Carb Syncing

[azcafe]

Hey All,

I just recently picked up new carb manifolds off of Mikes XS for my 72 BS38's. The new manifolds have the barbs in them. I was curious on how to go about syncing them. If I do the yard stick and and attached and end of the hose to each barb would I then adjust the air fuel mixture screw or the idle screw?

Thanks for the help.

 

[DogBunny]

Your 72 should have two separate idle screws, and you use them to adjust the sync. And if you have dual exhausts, you can do a perfectly reasonable job of syncing by just feeling and comparing the exhaust pressure coming out of the pipes with your hand instead of messing with sticks and barbs and such. I wrote about it, and I also wrote about adjusting the mix screws in this thread:
http://www.xs650.com/forum/showthread.php?t=33791
It will be a little different on your bike since you have two idle screws instead of one idle screw and a sync screw, but the basics are the same. Ignore the naysayers in that thread who criticize without ever trying it.
If you have a 2 into 1 exhaust, then you will need sync sticks, but you still use the idle screws for adjustment.
Get the sync close, then do the mix screws, then do the sync again.

 

[5twins]

You will actually have to sync twice. Once for the idle speed on the two carbs, then for the "at speed" sync when the throttle is opened. First you match the idle speeds on both carbs, then you match cable freeplay so both butterflies open at the same instant and both carbs run at matched speed throughout the RPM range. You must do the idle and mix screw adjustments, and idle sync, first, because adjusting the idle speed adds or removes freeplay from the throttle cables. That would throw off your "at speed" sync if you had done it first.

 

[DogBunny]

Yes, I think that is the second or third time that 5twins has corrected me on that -- I always forget to mention that you also have to match the cable free play.

 

[5twins]

Yes, that makes the butterflies begin to open at exactly the same time when you twist the throttle. It also has them open the same amount throughout their opening range compared to each other. This is what being "in sync" means.