Major differences when syncing carbs at various RPMs

[t3rmin]

Yeah I know that's gotta be it... I think maybe I had a vacuum leak on the intake fittings -- didn't screw 'em in hard enough or something. Thinking back, when I took it for a test drive with the fittings still in and capped, I think it was smoother. Now that the hex screws are back, so is the vibration. Perhaps there was an ever so small vacuum leak and I actually synched TO the leak, meaning the balance was better _with_ the slight leak.

I'll have to have another go at it.

 

[earlfor]

Your confusing throttle opening with vacuum. Yes, wind resistance and gradient have a big influence on throttle position, but required vacuum level is the same regardless of throttle position. There will be momentary variances due to induction geometry, surface rougnhess and a change in the slide position, but the engine normally operates at a constant vacuum level, or to put it another way, engine intake of air per revolution is always the same.

E

If you're accelerating up hill with a heavy load in 5th gear from 1500 rpm, you're outside the operating parameters of your engine. Good way to destroy an engine.

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.

 

[DimitriT]

Unless you can sync while riding or on a dyno, I don't see the point in doing a carb sync at different RPMs. Its really throttle position you want to account for, not RPMs, right? With your bike in neutral, the RPMs have nothing to do with throttle position compared to when you are riding it. The last time I synced carbs (on the 550 after a top end), I just went with about 1.5K RPM and got them all even. Before the sync I carefully cleaned the adjustment screws and springs with a brush and some compressed air. I didn't want any dirt interfering with the settings. Once the sync was done I put a drop of blue loctite on each jamb nut.

 

[earlfor]

No, throttle position doesnt have anything to do with vacuum level. Neither does rpm. The engine is an air pump. It will always ingest the same volume of air per revolution. It makes no difference whether the engine is running at 1 rpm or 10,000 rpm's. The only thing that changes is the time taken for each rpm. Vacuum and air volume per revolution are constant. Throttle position does not change that. The variance in vacuum levels noted between a synch at idle rpm and a synch at 4-5K rpm is due to intake geometry and surface conditions of the intake tracts, and differences in linkage play between each carb. If each intake had the exact same geometry, had the exact same inside surface and there was zero play in the linkages, then you could expect to see a synch done at idle rpm remain spot on at 5k rpm. Things are never made perfectly

My opinion is that most high rpm buzzing is due to a synch that goes slightly out of phase because of manufacturing variances that slightly change the flow properties. The only way to regulate intake velocity is through measuring the vacuum level. If you have a high rpm buzz, about 9 times out of 10, doing an accurate synch at the buzz rpm will make solve the problem. There is a vibration because power output from each cylinder is not the same.

E.

Unless you can sync while riding or on a dyno, I don't see the point in doing a carb sync at different RPMs. Its really throttle position you want to account for, not RPMs, right? With your bike in neutral, the RPMs have nothing to do with throttle position compared to when you are riding it. The last time I synced carbs (on the 550 after a top end), I just went with about 1.5K RPM and got them all even. Before the sync I carefully cleaned the adjustment screws and springs with a brush and some compressed air. I didn't want any dirt interfering with the settings. Once the sync was done I put a drop of blue loctite on each jamb nut.

 

[tconroy]

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"

All i know is I synced mine at 1750 and I get 58 to 60 mpg's and it can pull the front wheel in 2nd gear so I guess I will live with that!

 

[Guest]

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.

Nitrile rubber is usually OK to about 200 degrees Fahrenheit.

 

[nert]

No, throttle position doesnt have anything to do with vacuum level. Neither does rpm. The engine is an air pump. It will always ingest the same volume of air per revolution. It makes no difference whether the engine is running at 1 rpm or 10,000 rpm's. E.

That is true for a diesel there is no throttle plate. Intake is wide open. only thing changes is the fuel delivery. A gas engine throttles the amount of air. The air and fuel mixture, approximately 14.7 to 1.0 is required for efficient combustion. Increase the fuel, you must increase the air. Increase the air, you must increase the fuel. For max power, design problems has always been stuffing more air into a cylinder. That is why turbochargers and superchargers produce such a great power boost. They force a greater amount of air into the cylinder, along with the appropriate amount of fuel. Its easy to dump any amount of fuel into a cylinder.

earlfor
I don't doubt, you have good results with your current sync methods. possibly you should try a lower rpm sync and share unbiased results with us.

tconroy
yes, i agree. as suggested "my opinion" earlier post, a lower RPM sync, in theory should be more accurate than a high rpm sync.

My experience has shown VERY little turn of each sync screw can make a great amount of change in the sync tool reading. Visualizing the sync screw like a micrometer barrel, results in a mere thousands of and inch adjustment. Yes i can feel the difference after a good sync. But those carburetors that are greatly out of sync will find huge benefit from either method.

 

[earlfor]

I used to do a low rpm synch (as the books suggest). It was great if I wanted to cruise all day at 1700 rpm. Results (mine anyway) were better when I synched so the carbs all were "spot on" at my primary use rpm. I dont care if the synch is off a little at idle rpm. I hardly ride at all at anything below about 3500 rpm.

Earl

earlfor
I don't doubt, you have good results with your current sync methods. possibly you should try a lower rpm sync and share unbiased results with us.

 

[denydog]

Not to disagree with anyone's proven method but, I think I'll continue to synch my carbs at low rpm. At 1700 rpm, very minute adjustments of the throttle plate positions have a large effect on vacuum readings. Using these vacuum readings is a way to have all the throttle plates opening at the same time, allowing each cylinder to pull equally, or so I thought. A smoother Idle should be noticeable as well.

Along the lines of what NERT said, at higher rpm's I would think that small (thousandths of an inch) differences in throttle plate positions wouldn't have near the unbalancing effect that it does when the plates are only open a crack like at idle or slightly above. Another way of looking at it is .005 of an inch is a larger percentage change when the throttle is only open .01. But when the throttle plate is open an eight of an inch or more, how much difference is .005 going to make?

These numbers are totally made up for purposes of illustration, and so may be the argument. :-s Besides, I generally start out well below 3,500 - 5,000 rpm, so my idle and initial pull smoothness is of some concern to me..

 

[49er]

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.

 

[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