Topic: Let's discuss vacuum sync'ing.

[Scott S]

 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...    ) 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.

[flybox1]

This is one of those threads where I would think to try to give a informative answer, but then refrain as I know TwoTired would come aboard and do a much better job than I ever could

[hoodellyhoo]

"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."

  That's funny. Sounds like a few people I know.

I don't have much in the way of hard scientific evidence but I do have experience. I find the 350f to be very sensitive to carb synchronization. Suppose it has something to do with the small/intricate nature of the engine. Either way I had the carbs off and did a proper bench synch with a drill bit. Put 'em on and still had that little "chugga chugga" clutch rattle from uneven idle. When I put my manometer on for a proper synch there's a very distinct change in engine sound when everything in perfectly synched that I haven't quite gotten when synching my 750's. It's like the engine goes from pulsing to an electric "wrrrrr" and it's smooth as silk.

[RAFster122s]

Flybox1, TwoTired is taking a vacation from the group (needed a break) so you may need to stand in for him.

Vac sync is better since we are dealing with essentially air pumps (as most internal combustion engines are...)

[mrbreeze]

Best thing I could think of to show your dad that he is wrong would just be to show him. Get a bike that the carbs have just been rebuilt and bench synched. The bike will usually run if everything else is in order but it will exhibit signs of being out. You will have the clutch slap and usually after warm up a racing idle. I have never seen one in range after just a bench sync. The gauges are usually off and things will really smooth out as you get them all on the same page. The best tool I have for my bike is the Morgan Carb Tune. It costs more than gauges or Merc sticks but is so much easier to use. Mine has already paid for itself as far as paying a shop.

[andrewk]

Sounds like it will be tough to convince him.  If he refuses to believe that synchronization is useless when done in real-time (vacuum) I don't know what can be said to change his mind.  Hard to provide "definitive evidence" other than through experience.

I think the only way you'll convince him is to show him hard evidence.  Bench sync some carbs, then dial them in with gauges while he watches.  When the primary/clutch quits rattling when you get them right, and the revs build and fall quickly, maybe that will show him which is better?

Vacuum synchronization is used by every bike shop that wants repeat work.  There are merits to bench syncing, like reduced time dicking with the vacuum sync, but it's not a static setting.

[setdog]

What does the manual for that bike say to do? Does it go over vacuum sync?

My cb750 manual discusses vacuum synchronization therefore I would argue that since
The manufacturer elected to add it to the repair manual there is no argument, vacuum sync wins,
Regardless of whether you even make any improvement, the vacuum s give you measure able readings which then allow for adjustment of simply confirmation.

[Scott S]

 The thing is, he's SEEN me use the vacuum gauges to get all four carbs right. He's also seen me put the gauges on and the carbs are perfect.

 We're currently working on my girl's '85 FJ600. This bike sat for all of last season. It has CV carbs and has always, ALWAYS been a cold natured, hard starting #$%*.

 I pulled the carbs and cleaned them (they were fine, but just wanted to make sure they weren't the problem) and bench sync'ed them.
 It's time for a valve adjustment anyway, and the bike has that horrible, rattly sound like a bike does when the carbs are out of adjustment.
 The last time they were touched was when she first bought the bike. I found all four idle mixture screws at WAY different settings...from 2.5 turns all the way to 6.5 turns! Needless to say, that shop isn't in business anymore.

 She elected to have a respected local shop do the valve adjustment, in the interest of time, otherwise I'd vacuum sync the carbs myself. I feel sure that a valve/cam chain adjustment and carb sync will make the bike sound and run better and, hopefully, be easier to start, too.

[Scott S]

What does the manual for that bike say to do? Does it go over vacuum sync?

My cb750 manual discusses vacuum synchronization therefore I would argue that since
The manufacturer elected to add it to the repair manual there is no argument, vacuum sync wins.

 Yes, every manual I have shows vacuum sync'ing.

[bjbuchanan]

The idea behind the vacuum sync is that you have four separate cylinders, there is no plenum by which you have some type of equalization amongst the cylinders

Think of how inline sixes have a tendency to starve certain cylinders depending upon firing order with their single log style manifold and single carb.

So you have 4 separate cylinders, with each one mechanically different. Slight combustion chamber differences including valve shrouding, swirl marks, humps to ease the transition into the chamber. Each one is slightly different it doesn't matter how well dear old dad can work a grinder and eyeball stuff so it looks the same. That is why many heads are CNC ported when an optimum combustion and runner shape has been reached through testing using a flowbench. A flowbench works on similar principles so why is that so widely accepted as truth and a proof for port testing?

