while trying to start an old atc 200 i'm getting gas blowing out through the rear of the carb, if i take the carb off and spray starting fluid in the intake it will run until the fluid burns out...this makes me think the timing is not off. i switched the intake rocker(valve adjuster) whatever, with a spare i had and the bike started and ran...not for too long though...now back to the same problem...can't figure it out. if i manually turn the crank/flywheel i get suction as the intake valve opens but as the crank turns just before spark, back pressure through the intake......first time on the group any help appreciated

 

Sounds like your cam skipped a tooth. I don't know on that model, but on a 300ex there is 2 marks on the cam gear. Those have to be flat with the head when the crank timing mark is lined up.

 

Hmm. That's a bit weird. Are you sure the intake valve is closing completely? If not, the piston will just blow it back out they way it came in. Set it at TDC and the tappit (the rocker thingy you replaced) should wiggle around. If it's adjusted ook, it sounds more like the cam chain timing to me too.

 

i would make sure your not lean on the jetting either.

 

thanks for the info...just to be brief...when i switched the valve thing the one thing i noticed was the fact that the adjuster screw was out a lot farther than the exhaust was...is it worth messing with that adjustment or am i in for a lot of trouble...like you said(and it makes complete sense) that the valve has to open just a little too long for the blowback to happen.(but that would be the cam timing right?....thanks again for your time

 

This could be normal reversion. Sounds like it from your description. Eazy way to find out, do a compression, or leak-down test. This will tell you whats up.

 

The adjuster screw is there to set the tappit clearance. Turn it out as far as needed to have a gap between it and the top of the valve stem when at TDC (usually .005"-.009" depending on the engine). You checked that, right? The whole rocker should be loose at TDC since it shouldn't be pushing on the valve at all.

Another thing you can do is set it at TDC of the compression stroke and put an air chuck into the cylinder through the spark plug hole. If the tappit is gapped properly and air still comes out toward the carb, you've got a leaking valve/seat.

 

Its no hard to just open the inspection plates - including the cam chain inspection plate to see if the marks are lined up.