1985 yfm 80
 

Hello everyone I just bought a 1985 moto 4 80 cc atv for my son. It has good compression and good spark when the plug is outside the cylinder grounded to the case. When I spray starting fluid in the cylinder it will not fire at all. I checked the timing and both the cam sprocket mark and the fly wheel mark are correct. This makes no sense to me. Can anybody help? Thank you for your time

Starting fluid in the cylinder or the carburetor ? not sure about this model but I had a quad several years ago that would not start if the oil was low.

If it's an old plug just change it out. Seen some that would do the same thing. Would spark against the cylinder,but would ground out and loose spark when inserted in the cylinder. May or may not be your problem,but try the simple things first.

Thank you for your help. I got it to run. I shut it off and heard hissing and found that the crank case is building pressure. Any ideas what causes that? It tries to start back up if I hold throttle wide open then let the throttle close quick. Thanks again

If you have good enough ears to hear hissing from the crank case area,then it's possible crank seals/head or base gasket are leaking.That will stop a 2 stroke in it's tracks if they are.Best if you have a leak down tester for a 2 stroke.

Thank you for the quick reply. This is a 4 stroke 80 cc Yamaha
I'm thinking something with the valves or cam timing

Does make a difference between the two engines.. Check that the crank case vent is open. Dirt Cheap Yamaha, Honda, Arctic Cat & Polaris OEM Parts & Accessories – Cycle Parts Warehouse