Carb synching: At idle or part throttle?

[TheRadBaron]

I know that all the manuals and how-to's that I've read regarding carb synchronizing state that one should do the procedure with the engine idling.
I was thinking about it and it seems to me that it would be a bit better to synch the carbs at part-throttle since that's where the engine spends most of its time.  Granted, with the bike not moving and no load on the engine, the throttle opening isn't much different between idle and 3,000 RPM, but it's something.
I tried this theory out by synching the carbs with the engine running at around 3,000 rpm.  I got them synchronized at that speed, but now they are no longer very synchronized at idle.  The idle quality is still good, though.
What do you guys think?  Is it better to have accurate synching at idle or part-throttle?

[weymouth399]

At idle
Off rev you can suck the mercury into the engine, if your using mercury.
It should run fine. Be sure your valves are in spec first this can change your sync.

Bob W

[TheRadBaron]

My manometer uses stainless steel rods instead of mercury, so there's no danger of sucking.  I just got back from a test run.  The bike does seem to run a bit more smoothly with the carbs synched to 3,000 RPM.  Off-idle seems a bit smoother, and there is slightly less vibration at highway speeds.  I admit that there could be a bit of "power of suggestion" in my perception, but I really believe that the bike is a bit smoother now.

[Marsh White]

I tried this theory out by synching the carbs with the engine running at around 3,000 rpm.  I got them synchronized at that speed, but now they are no longer very synchronized at idle.  The idle quality is still good, though.
What do you guys think?  Is it better to have accurate synching at idle or part-throttle?

I'm a firm believer of syncing them while on the throttle a bit - 3K sounds about right.  This was not something I was told or read about - I just came to that conclusion on my own after a year or two of syncing 3 different FJs.  Trial and error - I can definitely tell a difference while riding.  Also - having the engine fully warmed up before you sync (from a short ride - not idling in the garage) helps too - IMHO.

[Dan Filetti]

Hmm.  Syncing the carbs is all about ensuring the butterflies/ throttle plate are in perfect alignment.  This is accomplished by the measuring, and bringing into alignment of carb vacuum relative to the other carbs.  How then is syncing at one RPM over another make any difference?  They're either parallel or they're not.  They simply can not be more parallel at one RPM than at another.  Ergo, the argument for this technique must be that performing the syncing at higher RPM makes for a more overall perfect sync.  If there is sufficient vacuum at idle to make a measurement, how is using the greater vacuum at higher rpm any more accurate?

This seems odd to me.

Dan

   

[Marsh White]

Hmm.  Syncing the carbs is all about ensuring the butterflies/ throttle plate are in perfect alignment. 

Nope.  Having the butterflies PERFECT will never actually be correct because of the valves (among other things).  Like when you do a compression test - and all of the cylinders are not exactly the same?  Having the butterflies open a bit brings the other pressures & clearances etc. into play better than just leaving them closed....at least - that is my thinking.  Anyway - how often do you ride your bike with the butterflies closed?  Why not sync them open a little bit - like they would be when you are actually riding? 

As the other poster said - his idle isn't as smooth when he synced with the throttle open - but it was better when actually riding.  That is my conclusion as well.  If it didn't matter - then why would the idle be worse when syncing while the throttle was open?  By your logic - the idle smoothness would be identical wheather you sync with the throttle open or closed.  Try it sometime - you will find that is NOT the case!

[andyb]

If you want the smoothest possible idle, synch at idle.  If you want smoothness when you're riding it, synch where you ride it.  Usually I try to synch things about where the speed limit is in top gear, if I can make a comfortable smooth spot there the bike won't gradually pick up speed without me noticing it

[Flynt]

I know that all the manuals and how-to's that I've read regarding carb synchronizing state that one should do the procedure with the engine idling.
I was thinking about it and it seems to me that it would be a bit better to synch the carbs at part-throttle since that's where the engine spends most of its time.  Granted, with the bike not moving and no load on the engine, the throttle opening isn't much different between idle and 3,000 RPM, but it's something.
I tried this theory out by synching the carbs with the engine running at around 3,000 rpm.  I got them synchronized at that speed, but now they are no longer very synchronized at idle.  The idle quality is still good, though.
What do you guys think?  Is it better to have accurate synching at idle or part-throttle?

I think that without having the engine under load (on dyno) you won't get the throttle open to a representative "cruise" position and keep it there without over revving big time.  I balance at idle, then crack the throttle slightly and compromise a bit to minimize any big differences in vacuum.  Then blip to see if there are any further big variations.  Bike idles great and has no funny imbalance (usually indicated by popping or even backfires if really bad) at speed.

But then I fine tune the idle jets individually too, so take my obsession with a grain of salt.

Frank

[Dan Filetti]

I want to hear what Dr. Raforth says on all of this.  In the end, that man has forgotten more about carbs than the vast majority of us will ever know. 

Having read and used his excellent instructions for carb re-building, and given that they do not (or did not 8 years ago, when I last looked at them) contain anything about raising RPM for syncing, I wonder whether; a) he just dumbed it down for the masses or b) he does not think it's important.  If 'b', my money would be on him. 

I fully acknowledge that 'a' is a clear possibility as well...

Just sayin.

Dan

[Scooterbob]

When recently cleaning, balancing, and synching mine (last weekend) I quickly learning a couple of things.  It turned out to be way more important as to the order of how they were synched (meaning all of them closing and opening exactly the same time) than it was to have them balanced on the carbtune or whatever other guage you use.  I had to read the instructions several times until I finally understood when it said to

"After reaching a stable idle, check the sync gauge readings. Perform an initial sync at this time. As a quick
summary, sync #1 to #2 then #4 to #3 then balance the 1,2 set to the 3,4 set. This procedure is using the
cylinder numbering which is from left to right as viewed from the seated position."

THAT seemed to make the biggest difference in anything I did.  After that, I just hassled with the mixture screws until the idle would act right after the "blip" test.  If you want to read about my latest headache, see this post --> http://www.fjowners.com/index.php?topic=3733.0

In the end, I upped the idle to about 1400 during the whole balance/synch process and then backed it down to about 1100 when I was done.