Carb sync at higher rpm's

[Venturous Randy]

On a ride the other day I noticed a vibration when under power. If I pulled in the clutch at speed, the bike was very smooth. I figured that before I started down the mechanical road, such as checking the u-joint, I would check the carb sync.

After a ride to get it up to temp, I shut it off and hooked up my four tube mercury sync tool. I started it up and it was slightly off so I tweaked it a little and my idle got a little smoother. One of the things I also do is bring the bike up to about 3000 rpm's and see how close they are at that point using the throttle lock to hold rpm's. I noticed this time that I had a bit of difference from carb to carb. I adjusted to bring them back even, but when I brought it back down to idle, I was off a little again.

I have not checked my diaphrams yet and am planning on replacing them but I was wondering if anyone else runs their engine to a higher rpm when syncing carbs. You just need to bring the the rpm's slowly down to idle if you are using a mercury sync or it will suck the mercury into the carbs.

RandyA

[buddy]

Morgan Carbtune is what I use and its awesome - came in last week. I had a vibe ration as well so I did the same thing you did and yes my 83 was a tad out after the 3,000 rpm test. I played with it untill I had it looking nice at 3,000 and at idle. Its not perfect dead on but boy it sure did wonders for the Motor, never knew these motors ran so smooth Morgan carbtune did wonders. The 03 RSV had a slight vibe ration on the hwy as well so I did carburetor adjustment on it to. The readings were not bad but just enough to cause a slight vibe ration...The 03 had the same readings as the 83 after the 3,000rpm test.I played with it a little to get it where it was real close and they both came out smooth running....

 

Buddy

[ToRide1]

I was shown how to do this back in the 70's and knowing the RPM's you run at most of the time in high gear and sync the carb's at that RPM.

[N3FOL]

I have synched my carb twice now. Everytime I synch the carbs, it is done at idle...but I do rev the engine time to time as I tweak the Carbtune to make it perfect.

[GeorgeS]

I do the low speed set, then take up to 3000 slowly, Record the readings, then I split the difference, and set them there. At least shoot for that.

 

I did this this the last time, it seemed to be better,

[GG54172]

You guys do realize that the throttle position and engine operating conditions at 3000rpm and no-load is nowhere near 3000rpm and loaded...right?

[RPG]

You guys do realize that the throttle position and engine operating conditions at 3000rpm and no-load is nowhere near 3000rpm and loaded...right?

 

Exactly! The mechanical sync of the carbs set at lower RPM should continue to be sync'd through the throttle arc. You would need to put the bike on a dyno under load to "safely" check for accurate sync for anything more than slightly above idle sync.

[GeorgeS]

I set them at Idle, the redo it at 3000, then try to split the difference.

 

All I can say, is it was and still is running great.

[Marcarl]

The theory behind the sync, is to get the vacuum all the same for each carb at idle, that way you know that all butterflies are in close position to each other,,, you'll never be dead on at all times, but close enough to spit at. When you open the butterflies (throttle) the opening becomes larger, much larger, and the slight difference from one to the other will have very little effect on the overall performance. Hence the sync is set at idle where the difference is most noticeable.

Hope this makes sense so far.

If your carbs are synced at idle and you have a vibration at 3- 4000 rpm. my suggestion is that you have other issues, such as diaphragms, dirty or partly dirty carb, plug issues, wire problems, or at worst ring problems.

[GG54172]

The theory behind the sync, is to get the vacuum all the same for each carb at idle, that way you know that all butterflies are in close position to each other,,, you'll never be dead on at all times, but close enough to spit at. When you open the butterflies (throttle) the opening becomes larger, much larger, and the slight difference from one to the other will have very little effect on the overall performance. Hence the sync is set at idle where the difference is most noticeable.

Hope this makes sense so far.

If your carbs are synced at idle and you have a vibration at 3- 4000 rpm. my suggestion is that you have other issues, such as diaphragms, dirty or partly dirty carb, plug issues, wire problems, or at worst ring problems.

 

 

Right, setting the butterflies at idle also make sure that every cylinder gets warm, and stays warm at idle. Since each cylinder is ready, tip-in throttle response improves.

[pegscraper]

The theory behind the sync, is to get the vacuum all the same for each carb at idle, that way you know that all butterflies are in close position to each other,,, you'll never be dead on at all times, but close enough to spit at. When you open the butterflies (throttle) the opening becomes larger, much larger, and the slight difference from one to the other will have very little effect on the overall performance. Hence the sync is set at idle where the difference is most noticeable.

Hope this makes sense so far.

If your carbs are synced at idle and you have a vibration at 3- 4000 rpm. my suggestion is that you have other issues, such as diaphragms, dirty or partly dirty carb, plug issues, wire problems, or at worst ring problems.

 

 

Setting the sync at anything above idle is not the way to do it. It won't really hurt much, but it won't give you the best job.