carb sync / idle mixture question <--- need some help

[Kevin]

Question for you motorcycle techs out there who do this every day, one (automotive) tech to another...

2001 Kawi ZX6R, CV carbs

Carbs sync adjustment is performed at idle, right?

When they're synced at idle, they become out of sync at higher engine speeds (no load). At 3 or 4k, they're out enough that the engine doesn't run smoothly. It's not horrible, it just runs like they're out of sync - stumbling a bit. Enough so that I feel it's not running right.

Is this indicative of an additional problem - incorrect idle mixture adjustment, a problem with the diaphragm, jets, needle, etc?

Should they stay in sync through the RPM range, or is this normal?

The carbs have been gone through - the idle mixture adjustment screws were all wrong when I took them apart - someone had removed the plugs and messed them all up. 2 of the 4 were loose, one was 1/4 turn off bottom, and the 4th was bottomed. Obviously, that wasn't right, so I had no reference when disassembling, and the service manual gives no adjustment procedure.

I'm not 100% sure I have the idle mixture adjusted correctly and I'm thinking that could be throwing off the sync. Using my automotive carb experience, I spent some time on them and adjusted each to the highest vacuum (using the manometer) with the smoothest idle, each independently of the others and kept going back and forth between the 4 carbs with the idle mixture and the sync until I was getting a consistent result and a nice smooth idle. But perhaps I should sync the carbs at 3k or 4k to baseline the throttle valves to each other (at that speed, they're off the idle circuit, right?) and then bring it down and adjust the idle mixtures until they're even, then they might stay in sync through the RPM range? The thought process being that if the mixtures are off between the four at idle, this would throw off the manometer reading at idle, giving me an incorrect baseline for syncing the throttle valves.

Thoughts, experiences?

Any help appreciated - I have plenty of professional experience with automotive carbs, (15+ years as an ASE tech) but four touchy motorcycle carbs are a whole different animal! I'd love to pick up some knowledge in this area.

[1BadSvt]

How many turns out are you on the idle air mixture? Also did you soak the jets or spray them out. I seen that issue onetime before there are so many things it could be. What rpm does it stumble at?

[1BadSvt]

Also does it run a little off on light throttle if so its the idle air mixture. Pm me your number I may be able to walk you through it. Also what are you synchronizing with a carbtune or a vacume gauge.

[chronicboy]

try the next size larger pilot

[Kevin]

How many turns out are you on the idle air mixture? Also did you soak the jets or spray them out. I seen that issue onetime before there are so many things it could be. What rpm does it stumble at?

I'm at about 1 1/2 turns out. I sprayed them out :roll: I don't have a proper carb-soaking setup. I haven't been for a test ride yet, so I don't know if it stumbles at any certain range. It's just what I was observing on the stand with the carbtune hooked up.



Also does it run a little off on light throttle if so its the idle air mixture. Pm me your number I may be able to walk you through it. Also what are you synchronizing with a carbtune or a vacume gauge.

It was running a little off under no load at about 3k, so yeah, light throttle. I'll go for a test ride today to see how it runs under load. I was using a carbtune - so anything higher than about 4k on the throttle and it'll start sucking fluid.

[1BadSvt]

Go 2 1/4 on the mixture screws. That is alittle lean and you really need to soak them a soak bucket at the parts store is 25$

[Kevin]

Go 2 1/4 on the mixture screws. That is alittle lean and you really need to soak them a soak bucket at the parts store is 25$

They were at about 2 1/2 and it was running like dogshit, rich as hell burned my eyes if I had it running in the garage too long. Lack of a soak bucket isn't the problem - I could have come up with something. I didn't have the strong chemicals - the stinky cleaner I've always used in carb cleaning machines (you know the ones with the bucket that swishes back and forth) I don't even know what that cleaner is, it's been so long now I can't remember. It was special stuff.

Anyway, just got back from a test ride - all is good, runs like a raped ape. Smooth and powerful through its entire range. I guess I done good. It could probably be better, but for carbs, I think it's as good as it gets. It's much quieter now too - I can put the other two discs back in my supertrapp.

But I'm still curious - should the carbs stay in sync at 3 or 4k rpm, or is it normal for them to wander out of sync at higher revs?

[Wrider]

Usually you balance at idle and at 4-5K just to make sure things are steady.

I'd say you probably have a small air leak post-carb.

[Kevin]

Usually you balance at idle and at 4-5K just to make sure things are steady.

I'd say you probably have a small air leak post-carb.

Thanks - that makes sense. I'll look for a vacuum leak - it wouldn't surprise me for this bike - it was about as screwed up as can be, in just about every way possible., and I didn't replace the orings or the boots.

[brennahm]

I'm going with possible air leak or clogged jets. Blowing out an even semi-clogged jet with just carb cleaner is not doing the job. Use a light...make SURE all your jets (pilots and mains) are squeaky clean. As said before 2-3 turns should be the zone on your mixture screw. If the carbs are surgically clean and you're finding you need to do fewer than 2 or more than 3 turns out...change the pilots. But that should be a last resort and not your issue.

