Topic: How important is vacuum sync?

[Kevnz]

I understand the theory and logic behind having carbs in sync, but , if you do a careful bench sync, is there much to be gained from a vacuum sync? Is there much actual difference if you have bench synched properly? Worth spending the money to buy a set of guages ? So far I've managed to get most of my bikes running pretty well without vac sync, but would spend the coin on a good set of guages if it would make a worthwhile difference? Opinions?

[bryanj]

Depends on how much your bikes are at idle, thats where the difference is

[dave500]

go get one synced and see what you think?so long as its done properly,smooths em right out,bit like just eye balling points,set them with a strobe and so much better.

[flatlander]

you don't need to sync these a lot unless you constantly fiddle with your carbs. if you do it only once after reassembly, try to make friends with someone who has a set of gauges and use those

[PeWe]

Bench synch work fine.
A little out of synch can act as carbs are wrongly jetted, slightly lean and run a little uneven up to ca: 3600 rpm on a CB750

[Deltarider]

I have a 4 gauge vacuumset and never use it. Such a set is useful in a professional workshop where it saves labour and so saves costs for the customer. A good bench sync can be just as good. Saves you a lot of noise and fumes too.

[TwoTired]

*IF* all four cylinders are identical, and the valves are adjusted the same for each cylinder, and your bench sync is perfect for each carb, the vacuum should be equal.

However, the machine is designed and adjusted by humans who are incapable of making anything perfect.  So, the vacuum sync compensates for imperfections in your specific example of the man made machine.

As mentioned, it makes the most difference at idle in run smoothness, minimizing transmission gear clack as the cylinders fire unevenly, as well as clutch basket noise and primary chain noise.  Some examples of machines benefit more than others, due to mentioned variables.  Some here claim to have perfect machines.  You are welcome to believe them.

Finally, new motors with multi cylinders don’t wear evenly, making volumetric efficiency drift with run time.  That’s why it is checked each tune up interval.  A vacuum sync means you don’t need to take the carbs off to do a mechanical sync.

Philosophically, multi cylinder bikes need specific tools to make them run well. Screwdrivers, wenches, air compressors, and other specialized tools are needed to repair and maintain them.  You either have these tools for the machines needs, or you pay a shop to do it for you that does have the tools.  What vehicle that you buy today doesn’t need specialized tools?  And why do you think a machine would never need those tools?

Yes, I do have vacuum sync gauges.  Not needed for adjustment at every check.  But sometimes they are, and I’m grateful not to do a bench sync to bring them all into smoothest idle as part of the routine tune up.  I bought my set way back when money was tight.  Not a daily tool.  But, I still don’t regret the purchase.

Cheers,

[maxheadflow]

I agree with twotired.  It's kinds like the difference of setting the timing statically with a test light compared to setting the timing with a strobe light and the engine running. 

[Deltarider]

The three times I have done a bench sync, I have double checked with vacuum afterwards. Each time I had managed to sync within 1,5 cm Hg, which is well within factory specs. Never had to readjust.
Ofcourse in a pro workshop just lifting the tank saves time, but when you have your carbs on the bench anyway... it will save you a lot of noise and fumes. Recommended.

[slikwilli420]

Think about it this way. You sync them at a very low slide opening (closed) which is why a bench sync is a good first step. The variance in the percent difference of each slide opening while closed could be enough to have rough idle. As the slides are opened, that variance in slide height is the same, but over a much greater surface area of the carb throat. If there is a 5% difference in the area closed, that area almost fully disappears at full throttle.

[Deltarider]

Syncing is to achieve an acceptable idle and that is it. Although sceduled in the maintenance scheme, syncing is rarely needed. In dealer workshops mecs wouldn't even bring out the gear, unless owners had specifically asked for it. They would however when they heard a sync was needed. The owner's manual already gives a clue (see below). I myself have done tens of thousands of km without resyncing and - to be honest - the times I have done a sync... it was more play and to satisfy my curiosity.

[maxheadflow]

It's interest that those here only vacuum sync at idle.  I've always done it at 4K to 5K RPM then check at idle.  For 1, it gives better resolution in the gauges from higher vacuum.   But what do I know?  I used to work at dealers in the 70s. That was the way everyone did it back then, at least in shops I worked in.  They weren't super large but kept 5 mechanics busy.  One shop had 8 mechanics. One had 2 or 3.

