://www.factorypro.com/tech/carbtune,CV,lower_rpm_engines.html

• 1. Top end (full throttle / 5000 rpm to redline) -

Best Main Jet must be selected before starting step 2 (needle height)!

� SelectBest Main Jet o To get the best, most even top end power (full throttle/after 5000 rpm), select the main jet that produces the highest top speed / pulls hardest at high rpm. � If the bike pulls harder at high rpm when cold and less hard when fully warmed up, the main jet is too large. Install a smaller main jet and retest until you find the main jet that pulls the hardest at high rpm when fully warmed up. This must be done before moving on to the other tuning ranges. � If the bike doesn't pull well at high rpm when cold and gets only slightly better when fully warmed up, the main jet is too small. � In order to properly tune the midrange and low rpm carburetion, THE MAIN JET MUST FIRST BE PROPERLY SELECTED, determined after 10 to 15 minutes of hard use! � Do not pay too much attention to the low-end richness when you are changing main jets - you still need to be using the main jet that produce the best power at high rpm. You will deal with the low-end / cruise later - after step 2.

• 2.Midrange (full throttle /2500 to 3500 rpm)

Step 1 (Best Main Jet) must be selected before starting step 2!

� Select best needle clip position o To get the best power at full throttle / 2500-3500 rpm, adjust the needle height, after you have already selected the best main jet. � If the engine pulls better or is smoother at full throttle/2500-3500 rpm in a full throttle roll-on starting at <1500 rpm when cool but soft and/or rough when at full operating temperature, it is too rich in the midrange and the needle should be lowered. � If the engine pulls better when fully warmed up but still not great between 5k-7k, try raising the needle to richen the mixture at 2500-3500 rpm. � If the engine pulls equally well between 2500-3500 rpm when cooler as compared to when fully warmed up, the needle height is probably properly set. � Do not pay too much attention to the low-end richness when you are changing needle clip positions. You still need to be using the clip position that produces the best full throttle / 5k-7k power in conjunction with the main jets (Step 1) that produces the best power at high rpm. You will deal with the low-end / cruise next.

• 3. Low end (full throttle / 1500 to 2000 rpm)

Step 1 (Best Main Jet) and Step 2 (needle height) must be selected before starting step 3!

� Float height (AKA fuel level & how to..) o To get best low-end power, set float height (fuel level) so that the engine will accept full throttle, without missing or stumbling, in 2nd gear from 1500 to 2000 rpm at minimum. � Float heights, unless otherwise specified in the installation guide, are measured from the "gasket surface" of the carb body to the highest part of the top of the float - with the float tang touching but not compressing the float valve spring. � If the engine has a "wet" rhythmic, soggy area at full throttle / 1500 to 2000, that gets worse as the engine heats up, lower the fuel level by resetting the float height 1mm greater (if the original was 13mm - go to 14mm). This will lower the fuel level, making full throttle / 1500 to 2000 leaner. � If the engine is "dry" and flat between 1500 to 2000 rpm, raise the fuel level. � Example: change float height from 15mm to 14mm to richen up that area. � REMEMBER, since the main jet WILL affect low speed operation, the MAIN JET has to be within 1 or 2 sizes of correct before final float setting. � Warning: If the engine is left with the fuel level too high,, the engine may foul plugs on the street and will be "soft" and boggy at part throttle operation. Adjust Floats to raise/ lower the Fuel Level. � Base settings are usually given if a particular application has a history of fuel level criticalness. The Fuel level height in the float bowl affects full throttle / low rpm and, also, richness or leanness at cruise / low rpm. � Reference: a bike that runs cleanly at small throttle openings when cold, but starts to show signs of richness as it heats up to full operating temperature, will usually be leaned out enough to be correct if the fuel level is LOWERED 1mm. Check out and RESET all: Suzuki (all), Yamaha (all) and Kawasaki (if low speed problems occur). Needless to say, FUEL LEVEL IS EXTREMELY IMPORTANT!!! � If there are low-end richness problems, even after lowering the fuel level much more than 1.5mm from our initial settings, check for needle wear and needle jet (part of the emulsion tube). See Worn Needle and Worn Needle Jet diagram. It is VERY common for the brass needle jets (in the top of the "emulsion tube") in 36mm, 38mm and 40mm Mikuni CV carbs to wear out in as little as 5,000 miles. Check them for "oblong" wear - the needle jet orifice starts out round! Factory Pro produces stock replacement needle jets / emulsion tubes for 36mm and 38mm Mikuni carbs. Click here

