Topic: carb sync question

[troybrewcity]

what would suggest a bike needs its carbs synced, also what causes carbs that were balanced to go out of balance?  for  example, a bike is running smoothly, plugs look good, but in the 5-6k rpm range the bike sputtters and coughs?  is this an example of carbs that have gone out of sync or something else?

[flybox1]

any rough acceleration, as you describe, would lead me to eventually re-synching my carbs, but there would be a list of things i would do first.
Completing the steps in a 3k tune-up, along with a thorough cleaning/bench sync of my carbs would be first on the list.  only then, would i do a vacuum sync of the carbs. 
If the bogging at 5-6k was still present, i would do a plug chop while that was happening to get a better look at what is going on there. 
best guesses....
clogged main jets/emulsifiers (which would be fixed during your carb cleaning)
points needing cleaning/timing advance off (remedied during your tuneup)

[midnightrider]

Bike not running smoothly would suggest carbs need synching, but could also suggest a range of other problems. 

When the carbs are out of synch, the cylinders are fighting one another because they are getting different amounts of fuel.  Vacuum in each cylinder effects the amount of fuel/air flowing from the carb, so... vacuum differences between cylinders = different fuel/air rates even when the carbs are open equally = carbs need to be synched, which is the adjustment of the slide on each carb to compensate for vacuum differences and even out the flow of fuel/air to the four cylinders.

I don't know enough to diagnose your symptoms, someone else here may, but they will likely need more info. 

I do know this: synchronizing carbs is what you do when you know that everything else (ignition, plugs, wires, valve clearance, timing, dwell) is good.

[Trav-i]

Also check your jets and emulsion tube.  At that point in the rev range you are transitioning to the mains from the slow jets.  Either may be dirty or clogged.  I would check your jets before getting into a sync job.  A sync should be the last thing you do after a full tune up with all the proper adjustments.

Travis

[TwoTired]

First symptom out-of-sync-carbs, is an erratic idle.  They can go out of sync because of wear, and adjustment creep, as well as prior owners trying to save a buck on maintenance.  Further, anything that changes individual cylinder efficiency can change the vacuum sync.

The RPM range you are talking about is more indicative of mixture ratio issues.  I'm guessing you changed the exhaust and/or air filter from stock.

Telling us how your bike differs from stock and when the last routine 3000 mile tuneup has been performed will likely get you more meaningful responses.  Also, we don't even know which year or model you are asking about.

[ofreen]

+1 on TT's post.  Poor carb synchronization will have really no noticable effect up at 5 to 6 thousand RPM and has nothing to do with your problem in that range.

[Mainer4]

I would suggest taking a close look at your plug boots, wires, and condition of your points.  I have experienced a breakdown in this RPM range and almost always found an issue with the ignition system, usually points.  In one case the rivet for the point loosened up.  Another case the points really needed replacing but I tried to get by for one more season by carefully filing and cleaning them up.  Worked for about 2K miles but eventually came back to bother.

Synchronization of carbs is generally indicated by acceleration deficiencies, erratic idle, poor shifting, and generally noisy (mechanical) sounding engine.  Your cam chain will let you know that carbs are in need of synchronization, it is much quieter when everything is running smoothly...

[troybrewcity]

Lets dig into this a little more, so as i understand it if the carbs were out of sync you would notice it through out the whole rev band.  so i were to look at the jets and emulsifer tubes as possible culprits, would the rev problem happen all the time, like when the engine is cold and hot, or just hot?  so lets say cold engine has no problem revving in the 5-6k range, but once the engine warms up and is at temp its sputtering just in that range, no problems in the band before, and none past... all insight is helpful.

[ofreen]

Lets dig into this a little more, so as i understand it if the carbs were out of sync you would notice it through out the whole rev band. 

No.  Carbs being out of sync would be noticable at idle and perhaps just above, but at wider throttle openings it will have no noticable effect. I am talking about carbs out of sync at typical degrees you are likely to see.  It would not cause the problems you are describing at 5-6000 rpm.

[dave500]

a rough feeling at any revs even no load cruising,viberaty tingly fingers?if your not sure what the po has done with this bike do all the ignition then sync the carbs.out of sync carbs will cause a noisy motor no load.