Let's discuss vacuum sync'ing.

My Dad and I are having a heated discussion about vacuum sync'ing carbs vs. bench sync'ing.

He believes, with all his heart, that you can't get any better than a bench sync; that all butterflies, throttles, slides, etc. are mechanically, physically exact and that anything you do after that will disrupt the equalization.

I believe that vacuum sync'ing will account for differences in the engine from cylinder to cylinder. Low or different compression, valves, etc.

Please educate BOTH of us on which one is better (vacuum... 8) ) and why. Please provide plenty of evidence, documentation, proof, videos, links, etc. My Dad is very skeptical of information on the internet. He's from the school of thought that "Anyone can put anything on the internet...doesn't mean it's right. I can type in that bench sync'ing is best and vacuum is wrong".

I personally have had very good luck with bench sync'ing. I've had a few bikes that were perfect right off the bat once I put vacuum gauges on them, or so close that it wasn't worth messing with.
I've also seen where all four carbs read differently.

We're currently working on a set of CV carbs. I tried to explain to him that, especially with diaphragm carbs, unlike mechanical slides, that even a bench sync won't make the diaphragms/slides rise uniformly. When I asked him "OK, I bench sync the butterflies and put the gauges on it and one carb/cylinder has 10 inches less vacuum.....what do you do?" HIs reply? "Take off the gauges and throw 'em away."

Help me out here. or, if I'm wrong, say so and explain why.
I'll be cross posting this on several forums to get a wide range of opinions.

Sounds like your dad won't be convinced regardless.

Best thing you can do is have him bench sync carbs, you check them with a vacuum gauge, and if they are still out, take the bike to a dyno. Once you have the horsepower numbers, sync the carbs with a vacuum gauge and take it back to the dyno.

If the 2nd dyno numbers are better, have him pay for both tests. If they are the same or worse, you pay for both tests.

If everything on each cylinder is exactly the same it should work, but the theory often varies from reality.
Vacuum sync adjusts things to actual cylinder variation, not just timing or valve adjustment but port variations
In some cases the variations may be miniscule and only noticeable with electronic measuring equipment but in many cases bike will be smoother and easier to ride at low rpm. Unless carbs are massively out of sync engine smooths out at 3~4,000 rpm anyway (assuming 9~10,000 rpm redline)
Carb sync mainly affects idle quality and small throttle openings

Knowledge is defined as a reasonable change in behavior . Willful ignorance in the face of facts makes this impossible .

If the carbs are bench sync'd but the vacuum readings on the running engine are different cylinder to cylinder that is all the proof needed .

So whose bike is it ? If the bike is his let him do what he wants . If it's yours do what you will .

I've been doing this for 30+ years now and refuse to deal with such ignorance .

~kop

kopcicle said:

...I've been doing this for 30+ years now and refuse to deal with such ignorance .

~kop

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One could deduct from this statement that your are probably quite lonely... 8)

mydlyfkryzis said:

One could deduct from this statement that your are probably quite lonely... 8)

Click to expand...

Where's the "Like" button?

Let's discuss vacuum sync'ing.

Ultimately the sync-o-meter or Uni-syn is going to be more accurate than the 2 finger method or drill bit method.
I sync mine by the 2 finger method, then fine tune with a sync-o-meter.

But if you sync your carbs using vacuum or flow meter only find that there is a noticeable difference in mechanical sync, then IMO you should look into the reasons this is happening.

In a way your dad is right, the vacuum sync and mechanical sync should be in sync.
Another advantage of the meters is you can also sync the carbs with tension on the throttle cable and locked in position at say 3k IMO the most important place to check sync.

I prefer to sync at low idle, drop rpm to check things are OK then slowly open throttle to around 4k
That will tell you if all the other mechanical parts are good, variations at different rpm means something is worn or wrong
I've had Kawasaki Z500's that will reliably idle at 400 rpm (yes, four hundred) when everything is set up properly
I don't leave it that low though ;D

The way I was taught, and I hope somebody tells me if this is wrong, I get them synced at idle, then have the throttle opened to 3k and sync. I go back and forth a few times to make sure they are even, if not, I start looking for leaks or other issues on the carbs that won't fall in line...

Sounds OK to me.
You shouldn't sync at 3 k though as airflow is smoothing out when throttle plates or slides are lifting
Syncing at 1800~2000 is always a bad idea as the ignition advance may cause problems with rpm fluctuations

One could deduct from this statement that your are probably quite lonely... 8)

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Not really and I deal with a better class of customer .

~kop

Well, I like Kop, we crusty old mechs need to stick together when we're so far outnumbered by the newbie hipsters

Let's discuss vacuum sync'ing.

kopcicle said:

Not really and I deal with a better class of customer .

~kop

Click to expand...

. I was actually implying there are lots of ignorant people. See my sig quote.


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. I was actually implying there are lots of ignorant people. See my sig quote.

Click to expand...

there are lots of ignorant people. See my sig quote.

~kop

I aint ingerant, I'z stoopid ;D

Let's discuss vacuum sync'ing.

ApriliaBill said:

The way I was taught, and I hope somebody tells me if this is wrong, I get them synced at idle, then have the throttle opened to 3k and sync. I go back and forth a few times to make sure they are even, if not, I start looking for leaks or other issues on the carbs that won't fall in line...

Click to expand...

Bang on in my books!

Let's discuss vacuum sync'ing.

crazypj said:

I aint ingerant, I'z stoopid ;D

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Not stupid. CRAZY. LOL.


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I try