Carb Stand from old carb holders.

Well in the dozens of syncs I've done, you can alter the sync by 5 to 10 mm of mercury just by wiggling the carbs around a little on the engine. If it goes to over 10, I like to re-sync, though they say up to 20mm is ok. But I notice a difference in the running by then. Installing the carbs, especially with an airbox, is going to require a sync check for sure.

I didn't mean a bench sync was totally useless on a good engine. I meant the bench sync was not a good means of getting a final sync. A bench sync is always a good starting point. I just think a final sync should be done on the bike (assuming compression and valves are all set and good), and might as well since you'll have to set it up to check anyway (thus saving the extra time you would use to precisely sync the carbs on the bench).

If one cylinder is unable to flow as much air as the other cylinders (a low compression reading will show this), for whatever reason, it's vac reading will be lower than the others when all throttles are at the exact same opening. That cylinder will do less work since it gets less air. What's worse is, a vacuum sync would have you close down that one throttle to get the vacuum reading back up to where the other cylinders are at. So you have then reduced the airflow to the weak cylinder even more, and thus it will do even less work. This increases the uneveness of the idle.

Using vacuum to sync throttles is based on the assumption that all cylinders are in the same condition (and all carbs and carb holders are in the same condition, meaning not leaking air somewhere).

I suggest that the smoothest sync would be to time the intervals between firing and adjust the throttles so the interval was the same for all cylinders, but I've never tried to do it that way. It wouldn't be too hard with a scope. It would be interesting to see what the vacuum ends up at if done this way.

loudhvx wrote: you can alter the sync by 5 to 10 mm of mercury just by wiggling the carbs around a little on the engine. If it goes to over 10, I like to re-sync, though they say up to 20mm is ok. But I notice a difference in the running by then. Installing the carbs, especially with an airbox, is going to require a sync check for sure.

I didn't mean a bench sync was totally useless on a good engine. I meant the bench sync was not a good means of getting a final sync. A bench sync is always a good starting point. I just think a final sync should be done on the bike (assuming compression and valves are all set and good), and might as well since you'll have to set it up to check anyway (thus saving the extra time you would use to precisely sync the carbs on the bench).

If one cylinder is unable to flow as much air as the other cylinders (a low compression reading will show this), for whatever reason, it's vac reading will be lower than the others when all throttles are at the exact same opening. That cylinder will do less work since it gets less air. What's worse is, a vacuum sync would have you close down that one throttle to get the vacuum reading back up to where the other cylinders are at. So you have then reduced the airflow to the weak cylinder even more, and thus it will do even less work. This increases the uneveness of the idle.

Using vacuum to sync throttles is based on the assumption that all cylinders are in the same condition (and all carbs and carb holders are in the same condition, meaning not leaking air somewhere).

I suggest that the smoothest sync would be to time the intervals between firing and adjust the throttles so the interval was the same for all cylinders, but I've never tried to do it that way. It wouldn't be too hard with a scope. It would be interesting to see what the vacuum ends up at if done this way.

Wiggling should not change anything is clamps are tight.
Firing interval? What are you talking about? You are going to change firing intervals by adjusting synchronizing? How many beers did you have?

Please keep it civil gentlemen. :whistle:
Steve

Jiggling carbs would cause sync disturbance because of the slop inherent in the design (pivot shaft, arm that goes ro slide, dampening springs in slide, slide adjusters and slide fit in the bore.

Not exactly sure what lou means, but considering he is one of the electrical gurus on here I tend to listen to what he says

floivanus wrote: Jiggling carbs would cause sync disturbance because of the slop inherent in the design (pivot shaft, arm that goes ro slide, dampening springs in slide, slide adjusters and slide fit in the bore.

Not exactly sure what lou means, but considering he is one of the electrical gurus on here I tend to listen to what he says

Well I'll just say this, if simply jiggling the carbs sets sync out of whack, I would guess between the engine vibrations and shock emitted from the chassis as the bike moves would never leave the bike in sync....

I would suspect you have some pretty worn (and leaking) cross shafts and linkages.

Anyway, back to the discussion of bench carb stands for maintenance.

Cory

Here's mine

With my old VM 28's all you had to do is look at them cross eyed and they would be out of sync. My 33's have internal linkage and I've had them off a few times, the bike still idles fine. I'm going through them again soon for fine tuning after all the other work has been done. This will be the forth time in 2.5 years.
Steve

bluej58 wrote: Here's mine

Nice! You could always use that one as a grave marker...."Here lies the carb tuner. May they rest in Pieces...."!

Lol

Cory

Heres mine...

Sure beats hanging them up with wire. Now that I have new holders I can make one for myself.
Steve

NakedFun wrote:

floivanus wrote: Jiggling carbs would cause sync disturbance because of the slop inherent in the design (pivot shaft, arm that goes ro slide, dampening springs in slide, slide adjusters and slide fit in the bore.

Not exactly sure what lou means, but considering he is one of the electrical gurus on here I tend to listen to what he says

Well I'll just say this, if simply jiggling the carbs sets sync out of whack, I would guess between the engine vibrations and shock emitted from the chassis as the bike moves would never leave the bike in sync....

I would suspect you have some pretty worn (and leaking) cross shafts and linkages.

Anyway, back to the discussion of bench carb stands for maintenance.

Cory

And that's the thing, jiggling does make them off kilter a minute amount, if it goes worse than that they need looked at. Ergo what lou was saying, and yes thread hijacking seems rampant. Don't know how this got off course.

For a carb holder I just use a spare gs750 head. Tried selling it, now it's a carb holder. Or IF I'm too lazy for that I use the handle of my outdoor recycling bin

The thread was going off course a little but back again. I for one don't mind a little dialog in my threads as long as it doesn't become abusive.
Steve