[Kevin]

I'm going with possible air leak or clogged jets. Blowing out an even semi-clogged jet with just carb cleaner is not doing the job. Use a light...make SURE all your jets (pilots and mains) are squeaky clean. As said before 2-3 turns should be the zone on your mixture screw. If the carbs are surgically clean and you're finding you need to do fewer than 2 or more than 3 turns out...change the pilots. But that should be a last resort and not your issue.

All the jets are squeaky clean - I've rebuilt plenty of carbs on cars, so I know all about getting these things clean. I went through these pretty thoroughly cause this bike wouldn't run when I first got it. It then sat for about 4 months (drained) while I built the bike, so it could have built up some scale in there somewhere. It's running pretty well now, but next weekend I might pull the tank off again and look for an air leak. I'd say it's 95% there, still stumbling a little bit at idle now and then, but overall, is running like it should. I'll see how it runs through the week when I get it into a better variety of riding scenarios - highway, stop & go traffic etc. If I find an air leak or it's still not running right, I'll yank the carbs and as long as they're on the bench, tear them down again and go thru them.

[1BadSvt]

The best way to find a air leak is to spray carb cleaner all around. If it revs up you found the problem. Now if it's not a air leak I am willing to bet you have the wrong Pilot jet or the 3 holes in front of the throttle blade (the ones your Idle air mixture feeds) are clogged. Also what size jets are in it? As stated before you idle mix screw SHOULD be at least 2 turns out. Bottom it out then back it out two turns all the way across. I am sure that is where your issue is. If you like you could ride the bike down here to the springs one day and I could knock them out in a hour. I do them all the time.

[1BadSvt]

Just to throw it out there do you have "POD" filters or stock air box or do you have a jet kit?

[Kevin]

The best way to find a air leak is to spray carb cleaner all around. If it revs up you found the problem.

Yeah, just like cars - done it before. Thanks for the suggestion. I'll definitely have to check it, if for no other reason than this bike was so screwed up and it's something I never checked.


Now if it's not a air leak I am willing to bet you have the wrong Pilot jet or the 3 holes in front of the throttle blade (the ones your Idle air mixture feeds) are clogged. Also what size jets are in it? As stated before you idle mix screw SHOULD be at least 2 turns out. Bottom it out then back it out two turns all the way across. I am sure that is where your issue is.

I know the holes you're referring to - they were all clear, carb cleaner sprayed cleanly through them, but that was 5 months ago, and it's been sitting with the carbs drained all this time. It wouldn't hurt me to go through them again. I have no idea if it's the stock jets or not. I'll look next time I have them down.


If you like you could ride the bike down here to the springs one day and I could knock them out in a hour. I do them all the time.

I appreciate that - I might take you up on it, if I end up banging my head against a wall in the next couple weeks. As an automotive tech, I know all too well there's the 'textbook' way, the 'backyard' way, and then there's the 'right' way (how it's done in shops every day). I'd love to learn the right way. It'd be a fun road trip. I'll buy the beer. :)


Just to throw it out there do you have "POD" filters or stock air box or do you have a jet kit?

Nope, stock airbox, K&N filter. No jet kit that I'm aware of, but I bought this thing in junkyard condition and I know none of it's history except that some backyard mechanic (and that's being nice) screwed everything up on it about as much as it can be screwed up. It's a wonder I was able to bring it back to life.

My other symptom is that I can't seem to find a happy spot in the choke to keep it running when it's warming up. It might be better now tho - I've only ridden it once, but noticed the choke was still really touchy this morning. I'll have a better feel for how it's doing after my adjustments by the end of the week.

[FZRguy]

Long but the absolute best thing I've ever read re dailing in CV carbs.

CV Carb Tuning
by Marc Salvisberg of Factory Pro

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

Follow steps in order....First dial in:

 1. Top end (full throttle / 7.5k to redline -
Best Main Jet must be selected before starting step 2 (needle height)!
 Select Best Main Jet
 To get the best, most even top end power (full throttle/after 7500 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 first - 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 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 jets (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)
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 throttle, without 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 Honda. 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've "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 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!

 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 to your solution will be installed in our computerized reference database. PLEASE CALL!
 We ask that upon completion of installation and tuning, that 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

[1BadSvt]

^That is awesome
Good write up there.

[Kevin]

Yup, air leaks there are. Repair I must. Diving in.

[stubbicatt]

I may be too late, or it may not make a difference, but I have a carb sync tool if anybody wants to borrow it.

[Kevin]

Interesting - 155 jets in the outer two carbs (cyl 1 & 4), 157.5 jets in the inner two cabs (cyl 2 & 3).

Is this normal???

EDIT: at least one source I found on the internet mentioned these specific sizes in the same spots as mine from the factory, so probably normal factory setup.