Backfiring Right Cylinder

[Take Warning15]

Hello,

I have a '75 XS650 with Pamco electronic ignition using the wasted spark system. Yesterday I took it for the first ride of the season and it ran great. I put about 70 miles on it at all different speeds. When I brought it home I noticed that the coil had come a bit loose. I took off the tank to tighten it up and everything looked good. However, now when I light it, it seems to only be running on the left cylinder. I can see it backfiring when I look into the right exhaust.

I swapped plugs and wires and the problem staying on the right. I also cleaned the carb on the right side. Timing checks out as well. When I did a compression test, I'm suddenly down to about 90 on the right, and still near 160 on the left.

Could this be evidence of the exhaust valve not closing all the way? If so, would it be a simple adjustment when it cools down, or does this point to a bigger problem? Also, what could lead to this problem after simply sitting for a few hours?

Thanks

 

[TwoManyXS1Bs]

Exhaust valve *could* be sticking. After shutdown, who knows where the engine stops. Might've stopped with that valve shoved open, and it likes being thataway now. Have you seen Gordon's adventure?

http://www.xs650.com/threads/help-i-broke-it-wont-start-warning-long.40457/

 

[pamcopete]

Does that model have separate throttle cables? Is it possible that you somehow broke or disconnected the right side throttle cable when you removed the tank? If the throttle was closed when you did the compression test you would get a low reading.

 

[Take Warning15]

Good question, Pete. The '75 does, but I have later model carbs on it that are connected. I'm thinking that the exhaust valve has been burnt and isn't seating correctly. Would that stand to reason? I still have to check the valve clearances, but I can't imagine why it was running when I parked it but not when I woke up the next morning.