[TwoTired]

The shops I frequented (dealer and independent) in California since 1975 all included a vacuum sync in their flat rate tune up offering.  I watched them do a few.  Sometimes it was just a check.  Sometimes they adjusted.  Yes, it was a matter of smooth idle, which impresses many not familiar with 4 cylinder bikes.

I'm sure some shops cheat their ignorant customers by skipping the procedure.  I try to avoid those, as they are prone to scamming customers out of more money and repeat unnecessary work which they may or may not actually perform yet still charge.

I once did a tune up on a 67 or 69 mustang with a 390 many years ago.  Tune up says to change plugs 12,000 miles.  But, #8 couldn't be reached if the car had AC.  Had the undo the motor mount and raise the engine to get to it.  Sure enough, this car had the original Autolite plug in it, eight years old and VERY well worn.  Even the Ford dealership didn't bother to change it.  The owner came back later and told me the car had never run so well for all the years he had it.  Nice to have happy customers.

I note the Honda owners manual list "carburetors" at 500 and 3000 miles -- Check and adjust if necessary.  Don't know how you do that without vacuum gauges.  Of course the maintenance manual lists and shows the procedure Honda dealerships were to follow.    But, we have members here with super special bikes and super ears, that can fix their bikes with simply a stern look while typing on a keyboard.  For me, its a matter of knowing I did all I could to make it right.  Some are willing to settle for "good enough".

A bench sync gets you close (maybe even good enough), for sure.  Vacuum sync to well tune the motor and know it's as correct as you can get it.

It's your bike, it's your choice.  You must live with your choices.

Best of luck!

[70CB750]

I heard of Ford with V6 where the rear bank had original spark plugs after 180 000 miles - always serviced at the Ford dealer, too.

[TwoTired]

I heard of Ford with V6 where the rear bank had original spark plugs after 180 000 miles - always serviced at the Ford dealer, too.

Yeah, gives a kind of different meaning to "trusted mechanic", eh?

Cheers,

[Deltarider]

I know in Holland it was common practice at Honda dealer workshops NOT to do a vacuum check when it was not needed and not specifically required by the customer. Well experienced mecs were and are well able to judge - after doing the routine maintanance - whether a vac sync is needed or not. I never have found it in my bills, because I wasn't charged. Again: it was common practice to save time and money and customers benefitted from dealer's common sense.
You may find telling what Honda wrote in the Owner's Manual: The carburetors were synchronized at the factory and should normally not require adjustment except when they have been disassembled.
And what is written there, matches exactly my experience over the last 41 years. 

[maxheadflow]

Back the late 70s we used to recommend a full service every 2500 miles.  It was $37.50 plus parts for 1 1/2 hours labor. 60% of the shop got 50% commission.  It was a gravy job if the bike was well maintained and could be done in 50 minutes to 1 hour depending things like finding loose wheel spokes.  It included valve adjustment , ignition timing, battery fill, oil change, carb sync and adjustment, cables, chain, tire pressure, light check, some fasteners and other stuff.  Lubing cables etc. Timing chain adjust. Short test ride.  If it was too expensive we'd occasionally do an oil change and timing set.

Here are carb adjustment recommendations out of the 78 owners manual.  Notice what the service table calls out Carb adjustment for every service. Also Honda expanded the interval to 3600 miles but oil change every 1800.

[craz1]

Now TwoTired is the person to know. I agree with his very good answers 100%. My Z1900 bikes need adjustment often ,at least once a year I check them. The linkage on those carbs are a little more fussy than the Cb750 carbs. We bench set my friends Cb750 and they were very close however we also checked them with my mercury gages and did have to fine tune them.