To give credit to the vacuum sync is taking all of the other factors in to effect, your environmental factors not just the single little piece you are working on. The carbs are just a part of the equation, he is considering them the whole equation which is just not the right way

Take two ports, one slightly smaller than the other. You take 1 single carb, and isolate all other variables like leaks and put it on one cylinder, the larger one. The carburetor will have less vacuum pulling through it on the larger cylinder. Vacuum goes up as the flow area goes down. On the smaller one you will have greater volume and charge speed going in to it

You can equate it to the difference between dual plane and single plane intake manifolds. Smaller runners on a dual plane increase charge speed which enhances low end grunt.

A single slide height for all 4 different cylinders will have different vacuums pulling do to different port and chamber volumes sucking on the mixture. You need different slide heights to equalize the vacuum amongst all carbs because as we all know CARBS SUCK.

A bigger "cylinder" after the carb will have less vacuum which will pull less fuel which will make it run different than other cylinders. Yes an engine is mechanical, a carb is basically mechanical but they are mechanical devices that function on the principle of vacuum

I know that isn't a very elegant post but TwoTired I am not. I'm not gonna go all over the internet pulling up diagrams and studies to prove the principle of what all our machines run on. It is a fundamental idea change that has to be made, not believing in one camp or the other. A close bench sync just means you have pretty similar cylinders, not much else. If it will lend some credibility I was a mech engineering major with a focus on automobiles b4 I switched to finance because I lost interest so I have taken all the thermodynamics, dynamics, physics 1 and 2 that kinda stuff

[bjbuchanan]

 The last time they were touched was when she first bought the bike. I found all four idle mixture screws at WAY different settings...from 2.5 turns all the way to 6.5 turns! Needless to say, that shop isn't in business anymore.

The mixture screws if aftermarket may have not been of great quality and rattled out. I had an issue with my keyster carb kit that the springs just didn't hold tension(enough atleast) on the screw to keep it in place so it would back out from vibration. I would after one single ride find screws at all types of crazy positions time and time again. I could only tell because I was trying to solve idle quality issues at the time.

I know this is a different model/situation but it is a thought. If you don't feel tension on those mix screws then you never know. Either that or the shop really does suck balls. Just a thought

[Duke McDukiedook]

A dynamic synch will give you a more accurate picture of vacuum being drawn since there will be differences between the carbs due to manufacturing imperfections from carb to carb. A dynamic synch will always trump a static synch.

[goldarrow]

In  tech support world, you're always wrong and customer is always right.  In family perspective, you're always wrong, parents are always right. 

Carb sync? Bench is great and cheap, vacuum is great but get expensive for more precision result. 

[seanbarney41]

everything stated so far is basically true...that said it seems the importance of a vacuum sync gets exagerated around here, with guys thinking they need to hook up the gauges everytime they go for a ride or that it is gonna miraculously cure all of their little problems in life.  My personal experience is that if you do a proper bench sync, it's gonna be real close unless you have some pretty uneven wear between cylinders....and it's gonna stay in adjustement for thousands of miles unless something disturbs the balance.  I am not saying that you shouldn't do a vac sync, it is definately an important part of tuneing and mantaining these bikes.  But, in a pinch(no gauges available), you can have a perfectly acceptable performance.  The vacuum sync is really only valid for the rpm it was done at anyway.  Are you more interested in watching your bike tickover smoothly in the living room or getting out there and riding.  Keep in mind this whole post is a counter exageration to the aforementioned exaggeration.

[iron_worker]

The amount of vacuum at the carb throat is in direct relation to how much fuel that cylinder receives. You want each cylinder to receive the same amount of fuel and thus thurst equally on the crank. This helps to smooth out the crank rotation in tead of accelerating and decelerating it several times per second.

The reason a mechanical bench sync will not provide the same vacuum (and thus fuel) in each carb is because these engines and carbs are primarly CAST. They have irregular surfaces with some deviation from part to part. Not every intake runner is exactly indentical and not every carb throat is either. I would be willing to bet if you had a CNC'd head with CNC'd ports and your carbs were also identical ($$$$$$$$$$$$$) then you could probably get away with mechanically measuring the height of the throttle slides to give more or less identical vacuum readings.

