Major differences when syncing carbs at various RPMs

[KEITH KRAUSE]

Also, you'll get a slightly leaner mixture because of less vacuum, not a richer mixture.

 

[Guest]

I thought you syncd the cylinders differently was because the outer cylinders run cooler than the inner because of the cooling fins

 

[49er]

Clymer manual for 79 850's details the correct sync for VM carbs when vacuum is equal. They then suggest that CV carbs from 1980 onwards are synced differently on 2&3 carbs.

 

[KEITH KRAUSE]

Well, if there's a crossover tube involved, any model any year with either carbs is synched with 2/3 a little less vacuum.

 

[49er]

Makes you want to run a tuned 4-1 with stock carbs and airbox, doesn't it?

 

[t3rmin]

Ok, finally got around to doing some more tuning today (weather finally permitting). This time I synced at 4-5k RPM/throttle held. It's definitely smoother through all RPMs than the 750 RPM/idle screw sync. More power, less vibration, and generally a more refined feel. My impression is the bike seems "happier" with this sync. Even idle and off-idle smoothness is improved, which somewhat surprised me, since I figured syncing at 750 rpm with idle screw would give the best idle.

Also, the previous sync was still pretty much spot on, even though I've removed the locknuts and am relying solely on blue loctite to keep the sync screws from vibrating out of adjustment.

 

[steve-lloyd]

Something that should be realized is that the 4000-5000 rpm buzz is more an engine balance issue than carb. balance. All reciprocating engines have what is known as a vibration period. It is not possible to balance an engine perfectly. Even though the GS uses an efficient gear driven balancer (unlike the near-useless chain-driven efforts used by Hondas for example) there is always remaining an engine speed range where the engine vibrates more. You might think a 4 cylinder engine should be smooth without a balancer with the pistons opposing each other as the engine rotates, but they produce what is called a "Rocking couple" (yes, really!!!) This all starts getting very technical. but the point is that all engines have a rough period, and on the 4 cylinder GS it's 4000-5000 rpm, and you will never get rid of it completely.

 

[Nessism]

Something that should be realized is that the 4000-5000 rpm buzz is more an engine balance issue than carb. balance. All reciprocating engines have what is known as a vibration period. It is not possible to balance an engine perfectly. Even though the GS uses an efficient gear driven balancer (unlike the near-useless chain-driven efforts used by Hondas for example) there is always remaining an engine speed range where the engine vibrates more. You might think a 4 cylinder engine should be smooth without a balancer with the pistons opposing each other as the engine rotates, but they produce what is called a "Rocking couple" (yes, really!!!) This all starts getting very technical. but the point is that all engines have a rough period, and on the 4 cylinder GS it's 4000-5000 rpm, and you will never get rid of it completely.

Note, the only GS engine that has a balancer is the 400/425/450/500 twin models.

Some in line fours have balance shafts as well but the GS family are not included.

 

[txironhead]

Isn't that what's termed the "engine harmonics"? Trying to scrape that last bit of high school auto mechanics class from the dark recesses of my brain.....