[Deltarider]

I'm with the Haynes manual (an exception ) that questions the purchase of a vac set unless you own more bikes. I personally regretted the money spent on mine - back in the day they were even more expensive - and so semi-donated my set to a friend who has helped me a lot with my yacht. The condition for him to have my set is, I can use it again when needed. So far it has not occurred. No doubt out of curiosity I'll have the carbs on the table in the future. I would like to verify the method posted by a German on Youtube, who disassembled his CB500 carbs to parts you never knew they were there. His vids are the best. On assembly he explains how to do the initial setting to arrive at a perfect bench sync. When tested afterwards using vac gauges, he didn't need to adjust. His method must have been the same way carbs were assembled at Keihin before they were delivered at Honda.
We can meander to all kinds of principles and laws of physics. Thank you, but I know the theory. I just thought it useful to share what was common practice in workshops where mecs have seen hundreds of these bikes back in the day. This was based on experience and less on checklists. To answer the OP's question: for me the expense for a set has not been worth it. Also I prefer not to stand in the noise and fumes of an idling motorcycle when it is not strictly needed.

[70CB750]

I understand the theory and logic behind having carbs in sync, but , if you do a careful bench sync, is there much to be gained from a vacuum sync? Is there much actual difference if you have bench synched properly? Worth spending the money to buy a set of guages ? So far I've managed to get most of my bikes running pretty well without vac sync, but would spend the coin on a good set of guages if it would make a worthwhile difference? Opinions?

Ten years ago I built my own set of gauges with parts from Surplus.com and stuff I already had - about $20 invested and I still use it. 

Nowadays you can get a set on ebay for around $40 - the cost is really not that great.  Sure you can get away with bench sync, but well synchronized carbs make the bike hum and it pleases my ear.

On top of that - it's a skill and for me that would be good enough reason to synchronize. 

My $0.02 only, of course.

[PeWe]

I synched my CB750 K2 with Morgan carbtune earlier this year when I had the tank off.
Carbs were bench synched 2018/19. Done around 6500 km after that.
 Carbs just a little off, adjusted
 small changes.
I think it runs a little smoother after that. No big difference, but the little extra.

I think a vacuum synch can compensate for  differences due to different wear. Maybe cam too if worn or grinded not equally?

[BenelliSEI]

I synched my CB750 K2 with Morgan carbtune earlier this year when I had the tank off.
Carbs were bench synched 2018/19. Done around 6500 km after that.
 Carbs just a little off, adjusted
 small changes.
I think it runs a little smoother after that. No big difference, but the little extra.

I think a vacuum synch can compensate for  differences due to different wear. Maybe cam too if worn or grinded not equally?

+1 to that. Whenever I use my gauges on a friend’s 4 cylinder/ four carb motorcycle, they are astounded by the improvement. Chain rattles disappear and the idle is sweeter. As mentioned, likely makes very little difference at WOT, but I’m a huge fan. It makes a difference, and completes the job. On occasion, no adjustments are needed, but very rare.....

[bryanj]

I have to reitterate that a synce with gauges is the LAST item in the service list after getting points gap and timing spot on, tappet clearances exact and cam chain adjusted so no slack.

When servicing from new if all these were done right the carb settings very rarely needed doing, of course all this is moot when the carbs have been totally stripped

[Deltarider]

Mecs in the dealer workshops would ofcourse carry out most of what's listed in the maintenance scheme, in particular oil, fast moving parts like spark plugs and filters. My breakerpoints were never replaced though, because they were still good. In my case they could have left the airfilter element alone too, but it is understandable no dealer wanted to risk its reputation by not renewing such a filter(done within 12 seconds!) and it possibly clogging up sometime in the next 12.000 kms to come, before the next service interval. That does not mean, when you do the maintenance yourself, you couldn't do years with the same airfilter element (I have done more than 10 years with such an element and it is still good). That's the advantage of doing things yourself. Syncing is always the last to be done... that is: if needed. In the 70s most workshops did not have exhaust gas equipment, so it always implied moving the bike away from the collegues. After all routine maintenance was done, mecs would start the engine and rev for a few seconds to check all was well. If they then judged a sync was needed, they would do it, not because it is in some list, but based on actual facts.
Two tips: when the Owner's Manual states "except when they [the carbs] have been disassembled", Honda means a disassembly where the sync is compromised, like when (re)positioning jet needles etc. Removal of the float bowls and removing parts there for cleaning or whatever will not affect the sync.
If you have carried out a sync, do not forget to put some loctite or similar product on the adjusting nuts.
 

[Whaleman]

I will bet that everyone who bench syncs and thinks it is good would be very impressed on how much better it is after I spent a half hour with my gauges. Until you use the gauges you have no idea how little of an adjustment can cause changes in the vacuum. Also, changing one can affect the others. Dan