2003 Yamaha Warrior 350 - Valve Issues

I have a 350 warrior that is experiencing fuel/air vapor blowback out of the air intake side of the carburetor followed by back fireing, powerloss and flames shooting out the back of the carb. Does anyone know what could be causing this???

could be valves or it might be rings. bad rings will have the crankcase build up pressure and the only way to escape is thru the carb. that might explain the power loss too.

It has a fresh set of rings and jug bored to match piston. One mechanic suggested loosening the valve adjustment to close intake valve longer??? It doesn't burn any oil and the plug burns with great carbon build up. So I don't think it could be rings, but I am open to any suggestions. You got anything else? Thanks!

Plugged case breather?

Hmmm??? I don't see how that can cause back pressure out of the carburetor, but I will check that out, Thanks. I am leaning toward a bent valve or tired valve springs becuase the cylinder just isnt taking all the fuel/air that the carb is throwing at it and it should cuz the exhaust chamber has a major port and polish and the thing tachs up like crazy?! This is frustrating!!!! :O

Who did the porting? The pressure has to go somewhere from the crankcase.

I ported it and it is real clean and smooth. I built racing motors for Yamaha Marine. (Two Strokes No Valves) I don't see how the crank case breather could be clogged but I will check it. My buddy is a Mecanical Engineer and he is saying "Valve adjustment or bent valve or valve spring"

I thought I rounded out the cam but he says it wouldn't even run. It is some kind of Valve timing issue cuz the fuel that the carb is throwing just aint goin into the chamber and is spittin fuel vapor out of the air intake side of the carb. So instead of having all suction, it is spitting back.

Seems like you would remember if you "rounded" the camshaft or not. It wouldn't run anyway. IMO, the problem is in your head. Wether it's the porting, the valves, not sure.

I would have to agree with you! I don't think it's the porting cuz it tachs up way faaster than used to just like a could porting will do and it was doing it before the port and polish even with the fuel vapor backlash deal. I am goin to adjust the valves again first, if that don't work I will pull the head and go through it. Thanx alot! ;)

And when I say "rounded the cam lobe" I mean wore it down so the valve doesn't open as much as it should and opens and closes too quickly. It loses duration.