Topic: Does Syncing the carbs really matter that much???

[ts354]

I guess the question should be; What do people mean when they say "sync the carbs?" 
Does that mean tune the carbs with proper jetting, needle placement, cleaning and sealing all vacuum leaks?  or
Get the mercury gauge out and fine tune the idle? 

I always think when people say; "sync the carbs", I think they mean; fix the carbs.

I think the biggest problem I run into on old motorcycles after a carb rebuild is the difference in compression between cylinders. (besides the easy stuff, bad gas tank, bad points, bad connections)

Does syncing the carbs do anything other than smooth the idle?  (unless WAAAAAAAAYYYYYYY off, like you messed up the linkage between the carbs on a multi)

[maxheadflow]

Syncing the carbs means setting the carb so each cylinder gets the same amount of air/fuel. It's more than some sort of Zen.

[Shtonecb500]

yes its important.

[TwoTired]

Syncing carbs really only matters when they aren’t in sync now. To get the motor to run smoothly, each cylinder should get the same air/fuel charge from separate carbs feeding four separate cylinders that only have one thing in common, the crankshaft.

The 500/550 does not have a primary chain tensioner.  If The cylinders don’t fire with even strength in sequence, the crank slows or speeds during that sequence, making the chain slack a then tighten.  Also, the trans gears are always meshed, even when not engaged, cylinders fighting for crank revolution dominance make those gears alternate tooth face contact.  I call this trans gear clack, which gets really loud during idle when the carbs are not vacuum synchronized, the trans in neutral, and the clutch engaged.

Cheers,

[BenelliSEI]

Very important. The “rattle” is even worse in a cb750 because there is little oil in the sump do deaden the noise......

[PeWe]

Feel the difference when carbs are properly jetted, ignition is right and carbs in good synch.
Sooo smooth at idle and when taking off. A little off can feel like wrong jetting up to normal cruising speed.

[MauiK3]

There is a big difference after the carbs are properly synchronized. There is less noise and much smoother idle and power delivery. The final process helps iron out all the slight variations that are inherent to the design and manufacture of the string of parts leading up to a running engine.

[kerryb]

When I dragged home my first 83 goldwing it ran, but barely.  It had been ridden hard and put away wet, with high mileage. After going through the systems it started to seem like the motor was toast...until I synchronized the carbs.  The transformation was amazing with a sudden loss of crashing and banging, vastly improved idle and starting.  Never did have to pull those carbs for cleaning!  A few screws and a cheap set of gauges worked miracles it seemed.

[jonda500]

I think yes, unless way off, syncing just improves smoothness at small throttle openings and low rpms. In a race situation I don't believe it would really matter very much at all. (but I would still do it anyway!)
John

[Deltarider]

As members here get older and older, idling seems to become more and more important. Sometimes I get the feeling owners rather sit in a deckchair listening to the idling than actually riding their bike. Youtube has countless vids of proud owners performing a walk around whilst having their bike idle for minutes and minutes. IMO, this is not a good practice. In other words: it's not what the bike was ment for.
Carbs, from the CB500 on, rarely go out of sync. It's not uncommon by just readjusting the points gap to arrive at a nice idle again. For the CB500 and CB550s with oldstyle carbs, Honda recommended an idle of 1000 rpm and 1050 rpm for the K3's which had PD carbs. My advice would be to have an idle of 1200 like Honda prescribed for the CB350/400F models.
Owners of a K3 model in particular should verify the tacho is true. I once worked on a CB550K3 that indicated no less than 1400 rpm on the onboard tacho @ a true 1000 rpm. Watch the vid below of a CB500K3. I guarantee you that where the tacho indicates a 1000, the idle is much lower. Poor bike that has to endure this for 3 minutes.

[maxheadflow]

Not only that but the motor sounds like the carbs are out of sync.   The motor is a CB500K four but the frame is looks to be a 77 / 78 cb550K.  It still has the clutch cable loop on the right side. 

[grcamna2]

I think yes, unless way off, syncing just improves smoothness at small throttle openings and low rpms. In a race situation I don't believe it would really matter very much at all. (but I would still do it anyway!)
John

+1

[PeWe]

Legal cruising speeds will not be nice as it can when carbs are in synch.
Chain rattling idle is a shame when it can sound better. Waiting for green light and during a short warm up before take off.

[Deltarider]

Not only that but the motor sounds like the carbs are out of sync.   The motor is a CB500K four but the frame is looks to be a 77 / 78 cb550K.  It still has the clutch cable loop on the right side. 

It's a CB500K3 all right. Only continental Europe had them. Not many though; some 4200.
But... there's somethng odd. I wonder who will be the first to spot that one.

[Deltarider]

I think yes, unless way off, syncing just improves smoothness at small throttle openings and low rpms. In a race situation I don't believe it would really matter very much at all. (but I would still do it anyway!)
John

A swiss race mechanic once told me he never used gauges, but synced by eye.

[jonda500]

I think the European 500K3 had the later crankcases and clutch setup - that bike must have an earlier engine fitted? I haven't seen an early rear mudguard with two number plate bump rubbers like that one has.
Maybe my tacho is exaggerating a bit too - that bike idling sounds very similar to mine! (my carbs have been vacuum synced)
John 

[Deltarider]

Well, Jonda, you have beat me! Actually I wondered who would be the first one to spot the carbs are the oldstyle and not the PD carbs the K3s had. I have clearly overlooked the clutch case is also the one found on the earlier CB500s. So... the bike is a bastard.
If you mean the four holes at the bottom of the rear mudguard, they are there to accomodate the rubber mudflap models in Europe had.

[bryanj]

If the clutch case is 500 so is the botto m end as crankcase shape is different so may even be a 500 complete motor i cant view embeded video links so hadnt noticed

[MauiK3]

We have a lot of humidity here so I don't start my K3 750 for any short periods because it will not dry the exhaust pipes out. When I start it I ride it and get it hot. Some have stopped by and want to hear it but I don't start it like that, it would be bad.

[Deltarider]

... and it seems to idle with the chokes fully closed for minutes which is odd.

[Sgt.Pinback]

Higly overrated.

It will only influence the idling.

[TwoTired]

Higly overrated.

It will only influence the idling.

So you are saying that at anything above throttle at idle, carbs that provide less air fuel mixture to some cylinders has no effect on engine power production.  And a dyno test would prove this?

Interesting theory.

And here I was, thinking that to achieve maximum power, all four cylinders should get the best air fuel charge they can attain at any throttle setting, and keep the spark plugs equally clean.

Cheers,

[PeWe]

I fiddled with pilot jets and fuel screws on Mikuni TMR carbs to solve hesitations and jerky movements between 2000-3600 rpm.
Finally gave up and synched the carbs that fixed it.