Carb synch, idle adjuster or throttle?

[Macmatic]

Finally got everything ticking over nicely on my bike and now I'm synching the carbs. (One of) the general rule of thumb seems to be to set them at around 2500 RPM and I got them nice and set there.... then I backed off the idle adjuster to set the idle at around 1200 and things were not and good as they were at 2500 but still all within 1.5 units. Inches of mercury? I'm using a Carbtune II.

THEN I used the throttle to bring it back to 2500 and things were all out of whack. #1 was drawing about 26units and 2-3-4 were all drawing about 23. Idled back down and used the idle adjuster and all was good again. 1&4 at about 24units, 2&3 at about 23.5.

Seems to me like I should set it by holding the throttle at 2500 since I want it synched when I'm riding, not sitting.

Any common setup tips? I did bench synch and everything is in the ballpark. I also searched but there are so many carb synch threads already its hard to find a specific piece of information.

Thanks,
/\/\ac

 

[80GS750]

you want to use the idle screw to put them up to about 3k....

then synch them.

do not use the throttle... that opens other parts of the circuits in the carbs and thats why the vacuum is way off then....

set them all at 3k, then lower it back down to regular idle rpm

doing it with the idle screw insures that the slides are down as far as they can be... and also very close in height in relation to each other..

if any of that makes sense...

 

[tkent02]

Why not try it both ways and use the way that gives the best results?

 

[Steve]

You may get several responses, and just as many different opinions.

The Suzuki manual calls for 1500-2000 rpm when doing a sync. That assures that all of them are open, but I find that I have better results when doing it at idle speeds, right around 1200.

Looking at the physical aspects of a carb sync, you are trying to synchronize all the carbs so they open the same amount at the same time. If you think of carb openings as a percentage, idle might be thought of as 3%. If you have one carb at 1%, two at 3% and the fourth at 6%, you can see why there will be imbalances. If you keep those same differences and run the bike at 3000 rpm (might be 8% throttle opening in neutral), they would be at 6%, two at 8%, and one at 11%. The difference between them becomes less significant. When you look at full throttle, they would be one at 95%, two at 97% and one at 100%. The differences become even less significant. This is why I do them at the smallest possible throttle opening.

.

 

[nert]

I agree with Steve. Accurate sync at a smaller throttle opening will yield less significant differance at greater throttle openings. So, Sync at about 2000-2500.

Doesn't mean that even lower is better. If you sync at a much lower RPM, then fuel idle mixture circuit, air bypass, worn throttle shaft, and general build tolerance air leaks can effect the results. You want to Sync the air volume passing through the carb throat at the lowest RPM that does not overly represent the other air sources listed above.

AND, yes lock/hold the hand throttle at the RPM you intend to Sync. (very perceptive of you) The Idle speed screw does operate the carb linkage rack in a slightly different way (it pushes the linkage). NOT the way we open carbs with the hand throttle (the cable rolls the linkage as it pulls). All performed at the same location, different forces at work here. Minor, yes but makes a difference. Sync them like you use them (hand control) for greatest operating accuracy.

When you set the idle speed with the idle screw, adjustments to the fuel mixture screws will compensate for the "tolerance" and "build" air leaks i indicated above. This will provide a nice smooth and steady idle performance.

Opening the throttle by hand will then benefit you from the adjustments made when controlling the hand throttle during your Sync. Again, optimum smooth, balanced and equal air delivery.

80GS750
"do not use the throttle... that opens other parts of the circuits in the carbs and thats why the vacuum is way off then...."

I am sorry 80GS750 to disagree with your statement above, but i am not aware of what you say. The VS carbs on my 1100's do not operate as you indicate. Perhaps yours are different.

(as always, my opinion)

 

[Macmatic]

80GS750,

I'm can't really agree with you on this either. We care about carb balance when we're using the throttle, not the adjuster screw. Since I get differing results with the idle adjuster and the throttle I'll go by the throttle since thats what matters on the road. Maybe your way works best for your application.
-----
Steve,
I've tried adjusting at around 1200rpm in the past but they never seem to be synched at higher rpm. This is about the fourth time I've synched carbs and the first time I thought of using the throttle... So I bet if I'd synched them at 1200 in the past using the throttle cable adjuster to hold the speed instead of the idle adjuster I would have had better results.
----
To make it "easy" to set balance the carbs at 2500 rpm (checked at 4k and 7k also) I used the throttle cable adjuster to take out the slack and hold the idle at 2500rpm. I think that this is the same as rolling on the throttle as far as the carbs know.

I set 1&4 to 24cmHg and 2&3 to 23.5 and then checked the balance at 2500, 4000 and 7000rpms. Balance was good at those three speeds but somewhat farther out at 1200 rpm resting on the idle adjuster. They were all still within 2cmHg and it idles well.

