back fire thew exhaust

[sparky01]

Just rebuilt my 81-650 with a new big fin cylinder, new timing chain and at idle you crack the throttle it back fires through the exhaust (2 into 1). As you let the clutch out and give it gas it stumbles and misses until about 1/4 throttle and then it runs fine. When I was installing the timing chain at tdc the mark on the cam was off 2 degrees from being straight up. I put the mark slightly towards the front of the motor as the timing chain stretches it should be straight up. I am running a set of carbs (2 cycle) from 650 central, can this be the problem. Running new stock head from hoots racing. Compression is 135-135, as rings are not seated all the way in. I am frustrated and need some HELP.

 

[scabber]

Check for air leaks vaccume barbs capped carbs synced check timing with light.

 

[sparky01]

could it be the carbs

 

[Patches]

You are going to need to:

1. Check for air leaks
2. Tune your carbs properly

Check out the carb tuning manual in the tech section as this is a must for your upgrades.

 

[XSLeo]

Is the big fin kit the 650 or 750 kit? That compression number seems a bit low for the 650, a lot low for the 750. Did you hold the throttle wide open when checking?
My second over top end had 165 psi compression, the 750 kit had 195 psi.
650 central has the VM34's as a kit, Are these what you have?
On the carbs www.amckayltd.com/vm34-36.pdf should help.
Sounds like you need to tune the carbs. The pilot jets or idle mix is off.
Even great carbs from great people can only come close, no just bolt on and run.
Leo

 

[sparky01]

i is a 650 and i checked timing and it was retarded re set and tuned the carbs and it runs great thanks all. now the comp. is up to 140-140 i guess the rings are seating