TOPIC:

To have them on a swivel to make them able to be tilted from side to side could help trouble shooting needle valves too. Hmmmm.
Steve

I was thinking just the same.
Although I think the vaccum hose should connect to the middle of a "vacuum box" attached to the carbs.

If you have carbs perfectly synced on the bike, just pulling them off and putting them back on will alter the sync. So I would think bench syncing would be altered by installing the carbs. So you'd have to sync again anyway to make sure they were still good, or make any minor changes.

The only time a bench sync makes sense is if the engine condition is really poor. When one or more cylinders are in worse condition than the others, vacuum syncing is not a good way to sync the carbs. It will result in you adjusting the carbs opposite of what the engine needs.

Poor cylinders breathe less. They will pull less volume. A vacuum sync will have close the throttle on the weak cylinder to increase the vacuum reading. This means the cylinder that breathes less, will now be getting even less air/fuel. This will make it do even less work. A vacuum sync only makes all of the cylinders have the same vacuum, but what you really want is for all cylinders to do the same work to get a steady idle. If all cylinders are in the same condition, then same vacuum will ensure the same ari/fuel, and the same work. If a cylinder is significantly weak, having the same vacuum makes that cylinder even weaker.

So with a weak cylinder, bench sync, then don't adjust after install.

loudhvx wrote: If you have carbs perfectly synced on the bike, just pulling them off and putting them back on will alter the sync. So I would think bench syncing would be altered by installing the carbs. So you'd have to sync again anyway to make sure they were still good, or make any minor changes.

- I don't agree with this comment. As long as anything that does not physically affect carb movement or cylinder breathing, sync should not be affected just by R&R'ng the carb rack as a unit. Valve clearance, cylinder sealing abilities due to ring wear, carb slide position, carb linkages not bending, vaccum leaks, etc... If all your intake components are in good, serviceable condition then you should not really affect sync.

The only time a bench sync makes sense is if the engine condition is really poor. When one or more cylinders are in worse condition than the others, vacuum syncing is not a good way to sync the carbs. It will result in you adjusting the carbs opposite of what the engine needs.

- If you tear apart the carbs for a major cleaning or repair/replacement of components you will need to static sync them. This comprises of using either a feeler gauge or a set of pin drills to set the slide height in the body when at rest. You want to ensure that they are all even so you are not chasing them around as you set sync dynamically once the engine is running and at temp.

Poor cylinders breathe less. They will pull less volume. A vacuum sync will have close the throttle on the weak cylinder to increase the vacuum reading. This means the cylinder that breathes less, will now be getting even less air/fuel. This will make it do even less work. A vacuum sync only makes all of the cylinders have the same vacuum, but what you really want is for all cylinders to do the same work to get a steady idle. If all cylinders are in the same condition, then same vacuum will ensure the same air/fuel, and the same work. If a cylinder is significantly weak, having the same vacuum makes that cylinder even weaker.

- The cylinder is generating a vacuum signal based off of atmospheric pressure. If we for the sake of this discussion agree that will stay fairly constant for a given localized area of the country, that leaves the engines restriction and ability to seal the variables that would affect the carb signal on that particular cylinder. Now what is going to weaken that signal? Excessive ring wear, poor valve sealing, improper valve clearance, cam lobe wear, etc.. This signal directly affects that amount of fuel being drawn from the carb as that cylinder operates. The weaker the signal, the less fuel it takes in and leaner it gets. If I begin to close that carb slide position, this creates restriction in intake tract of that raising the vacuum. This does restrict the air, but pulls more fuel from the jets. Ever "tickle" the carbs on an old bike that has no choke to get it started? If you place your hand VERY near the carb inlet while it is running or while trying to start, it applies a stronger vacuum signal to that carb and thus it pulls more fuel. Syncing is all about getting each cylinder to apply the same amount of work output. Sync is really about matching the fuel to the amount of air you are getting for that particular cylinder.

So with a weak cylinder, bench sync, then don't adjust after install.

So I think we agree to a point per my comments above, that it would be another way of what the manual advises you to do a static sync after tearing apart the carbs for a major repair or cleaning. As far as individual comments to your points, Please see the above in BOLD. Either way, the carbs need to be sync'd to the engine and consequently the cylinders that they are supplying air and fuel to.

FWIW....

Cory

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.
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