Out of time for today but tomorrow I'm going to finish re-wrapping the wiring and putting things back together. Tuesday will be the test ride and I'll post back if I find problems with the balance as I've done it.

Thanks for all the input, I know this is almost like asking what brand of oil is best...
/\/\ac

 

[Steve]

I've tried adjusting at around 1200rpm in the past but they never seem to be synched at higher rpm.

Another thing to keep in mind is that while the bike is in neutral, not under load, your throttle openings will be much smaller than they will be on the road, under load. I barely have to open the throttle to hold 5,000 rpm while on the centerstand, but it takes about 1/2 throttle or more to hold the same engine speed at 70 mph, so the 'sync' openings might be ever so slightly different.

In the end, do whatever make your bike comfortable for you. Hope it's as smooth as my wife's 850L when you are all done.

.

 

[tkent02]

These bikes are all old.
They are all different.
What works well on one may not work at all on another, even the same model.
Experiment.

 

[Macmatic]

I wont have a chance to get out and ride until Tuesday but I'm pretty confident in this synch. When resting on the idle screw things are a little uneven but as soon as I take up the slack in the throttle cable enough to raise up to 1,500 rpm or so the readings level out again.

I THINK what I've done is balance them in the work condition, open with the throttle and the imbalance I see in the rest condition is due to the spring pressure. The difference is acceptable even on the idle screw but pretty much gone as soon as I'm actually on the throttle.

If that made sense.

/\/\ac

 

[nert]

surgically speaking, i talk about sync at 2000 to 2500 RPM. BUT in practice.....
i continually make adjustments, and roll the throttle from a low RPM to a higher RPM, including a couple throttle blips, (don't do that with mercury or water) and get the best overall even rise, fall and steady hold for all four carbs. I takes a bit of work, but i get the best results this way. I ride on the road, not in a hypothetical laboratory.

(3) things need to be considered.

1. These are probably not the easiest skills to develop, or communicate to a person who works on only one bike, and only rarely. (like everyones feel on "feeler gauge" accuracy is a little different, or tighten to "mechanics tight" may vary from individual to individual)

2. When you hold the throttle at 2000 rpm on the center stand, (no load) the throttle position is less than it is at 2000 rpm driving down the road, or up a road, or different gears. The air flow for throttle position is not consistent for load. So if you Sync for one throttle position, and one load/no load, then for those conditions, it is the absolute best adjustment. How often you obtain that same load/no load/RPM while driving is probably a VERY small percentage of the time. SO, i go for best sync through a range of throttle opening.

3. Normally every time you turn one of those screws and tighten the lock nut it orientates the linkage and levers by some very small amount making accurate adjustment so much more elusive. Returning the throttle to idle and continuous operation from the hand throttle, cause all the setting to normalize as they would while you are riding.
I have performed on many carb racks the tension screw modification to eliminate the lock nut from the adjusting screw which can actually add pleasure to "running the rack". So one can easily go from, adjusting screw to adjusting screw, single handedly while throttling with the other hand and observing your gauge set.

STEVE
"Another thing to keep in mind is that while the bike is in neutral, not under load, your throttle openings will be much smaller than they will be on the road, under load. I barely have to open the throttle to hold 5,000 rpm while on the centerstand, but it takes about 1/2 throttle or more to hold the same engine speed at 70 mph, so the 'sync' openings might be ever so slightly different."
Sorry Steve, didn't mean to steal your thunder. I guess i was "composing" when you slipped that in. I reply much slower than others.

Macmatic, i think you've got it!!

 

[80GS750]

im running VM series carbs...

the reason i say to run it at 3k for sync is becuase the motor is pulling way more vacuum at higher rpm, and its easier to dial in the gauges....

thats my experience anyway...

i was instructed by keith kraus to do it this way with VM carbs, and it worked out great for me

 

[Macmatic]

80GS750

Heh, I don't think I'd know a VM if it bit me. The oldest bike I've worked on so far is my friends death machine '82 GS750.

I have performed on many carb racks the tension screw modification to eliminate the lock nut from the adjusting screw which can actually add pleasure to "running the rack". So one can easily go from, adjusting screw to adjusting screw, single handedly while throttling with the other hand and observing your gauge set.

That sir, is an epiphany.

I hope I'm not pulling these carbs again soon but I've got a spare set so I'll be gathering parts for one method or another. I'm going to take a look and see if I can find some adjuster needle springs from 2-cycle carbs that will fit the proper size pan head screw, if not I'll be drilling and clipping....or something.

Thanks!
/\/\ac