IW

[Deltarider]

I have a set of 4 vac meters and I never use them. Three times the last 15 years or so I've bench synced and all three times I have been able to sync within 2cm Hg when checked afterwards (well within factory specs). I've learned this from two mechanics (one Swiss, one Dutch) who assisted racers. Both of them said bench synching is good. If after your bench syncing vacuum differs too much and you start readjusting you're trying to fix something rotten in your engine, something that needs to be adressed properly.  Vac syncing is all about not having to remove the carbs.

[Dave Voss]

I'll start by saying that both you and your father are correct, but perhaps neither of you are giving the other credit for your perspective halves of the total synchronization equation?

If each cylinder is exactly the same, and each intake valve is exactly the same, and each exhaust valve is exactly the same, and the compression in each cylinder is exactly the same, and the back pressure in the exhaust is exactly the same, and each intake runner is exactly the same, and each carburetor is exactly the same, etc. etc. etc. then a mechanical bench synchronization should result in exactly the same intake vacuum.

Now, count how many 'exactly's there are in the statement above, and consider how often anything ever really turns out to be 'exact' and it will be easy to realize that there will be differences in the vacuum no matter how careful and meticulous you are about performing a mechanical bench synchronization.  You can think of this as the 'static' synchronization, which sets the 'course' level.

After that, hook up the gauges and make the necessary adjustments to equalize the intake vacuum readings, as this is how the differences between each cylinder (anything and everything above that isn't actually 'exact') are compensated for.  You can think of this as the 'dynamic' synchronization, which sets the 'fine' level.

So, both are important and necessary.  Also, if the mechanical synchronization is too far off, there may not be enough range of vacuum adjustment to compensate.  It is also important to know if the engine is running on the idle circuits or the main circuits when performing a vacuum synchronization, because the procedure may be different.  I like to hold the engine rpms at about ~2k when making adjustments, as in my experience that helps to minimize any hiccups during transition from the idle to the main circuits due to the synchronization being off a bit.

The above is generic in description, and not necessarily specific to Honda CB motorcycles, the theory is the same for any engine that uses separate carburetors (or separate barrels in the same carburetor) for each cylinder.  I hope the explanation helps.

[dave500]

bench sync gets you ball park and running,vacuum makes the final proper adjustment,,period,be sure your ignition is 100%spot on first though.

[Scott S]

FWIW, the bike that started this discussion (well...got it rolling...we had already been in disagreement over it) is an '85 FJ600. Always been cold natured and finicky. Sat all last year. It has that "clunky" sound at low RPM's that I know out of sync carbs can cause. It has four CV type carbs.
 Pulled the carbs to make sure no slow jets, etc. were plugged that would cause the hard start issue. Bench sync'ed. Will adjust valves and cam chain next, and then I mentioned sync'ing the carbs. That's what got the ball rolling.
 This is a high revving sport bike (Fastest production 600cc bike in 1985) and I can't help but think that the diaphragm slides need to be sync'ed with the engine tuned, warm and running.

 BTW...EVERY manual I have says to sync using gauges, be it vacuum, manometers, etc. The factory Honda manual for my 500/550's, Chilton, Haynes, etc. Be it Yamaha, Honda or even the Weber/Dell'Orto manuals I have for my Hi-Po VW's...they ALL use some form of vacuum to sync.

[77Pinto]

bench sync gets you ball park and running,vacuum makes the final proper adjustment,,period,be sure your ignition is 100%spot on first though.

X2!  Could not say it much better.

And be sure your valves are adjusted before the sync.


Bill

[JimJamerino]

I was thinking about this yesterday, and I wondered if your dad would only do static timing on an engine.  It's kind of the same principle... you static time to get the timing close enough to get the engine started, then use a timing light to get it perfect.  Bench sync to get started, then vacuum sync to get it right.

[pangloss]

Bench sync ONLY for me I'm afraid..!! Everything must be set up perfectly tho.( or as near as possible)  vacuum sync can mask a lot of problem areas, particularly ignition problem in that It's done mainly to attain good idle..the advance retard unit can be slapping around a lot at idle speeds and I consider this to be main reason for idle problems

[dave500]

Bench sync ONLY for me I'm afraid..!! Everything must be set up perfectly tho.( or as near as possible)  vacuum sync can mask a lot of problem areas, particularly ignition problem in that It's done mainly to attain good idle..the advance retard unit can be slapping around a lot at idle speeds and I consider this to be main reason for idle problems

#$%*?thats why you sort your ignition fully before you sync the carbs.

[pangloss]

Aaahh......!!  But how many do..?? Those flailing bobweights and springs . All hidden away down there behind the points...

[dave500]

judging by your reply you might also be one of them?thats why you unhide them and set up your ignition properly first.