• 4. Idle and low rpm cruise

� Fuel Screw setting (AKA mixture screws) � There is usually a machined brass or aluminum cap over the fuel screws on all but newer Hondas. It's about the diameter of a pencil. Cap removal details. Newer Honda carbs have no caps, but use a special "D" shaped driver, usually supplied in the carb recal kit. We do have them available separately, too. Call 800 869-0497 to order. � Set for smoothest idle and 2nd gear, 2000 rpm, steady state cruise operation. Set mixture screws at recommended settings, as a starting point. For smoothest idle, 2nd gear, 2000 rpm steady state cruise , and 1/8 throttle high rpm operation. (pj tuning information) � Pilot fuel mixture screw settings, float level (but you "fixed" the fuel level in Step 3 - which you have already done!) AND pilot jet size are the primary sources of mixture delivery during 2000 rpm steady state cruise operation. � If lean surging is encountered, richen mixture screws (turn out) in 1/2 turn increments. Alternative pilot jets are supplied when normally required. � Pilot fuel mixture screw settings, float level and pilot jet size also affect high-rpm, 0 to 1/8 throttle maneuvers. Too lean, will cause surging problems when the engine is operated at high rpm at small throttle openings! Opening the mixture screws and/or increasing pilot jet size will usually cure the problem. � NOTE: A rich problem gets worse as the engine heats up. � If the throttle is lightly "blipped" at idle, and the rpm drops below the set idle speed, then rises up to the set idle speed, the low speed mixture screws are probably set too rich: try 1/2 turn in, to lean the idle mixture. � NOTE: A lean problem gets better as the engine heats up. � If the throttle is lightly "blipped" at idle, and the rpm "hangs up" before dropping to the set idle speed, and there are no intake leaks and the idle speed is set at less than 1000 rpm, the mixture screws are probably too lean: try 1/2 turn out, to richen mixture. Be sure there are no intake leaks and the idle speed is set at less than 1000 rpm!

• Successful carb tuning is a combination of science, art, intuition and a lot of wizardry.
• These tuning kits have been thoroughly tested to ensure easy, trouble-free, optimized performance.
• Please note: If you have installed the kit and gone through the optional screw settings, clip positions and main jets, and still have a persistent flat spot/problem, we ask you to call us. Unique engine/exhaust/filter/altitude/temperature combinations may require individualized setups. We are here to help. The information gained to your solution will be installed in our computerized reference database. PLEASE CALL!
• We ask that upon completion of installation and tuning, you call us with specifications of your installation, (pipe brand, filters, advancer, altitude, humidity, temperature and final carb settings) to be entered in our TUNING DATABASE. The database allows us to include the "most used" jet sizes and setup specifications in every kit.
• Use (415) 491-5920, (800) 869-0497 or fax (415) 492-8803.

Thanks!

Marc W. Salvisberg

Carburetor Jetting

Fundamentally, increasing fuel flow means installing larger jets in the carburetor. Sounds simple, doesn't it? Trust me, I've been doing it now for more than 30 years, and it really is that straightforward.

Picking the right jet is the hard part. Fortunately, that element of the job has been made much easier by the proliferation of jet kits now available. These kits contain all of the bits and pieces you'll need plus detailed instructions, and they are available to fit nearly every current and not-so-current motorcycle I can think of. But this would be a damn short article if we left it at that.

