Vacuum Sync - Running motor only??

Just something that I have been thinking about.

Is it absolutely necessary to do the vacuum sync on a running motor, or could it possibly be done using a remote source to create the vacuum?

My understanding is that it is not so much the source of the vacuum, as it is the consistancy of the vacuum across the rack. I am aware that different wear in the cylinders will give different vacuum readings. Understandably this would not eliminate the need to do a final colortune, and vacuum sync on the bike while running. Just trying to minimize some of the inconsistancy in the vacuum sync process.

Any thoughts, theories, or ideas?

Ghost

 

Sure.

You're not really synchronizing the carburetors, so the answer is no. OK, maybe more like, "sure, but it wouldn't do any good."

If you were to scientifically synchronize the carbs themselves, using say, a flow bench, then reinstall them the bike would run like crap. We honestly DON'T want the carbs sync'ed to each other.

HUH? Yes. It's those minute differences in the cylinders' individual capabilities that we're trying to compensate for.

What we are really doing is using the CARBS to synchronize the CYLINDERS' output as closely as we can.

If valve shims came in finer increments, it would be possible to synchronize the cylinders without using the carbs to do it; but since we have to live with valve clearances that are withn a range rather than "spot on" we have to use a different method to sync our motor. So we "throttle" the individual cylinders by using the carbs to set their outputs as close as possible, by reading manifold vacuum.

So while you could synchronize the rack of carbs themselves, it wouldn't achieve the desired purpose, which is to synchronize the cylinders. For that, we need manifold vacuum to measure which is produced by the motor when it's running.

Because of what we're really trying to do, synchronize the cylinders, it means that valve clearances are of utmost importance in the equation. Len takes it to an even deeper level here: http://xjbikes.com/Forums/viewtopic/t=14608.html Give it a read.

 

A very good read, and as always very good info from Len. Thanx Fitz!

I understand the necessity of the running vacuum sync on the engine because of the differences in each cylinder, and agree that the step cannot be skipped. Valve clearances included.

Rereading my original post I can see where it could be misunderstood. I am looking at the bench sync process.

This is my understanding of the bench sync process. To get the entire rack in a basic sync by getting the butterflies aligned.

My question then is would it be of any benefit to do a vacuum sync on the rack during the bench sync process?

Yes, this would require the use something on the order of a flow bench. This is what I have been thinking about. A more consistant bench sync through the process of a bench vacuum sync.

To me this would serve multiple purposes. Identify problems such as misaligned butterflies, improperly aligned rack before installing the rack onto the engine. This would also provide a more consistant starting point from which to do the running vacuum sync, and any adjustments should be minor at that point.

I hope this clears up the confusion.

Ghost

 

Yeah - you COULD sync them on a bench - and (as you said) it would just be a basic synch - mostly for testing and to get the engine running.

But you'd still have to do it on the bike with the engine running to get it done properly.

 

Nope. The current bench sync process, if done correctly, will get them as mechanically close as poossible. Once back on the motor, any adjustments SHOULD be minor at this point; simply to compensate for the differences in the cylinders.

Remember what it is we're doing (from Len's piece

The only vacuum we care about is that being produced by the running motor so that we can make those (hopefully) minor adjustments to compensate for differences in the individual cylinders' characteristics. Starting out with a mechanically synchronized, theoretically identically adjusted bank of carbs is sufficient.

Remember, the carbs don't produce the vacuum; the engine does. They respond to it while at the same time controlling the intake flow that allows it to run; and thus varying the vacuum signal we're reading. A flow bench would hopefully produce the same vacuum for all 4 carbs in a rack and be unaffected by any adjustments we would make so they would all end up the same anyway. The flow bench wouldn't "respond" to adjustments to the carbs like a running motor does, so in theory what you'd have is a mechanically identically adjusted rack.

Which is what we do now.

But our motor doesn't produce the exact same vacuum on every cylinder; so we don't WANT them to be exactly the same. So we put the rack on the motor and proceed to UNSYNCH the carbs until the vacuum signals from the MOTOR match.

This also points up the need for the valves to be in spec. If we take a nicely mechanically synchronized rack (even one from our theoretical flow bench) and put it on a motor whose cylinders WAY don't match because of wonky valve clearances, it's not going to want to run very well (if at all.) And we'd have one heck of a time "unsynchronizing" the carbs to get these wildly mismatched cylinders performing anywhere near each other. Which is why your valve timing has to be right (via clearances) to get a decent vacuum sync.

The bottom line is this: In the end, we want a rack of carbs that's been de-synchronized to "match" our motor. The best way to get there is to start with a rack that's as identically adjusted as we can get, and then tweak it into place. There's no benefit to having that "soon to be adjusted away" starting point be uber-precise.

It's like dry-setting float levels. You can be as precise as you want measuring them dry, but you still gotta check 'em wet and adjust accordingly; to compensate for any small differences in each carbs' needle and seat.

Good thoughts though. If the flow bench could somehow respond to changes in the carbs the way a running motor does, it might even work. But it would still be moot as soon as the rack got installed on our less-than-perfect motor.