Carb sync RPM

[Premature Accelerato]

Although the manuals I have read state that carb sync should be done with a hot engine and at idle speed, would there be any benefit or danger from balancing the carbs at a higher rev range, say around 5000rpm. I ask this as the bike I am tuning is used mostly for track days so it is usually in the high end of its rev range. I am using a mercury tube balance and am aware that if you rev it too high it will suck the mercury in to places you dont want it to go but I know that at 5000 it has a long way to go before it gets any where near the top of the tubes.

[bogan]

I assume the carb sync means adjusting the throttle butterflys for the same intake vacuum.

Its done at idle because that is where there is high vacuum which is sensitive to small changes in butterfly alignment. Trying to do it at higher revs will be more difficult and the result will not be as good. So stick with the manufacturers recommendations this time :niceone:

[Coolz]

I don't know what sort of bike you ride but on my twin carb vtwin you first do the idle sync by adjusting the cable between the carbs. Then you do the off idle sync at higher revs by adjusting the accelerater cable at the front carb.

[Premature Accelerato]

Hi Bogan. Ok so I will keep to the spec. Pardon my ignorance but how come the vacuum is greater at low RPM. I would have thought it would take more vacuum at higher rpm to lift the mercury higher in the columns. Am not that great on physics.

[bogan]

Hi Bogan. Ok so I will keep to the spec. Pardon my ignorance but how come the vacuum is greater at low RPM. I would have thought it would take more vacuum at higher rpm to lift the mercury higher in the columns. Am not that great on physics.

Yeh higher lift would be higher vac, is it sustained at revs though, or just on decel? I would expect the vac to decrease as the throttle is opened, under load anyway, might go the other way when unloaded.

[Premature Accelerato]

When the rpm are incresed the column lifts, it then stays at its new level as long as the revs are held steady. The column does lift a little when the revs are dropped and then the column then settles at a new lower level.

[ducatilover]

Vacuum decreases with more throttle. Vacuum is formed between the cyl and butterfly, so less butterfly in the way = less vacuum.
Just sync it at idle

[Paul in NZ]

Vacuum is irrelevant - its the difference between the readings thats important.

On the Guzzi it has slide carbs (ie non cv) so you balance it at idle on the slide stops and idle mixture for a smooth idle and at higher rpm on the cable length for smooth running.

smooth in this case being a relative term