Major differences when syncing carbs at various RPMs

[Jethro]

Boy this thread is too technical for me... I have found that even when the carbs are out of synch, if you drop a gear and open the throttle up WIDE FREAKIN' OPEN the poor synch is much less noticeable. Vibration and the view in your rear view mirrors becomes a non-issue when you are in 4th gear at 9500 RPM's...

On a serious note, I synch the carbs at about 1800 rpm. It is always off when I check it at 4000, but whatever. At 1800 it is still pretty close at idle and not terrible at 4000 rpm.

[DimitriT]

Definitely an interesting thread..

Earl, I agree that throttle doesn't affect volume (I can't argue that the cylinder's volume isn't changing due to throttle). But I don't think it's volume we are trying to control with a sync. I think it is the amount of air (and fuel) per stroke we are controlling (by amount I'm talking mass). And the throttle definitely affects this mass.

One thing that I never thought about, but which someone mentioned, is how the throttle cable linkage could affect the sync. The vacume as measured when the throttles rest on the idle adjustment knob may be different then when they are being pulled by the throttle cable. Next time I do a sync I'll try and test for this and see if there is a difference and how bad it is.

[nert]

Here's a question: will regular nitrile (or whatever) o-rings on the intake tap nozzle thingies survive the heat if I leave them on and capped? I'd love to just leave those suckers on all the time so I can sync more easily.

I made little manifold extensions from 3/16 brake line, a metric nut, and vacuum cap, X 4 = 1 bike. then x 3 = all 3 of my 82 gs1100glz's.

(1) 6" x 3/16 brake lines, cut in half gives you (2) 3" adapters.
I run a metric die over the the non-flared end of tubing.
Install the metric nut and then i use the OE washer that comes on the plugs.
Screw in the adapter, snug the nut, install a vacuum cap over the flared end.

Option:
I bought a long length of line as i had so many (12) to make, and just flared my own ends. I also put a nice radius bend on the tubing so they would extend out to the side of the carbs for easier access.

Total cost, including the brake line, nuts, vacuum caps, and metric die was under $15.00

I would show a picture but have had no luck at posting an image.

[nert]

Definitely an interesting thread..

Earl, I agree that throttle doesn't affect volume (I can't argue that the cylinder's volume isn't changing due to throttle). But I don't think it's volume we are trying to control with a sync. I think it is the amount of air (and fuel) per stroke we are controlling (by amount I'm talking mass). And the throttle definitely affects this mass.

Correct the cylinders volume does not change, as in "cc", "cid", or displacement.
The throttle DOES control the amount of air entering the cylinder. And more correctly defined as air mass as you indicated above. The metered air mass than expands to fill the cylinder volume with more or less density relative to its mass and cylinder volume (the mass of air throttled in will fill the entire cylinder either thinly, high vacuum small throttle opening, or densely, low vacuum large throttle opening). Again the difference being a diesel engine which has no throttle plate, or means of metering air. Wide open all the time.
I think the horse may be dead. I am!

[earlfor]

I was trying to keep things simple and illustrate a point. The point being that vacuum level is a measurement of velocity, and velocity is governed by volume and throughflow area. For that matter, there is nothing anyone can say about anything that I cant cite an exception to if I want to be picky. ehehehe

Your words, "I suggested that the throttle positions were greater when accelerating between 1500-4500rpm in a high gear, not 5th gear." Yes, the throttle will be in a more opened position, but where are the slides positioned? The slides must be in the same position at any given rpm regardless of throttle position or gear selected. The vacuum/intake velocity will only be correct at one slide height relative to intake air volume. Throttle position has nothing to do do with it, at least not on a CV carb'd GS.

But anyhow, my original intent was only to offer an alternative way of doing a carb synch and one that has worked for me in eliminating buzzyness at higher rpms. Whatever works for anyone is fine. :-)

E.

Dam interesting post guys.
Earlfor, your suggestion that an engine only ingests the volume that is displaced in each cylinder is incorrect. Highly tuned natrurally aspirated engines sometimes reach 85-90% efficiency.
By enhancing the scavenging process, an engine with a static compression of say 10-1 could end up with a running ratio of 13-1 or higher. This is the case when camshafts that produce moderate to high overlap between the inlet and exhaust valves are used to increase cylinder scavenging along with well designed intake and exhaust systems. The only way to improve on this efficiency is to use forced induction.
I suggested that the throttle positions were greater when accelerating between 1500-4500rpm in a high gear, not 5th gear.

[tconroy]

Would it change things when your bike is under a load at higher rpm's than sitting at a high idle when syncing? just a thought.

[denydog]

Not sure if this relates at all, but I just thought back to when I was a kid riding around with my Dad. His 51' Chevy had vacuum powered windshield wipers. I can still remember ervery time he stepped on the gas, the wipers would stop until the engine caught up and vacuum was restored. I know throttle position (and engine speed) was relevent to vacuum in that situation .

[t3rmin]

Not to mention vacuum timing advance in older cars.

[nert]

Would it change things when your bike is under a load at higher rpm's than sitting at a high idle when syncing? just a thought.

sure would. wow, that sounds like fun. guess we have to use the buddy system. One guy charging up the hill, and the other guy holding gauges and a screw driver.

[knotzilla]

What is a vacuum cap? Ted

[txironhead]

sure would. wow, that sounds like fun. guess we have to use the buddy system. One guy charging up the hill, and the other guy holding gauges and a screw driver.

Now THAT I'd like to see.....especially if I had my camcorder handy!:-D

[txironhead]

This is where having a buddy with a rear-wheel dyno would come in handy.....wish I had one!

[nert]

What is a vacuum cap? Ted

a vacuum cap is like an eraser tip with a blind hole in it. mmmm, somewhat like one of the rubbers that go on the end of a crutch. You could slip it over an open tube or fitting, and effectively seal the opening. Presuming its size was correctly chosen. It slides on and you can pull it off. Its the female equivalent to a plug!

are you intersted in the fabricated manifolds, are just the language?

[KEITH KRAUSE]

You guys are making too big a deal out of this carb synch stuff.
I just ask the bike "where would you like me to synch you", and they always say "oh, right around 3,500 rpm's Keith". Can't argue with that.

[t3rmin]

You guys are making too big a deal out of this carb synch stuff.
I just ask the bike "where would you like me to synch you", and they always say "oh, right around 3,500 rpm's Keith". Can't argue with that.

You're supposed to consume the beer WHILE working on the bike, not before!