Rather than just show you how to install a jet kit, let me walk you through the basic steps used to jet a bike from scratch. Even though the kits greatly shortcut the jetting process, individual circumstances may dictate some fine-tuning from bike to bike. If you know how to evaluate jetting, you'll know what to look for and what changes need to be made. Because the most popular carburetor in use is the variable-venturi, slide-type carburetor, that's the model we'll use. For the sake of convention, we'll assume you have either installed a jet kit and want to make sure the jetting is spot-on or you suspect some jetting changes need to be made.

Presumably, you already have some fundamental understanding of a carburetor's internal workings. If you don't, I suggest you bone up on a little theory first. Mr. Bernoulli was a pretty bright guy, and his insights are well worth discovering.

It Just Ain't Running Right

Deciding where the problem lies is the first step. One of the more common mistakes the novice tuner makes is trying to jet based on engine rpm. Throttle position dictates which carburetor circuit is being used at the time, so jetting is always predicated by throttle position, never rpm.

Experienced tuners and riders are pretty good at determining which carburetor circuit is at fault. If you're new to this, a visual aid may prove helpful. A throttle index will show you exactly which circuit is in play at any given time. Using masking tape, place a datum point on the throttle housing. Next, use another piece of tape to divide your throttle into quarters. As an alternative, you can mark the quarters on the housing and use a colored pin stuck into the grip to indicate the throttle's position.

Your markings should indicate 1/4, 1/2, 3/4 and wide-open throttle, although the last is usually pretty obvious, as is the closed position.

Too often a carburetor jetting chart seems to indicate a very clear delineation among the effects of the various jets and circuits. There isn't; in fact, there is always some overlap, and bear in mind that carburetor fuel-delivery circuits are cumulative. In other words, even when we're running wide open on the main jet, the pilot jet is still supplying some fuel. For this reason, we start by addressing the pilot jet first and the main jet last.

The Beginner's Guide to Jetting

Pilot circuit: 0 to 1/4 throttle

Start by bringing the engine up to operating temperature. Open the throttle slightly. Does the engine pick up speed smoothly? If it doesn't, the fuel-mixture screw may be out of adjustment or the pilot jet may be either too small or too large. To determine the pilot screw setting, adjust the idle-speed screw so that the engine is idling slightly fast, somewhere between 10 and 20 percent above the recommended idle speed. Using either the manufacturer's or the jet kit's recommended pilot screw setting as a starting point, turn the fuel screw left or right between 1/4 and 1/2 of a turn. Select the screw position where the engine speed increase is the greatest. Readjust the idle speed to the correct level.

Next, take the bike for a ride, preferably on a relatively quiet, straight section of road. Using your throttle index, run the bike at 1/4 throttle. If the pilot jetting is correct, the engine should run smoothly, and it should be easy to maintain a constant speed without varying the throttle position. If the bike surges or hunts, the pilot jet is probably too small (lean). Other indications that the pilot circuit is lean are popping or spitting through the carburetor when the throttle is opened and popping or backfiring through the exhaust when the throttle is shut. If the pilot circuit is rich, the engine will have a rough idle or may not return to idle without "blipping" the throttle. You may also notice a distinct smell of raw fuel and some eye irritation from the exhaust.

Low- to high-speed transition: 1/4 to 3/8 throttle

Although the throttle cutaway plays a significant role between 1/4 and 3/8 throttle, it is seldom changed when rejetting a streetbike, particularly a cruiser. For that reason I'll gloss over it. Basically the only time the cutaway needs changing is when the transition from the low-speed fuel circuit (pilot jet) to the high-speed circuit (needle, needle jet and main jet) gets seriously out of whack.

Medium throttle opening: 1/4 to 3/4 throttle

The fuel delivery at this stage is controlled by the jet needle and the needle jet. At one time all carburetors came with adjustable jet needles. Once the smog Nazis got involved, fixed-position needles became the standard. Jet kits will usually include new needles that can be raised (to richen mixture) or lowered (to make it leaner) to suit or, less often, shims to change the height of your stock needles.

