Major differences when syncing carbs at various RPMs

[t3rmin]

This has been somewhat discussed before but I want to bring it up again and maybe get into some more detail on this one point.

I was syncing the carbs today with my "carb stick" and getting some wonky results. When synched perfectly at idle, it was way off at 4k RPM and when synched at 4k RPM it's pretty far off at idle. It's off enough that another person doing a sync at the other RPM would look at it and think it hadn't been synced at all (about 2 inches of variation between highest and lowest and it's no longer at all even between 2-3 and 1-4).

One thing I noticed is there did seem to be a difference between turning the idle screw up to 4k and actually holding the throttle there with my hand. Somebody mentioned this before that they thought the idle screw put slightly different (uneven?) stresses on the throttle linkage and that the handgrip throttle was a better way to sync 'cause that's what you use while driving. Can anyone corroborate this?

So I synched it at 3-4k using my hand on the throttle. Haven't ridden it yet but I hope it'll be smoother since this is the first time I've synched it like this.

[tconroy]

This has been somewhat discussed before but I want to bring it up again and maybe get into some more detail on this one point.

I was syncing the carbs today with my "carb stick" and getting some wonky results. When synched perfectly at idle, it was way off at 4k RPM and when synched at 4k RPM it's pretty far off at idle. It's off enough that another person doing a sync at the other RPM would look at it and think it hadn't been synced at all (about 2 inches of variation between highest and lowest and it's no longer at all even between 2-3 and 1-4).

One thing I noticed is there did seem to be a difference between turning the idle screw up to 4k and actually holding the throttle there with my hand. Somebody mentioned this before that they thought the idle screw put slightly different (uneven?) stresses on the throttle linkage and that the handgrip throttle was a better way to sync 'cause that's what you use while driving. Can anyone corroborate this?

So I synched it at 3-4k using my hand on the throttle. Haven't ridden it yet but I hope it'll be smoother since this is the first time I've synched it like this.

I go by the book and the book say's to do it at 1750 rpm's.My bike runs exellent at all speeds.

[t3rmin]

Well I got 'er back together and took it for a test drive and it freakin' pulled the front wheel off the ground when I shifted into second one time. Dang!

[chef1366]

Well I got 'er back together and took it for a test drive and it freakin' pulled the front wheel off the ground when I shifted into second one time. Dang!

Sounds like you did something right:-D

[earlfor]

I usually do a preliminary vacuum synch at around 1500-1700 rpm to get everything in the ballpark. Then I do a final synch in the rpm range I ride the most in. For me that is about 4000-4500 rpm. I like having my carbs come into perfect synch at the rpm I use the most. They will usually be out of synch a bit at idle rpms though done this way. I dont use the idle adjustment screw for maintaining engine rpm because the idle screw activates the throttle linkage differently than does twisting the throttle. (you can set the idle adjustment screw to hold 1700 rpm and synch the carbs, disengage the idle adjustment screw and hold 1700 rpm with the throttle and you will find the carbs will not be in the same synch with throttle as they were with the adjustment screw. I always synch on throttle.

Earl

[KEITH KRAUSE]

I synch around 3,500 rpm's, using the throttle. Always have. It made more sense to me than synching at the much lower factory recommendation.
Remember, if your model has the stock exhaust requiring the middle cylinders to have a little less vacuum, I suggest you follow that. I've never seen in person the small "official Suzuki vacuum gauge" that uses the balls but I think adjusting the middle cylinders about 3/4" lower on the full size gauges I've used is very close to the "lower by 1/2 the ball diameter" that the small gauge says.
If you set the levels well at idle but they become significantly uneven as you increase rpm's to say, 4,000 rpm's and vice-versa, most vacuum gauge side info will tell you there's a problem. Low compression, incorrect valve clearances, intake leaks, worn slides or worn carb bodies, etc, can cause this. Anything that effects flow/vacuum actually. Even some restrictive exhausts.
Sometimes, especially if you're happy with the performance, it's just not practical to worry about it on our older bikes. But compression, valve clearances and intake leaks should always be checked/repaired before any synch, along with tuning the mixture/pilot air/pilot fuel screws for highest rpm.

