high rpms

[Shockman54]

ok, so after tearing apart my carbs completely, resetting the float height to factory, going back to the airbox, and putting the stock exhaust on, she revs great. but now the idle stays high. if i rev up, it stays wherever i rev it to. anyone have any ideas as to why?

[TopCatGr58]

Yep, they need VACUUM synchronizing, not just bench synching!

T.C.

[Shockman54]

thanks, much appreciated. i just wanted to make sure that it wasn't anything broken

[natemoen]

A hanging idle like that is caused not only by out of sync carbs but also a lean running condition.

Open up your idle mix screws to 2.5-3 turns and start syncing and adjusting from there.

[Stralya]

My mechanic (I say 'my' because I've paid him so much money I must own him) insisted that bench synchronising was by far the superior over vacuum synching on the bike. He told me you only do it on the bike if you don't have the time or inclination to take the carbs off the bike. Now that he's re built my carbies using kits from Georgefix, I have a similar problem. Throttle response is excellent, however, backing off is very slow, the revs come down but very slowly. Looks like it's time to break out my new synch tool.

Dan.

[crazy steve]

My mechanic (I say 'my' because I've paid him so much money I must own him) insisted that bench synchronising was by far the superior over vacuum synching on the bike. He told me you only do it on the bike if you don't have the time or inclination to take the carbs off the bike...

Time to find another mechanic... There's no way to properly sync the carbs off the bike, period.

[DGXSER]

Dan, do not feel to bad, your mechanic has been listening to way to many "experts" that have no real clue. But I would start paying him alot less, like no more of your money. It is amazing how often I see or hear folks say to bench synch the carbs and you will be fine.

Bench sync is simply setting all the butterflys to the exact same opening. takes no account for engine compression changes from cylinder to cylinder, or carb performance changes from carb to carb, or any other differences in actual operation. The running sync addresses the reality of life that no matter what you do each cylinder is goign to perform a little differently. And so you adjust the butterfly plate opening to equal out the performance in running idle condition. relying solely on the bench sync would be the equivalent to setting your mixture screws to three turns out on each carb and calling it tuned. Preset them correctly and no need to ever adjust, they will be perfectly tuned.

So, if your plan is to let your carbies sit on the bench and look pretty, the bench sync is the right tool and perfect for you. However, if you plan to put them on the bike and run it, you will want them synched under running conditions.

[ViperRon]

Dan in the relm of mechanics there are two extreems and to the top I assign the title of being a GURU and the bottom GOOBERROO. Goober being tied to the mechanic in Maybery on the Andy Griffith show years ago and (OO) the sound you make everytime you pay him to work on it.


To say that mechanical sync is superior to vacume sync in running operation is a Gooberoo statement.

To you I suggest another mechanic or you can bohica (bend over here it comes again) till your bike is Fubar (F*%$ed up beyond all repair).

For your condition I would check to make sure that the carbs are bolted down tight and no vacume leak. The engine reving high like that tells me its getting too much air from somewhere and for the idle speed to increase id look for vacume problems first. Check the sync then adjust the idle mix and resync carbs.

However if that does not get it straight then other things in the carbs may off.

[Tat2demon]

A hanging idle like that is caused not only by out of sync carbs but also a lean running condition.

Open up your idle mix screws to 2.5-3 turns and start syncing and adjusting from there.

I dont want to derail the thread but is that for all XSs or just his 79? My Haynes says 1.25 turns out. Wonder if thats why I was having high idle after synch too.

[natemoen]

I dont want to derail the thread but is that for all XSs or just his 79? My Haynes says 1.25 turns out. Wonder if thats why I was having high idle after synch too.

it applies to all internal combustion engines. Personally I feel that starting it at about 2.5 turns out on these carbs is closer to what they need.

[Tat2demon]

Wow I was way off then. Back to the garage tonight when it cools off.

[crazy steve]

The 1.25 number is correct for the early carbs, the late carbs need more turn out....

[natemoen]

The 1.25 number is correct for the early carbs, the late carbs need more turn out....

Mine is waaayy to lean at 1.25

[Stralya]

What's a 'turn'? Is it a full 360 degree turn or 180? And what screw do I turn anyway? I've got the synchroniser on the bike and I'm turning the idle mixture screw but nothings happening. I turned what I thought was the idle speed screw and the bike revved it's head off and the gauge went off the scale. I can only see three of these though. Is there a step by step procedure for idiots?

Dan,
Idiot.

[crazy steve]

http://www.youtube.com/watch?v=2nwP6...eature=related

The idle mixture screws are on each carb. The sync screws are located between the carbs on the linkage, but there's only three of those. On the XS, carb three is the one you sync all the others to. Generally, you'll sync 4 to 3 and 1 to 2, then sync 1/2 to 3/4. After you do a initial sync, you can then go back and adjust each idle screw to the highest vacuum reading, then do your final sync.