To evaluate the needle/needle-jet performance, run the motorcycle in second or third gear. Roll the throttle on from 1/4 to 1/2 throttle only. The engine should accelerate cleanly without sputtering or bogging. If so, you're all done with the midrange; if it doesn't, you'll need to do some experimenting to determine if the needle position is too rich or too lean. Assuming you've installed adjustable needles from the kit, begin your experiments by trying some alternative needle-clip positions, perhaps a step richer to begin with. If that makes the situation worse, try a step leaner. If the jetting is stock and problems crop up, it's time to order the appropriate jet kit.

Top end: 3/4 to wide-open

At this stage of the game, the majority of the fuel flow is controlled by the main jet (remember some fuel is still flowing through the pilot jet). The time-honored method of determining main-jet size is via a plug chop. If you've never done one, a plug chop is pretty simple. Start with new spark plugs. Run the bike flat out (full throttle), in as high a gear as possible, for eight to 10 seconds (don't hold the bike at the rev limiter, as it'll give you a false reading). Without cutting the throttle or slowing down, simultaneously pull in the clutch and hit the kill switch. Coast the bike, with the clutch still in, onto the shoulder and remove the plug(s). The plug insulator should be tan to grayish-white. A white plug indicates a lean mixture; a dark gray or black one means it's rich. You then select the main-jet size accordingly.

Unfortunately, plug chops present a few problems. Some bikes require an inordinate amount of work to remove the plugs, at least more than you want to do on the side of the road with a smoking-hot bike. And in most parts of the country, the law takes a very dim view of banzai runs down the main drag followed by an impromptu tuning session on the side of the road.

The alternative, and the method I personally prefer, is to use a dyno. It's certainly a lot easier and arguably more accurate, but it may not be convenient.

Is It Lean or Rich?

One of the reasons I'm so stoked about jet kits is because they remove 95 percent of the guesswork. Be that as it may, even the best kits require some fine-tuning. Essentially a jetting problem can always be reduced to one of two choices: is it lean or is it rich?

To determine if the bike is lean (too little fuel for the amount of air reaching the engine through the carburetor) at any given throttle opening, partially cover the air-filter intake with a piece of duct tape; if the carburetion improves, it's running lean. If you suspect the bike is running rich (too much fuel in the air-fuel mixture), remove the airbox top or the air cleaner element; if the changes are for the better, the bike was running too rich.

If your bike is equipped with a manual fuel shut-off (or you're clever enough to disconnect and plug the fuel lines), take the bike for a ride with the fuel turned off. If it runs worse as the float bowl drains, it's lean. If it gets better, it's rich.

Troubleshooting

A lean condition is the result of too little fuel and too much air. Slightly lean conditions create drivability problems. Worst-case scenario: lean conditions can and do destroy engines. Holes in the pistons, burnt valves and trashed main bearings are the direct result of lean mixtures.

Rich mixtures are the end result of too much fuel and too little air. Rich mixtures waste fuel, contribute to carbon buildup and pollute the air.

• • Typical Lean Conditions:

- Poor acceleration; the engine feels flat.

- The engine won't respond when the throttle is snapped open, but it picks up speed as the throttle is closed. (A too-large main jet also mimics this symptom.)

- The engine runs hot, knocks, pings and overheats.

- The engine surges or hunts when cruising at part-throttle.

- Popping or spitting through the carbs occurs when the throttle is opened. Or popping and spitting occurs through the pipe on deceleration with a closed throttle.

- The engine runs better in warm weather, worse in cool.

- Performance gets worse when the air filter is removed.

Typical Rich Conditions

- Engine acceleration is flat and uneven and loses that "crisp" feel.

- The engine "eight-strokes" as it loads up and skips combustion cycles.

- The engine's idle is rough or lumpy, and the engine won't return to idle without "blipping" the throttle.

- The throttle needs to be open continuously to maintain acceleration.

- Black, sooty plugs, a sooty exhaust pipe and black smoke from the tailpipe that stinks of unburned fuel.

- Poor fuel economy.

- The engine works better when cold. Performance falls off as it warms

Jetting kits and sources: http://www.factorypro.com/Prod_Pages/prodkt07.html

http://www.carbparts.com/mikuni/mikuni_tuning_info.htm

http://www.thumpertalk.com/forum/showthread.php?t=392117