[GregM.]

I'm not sure how to ask this... But, is carb synching a proportional thing??
Meaning, if you get all synched up at idle, or 1750 RPM... Would you then expect to run really smoothly for a couple thousand RPM up from that?

Conversely, if you synch at 4000 RPM... would you expect to run really smoothly a couple thousand RPM above, and below that number?? But then at idle, and up near 7 grand, you might be running kinda rough.

Maybe ideally, you're supposed to be smooth all through the power range, with a synch at 1750. But with 20 - 30 year old bikes with questionable up-keep - what's the real world advice here.

Or, am I completely off on this - barking up the wrong tree all-together.
Any enlightenment would be great. Thanks.

[KEITH KRAUSE]

The factory tuning recommendations are always based on a new motor that obviously meets all required spec's. If the motor is in that kind of condition then a good vacuum synch at the factory recommended rpm will be within spec at all other rpm's. Working with all new parts makes things a lot easier.
On older bikes with all the issues that can come up over the years, something like carb synching may need some adjustment/compromise to work in the real world. Unless you're willing to replace EVERY part that has anything to do with a good synch, the best we can do is be sure the bike meets basic compression requirements, has good spark/timing, good fuel flow, clean parts that have no serious wear, etc. If you have that, then I suggest synching at an rpm that's closer to where you're normally running the bike, which is closer to 4,000 than typical factory idle rpm's. In most cases, that will work well.

[GregM.]

Sorry to hi-jack a little here fellow Washington-ian...

But Kieth (or any of you other experts) - Speaking of compression... do you have any info for me on what good comp. #'s would be. Bike below with 23K.

Maybe an excellent range - "Dude you have a tight engine". Set of #'s.

An OK, or good range - "Dude, it's not great, but you can probably live with that, and get your bike reasonably tuned-up without major work. Set of #'s.

A not-so-good range - "Dude, your engine's in trouble... it's gonna take some major bucks if you ever want that thing to run half way decent". Set of #'s.

Thanks mucho

[nert]

I usually do a preliminary vacuum synch at around 1500-1700 rpm to get everything in the ballpark. Then I do a final synch in the rpm range I ride the most in. For me that is about 4000-4500 rpm. I like having my carbs come into perfect synch at the rpm I use the most.

Earl

Syncronized carburetors have much to do with equal volumes of air flowing through all the carburetors at any given throttle opening. I very much doubt that a high idle, no load speed of 4000-4500 rpm is gonna have the same throttle opening when pushing 600lbs, 60 mph down the highway. If that where true we might all be getting at least 150 mpg. So when driving in high gear at 4000-4500 rpm there is a much greater percentage of throttle opening compared to where they where syncronized. Idealy the air volume would remain linear and equal as the throttle is progressed. Mechanical differances in carburetor throat, throat finish, butterfly dimensions and shape, throttle shaft play, slide spring tension, lingage arms, alignment of the carbs in the rack as fastened to the angle braces all effect performance of equal air flow when the throttle is twisted. Earlfor you probably have a good set of carbs. Much of this has to do with manufacturing tolerances and quality, material choices, etc = enginnering = cost.

I always use the hand throttle for syncing. Since i employ the famous nert sync screw modification, its easy for me to enable changes. I will make a prelim sync at under 2000 rpm. Then i fine sync through various rpm ranges, attempting to get the best overall, close proximity sync through all ranges of opertion. I am careful to "blip" the throttle after any adjustment to "settle" or "neutralize" any screw tension, bind and linkage tweaks.

I think to put it short. i apply nerts second rule, "do the best you can with what you got"

[earlfor]

Since my method has worked fine on every bike since about 1970, it probably isnt that I have always gotten a set of "good" carbs. :-) BTW, intake velocity is governed by throughflow area, and volume is controlled by engine rpm. Load will not vary those two things.

Earl

Syncronized carburetors have much to do with equal volumes of air flowing through all the carburetors at any given throttle opening. I very much doubt that a high idle, no load speed of 4000-4500 rpm is gonna have the same throttle opening when pushing 600lbs, 60 mph down the highway. If that where true we might all be getting at least 150 mpg. So when driving in high gear at 4000-4500 rpm there is a much greater percentage of throttle opening compared to where they where syncronized. Idealy the air volume would remain linear and equal as the throttle is progressed. Mechanical differances in carburetor throat, throat finish, butterfly dimensions and shape, throttle shaft play, slide spring tension, lingage arms, alignment of the carbs in the rack as fastened to the angle braces all effect performance of equal air flow when the throttle is twisted. Earlfor you probably have a good set of carbs. Much of this has to do with manufacturing tolerances and quality, material choices, etc = enginnering = cost.

I always use the hand throttle for syncing. Since i employ the famous nert sync screw modification, its easy for me to enable changes. I will make a prelim sync at under 2000 rpm. Then i fine sync through various rpm ranges, attempting to get the best overall, close proximity sync through all ranges of opertion. I am careful to "blip" the throttle after any adjustment to "settle" or "neutralize" any screw tension, bind and linkage tweaks.

I think to put it short. i apply nerts second rule, "do the best you can with what you got"

[SqDancerLynn1]

If he compression readings are below 100 psi your in trouble I should say your motor is in trouble. my 850 40K mi still has acceptible reading 125 psi across the board

[t3rmin]

Well I haven't done a compression check and honestly I'm not really sure I want to. The thing runs GREAT, has lots of power and is relatively smooth. It doesn't use any oil that I can tell and there's zero blue/black smoke from the exhaust at all at first start or while riding hard. If it's running this strongly and still has some ring issues or something, I'm better off just not knowing. ;-)

Like Nert said, with a 25yr old bike, you make do with what you've got and don't necessarily sweat the small stuff if you don't have to. My carbs are clean as a whistle, my intakes and intake o-rings are in excellent shape, I just finished dialing in my valves to 0.05mm across the board, I've got a brand new Dyna S ignition and good coils, etc., etc. All the basics are accounted for and solid, so I'm pretty satisfied at this point. Besides, from the responses here it doesn't seem like I'm seeing anything too out of the ordinary.

One thing I probably will do is re-sync one more time with even more emphasis on 4k RPM with a hand on the throttle. I was a little tentative and I think I left it at a bit of a compromise with lower RPM sync. Next time I'll get 1-4 spot on 1 inch above 2-3 at 4K throttle-held RPMs. It's still got some of that annoying 4k RPM buzz but otherwise no complaints.

[GregM.]

Thanks t3rmin, for letting me learn here about this too.

Maybe we oughta set up a ride around Mt Rainier next season, I'd like to see that "clean-carbed" bike that's poppin' wheelies for you

Thanks Lynn, I'll get a comp. check and hope for around 125 PSI. BTW, I read here about a "leak-down" test?? What's that? I probably should check that out too.

Then a carb synch at mid-RPM ranges... maybe that'll help with the major vibration/BUZZ that starts in at 5 grand, and just gets worse.

So much to learn... all teachings appreciated.

[49er]

I support Nert on this issue.
The increased load produced by accelerating 600lbs up a gradient from 1500 - 4500rpm in a high gear requires a far greater throttle opening than a static 4000 rpm sync. Wind resistance and gradient have a big influence on throttle opening and response.
Keith is right when he points out that the lower factory recommended sync rpm's are suited to an in spec new motor with all clearances correctly set.
On the VM carbs, I set the pilot air screws to the highest idle speed, then sync at 2000 rpm on the idle stop. I don't see any advantage in holding the throttle at this speed when the idle adjustment screw is adjacent and both are offset to to the middle of the shaft assy anyway.
I then return the throttle to 1100 rpm and re-adjust the air screws to gain the highest rpm. One or two usually require slight re-adjustment. I bring the idle back to 1100 rpm and then check throttle response staticly through the rev range. When respose is good I then road test and do plug chops at various rpms. Sometimes an adjustment to the pilot fuel screws is called for especially in the 2000-4000 rev range.
When running a 4-1 exhaust, it is common for the idle - 1500 rpm range to be more irratic than for the standard exhaust. Having said that, a good 4-1 on a well tuned motor should still idle evenly provided any other after market indulgences have been well chosen and balanced.