Why Won't This Engine Run?
 

I picked up a 110cc quad. Compression tested at 160psi. Had spark and fuel, but couldn't get it running. Checked timing marks and adjusted valves. Still no run and made a banging noise when turning over. Removed it and pulled head to be sure piston was at TDC and timing marks all lined up. Yup. Pulled right side cover thinking something broke inside, but no metal fragments. Bolted it back into a bare frame with no exhaust. Wired it up and ran fuel. Fired right up and ran great. Then I hooked up the exhaust and the banging was back and it won't run. This is not the same exhaust as before, so it has to be the engine. What can be wrong?

Sounds like it has a blocked exhaust pipe check to make sure theres no blockages.

If I was using the same exhaust I would agree, but it was a different one. I also put a different engine in the quad that this engine came in and it ran fine with that exhaust. I think it has to be an engine issue.

I know this is an dumb Ideal but put new NGK spark plug in it

What about the air system? air box? air filter?

I'll try a new NGK plug. There was no air filter/air box on there.

Got to have aif filter or air box assembly for mixture to be correct unless carb is jetted to run without

Apparently this wasn't a dumb idea at all! I put in a new NGK plug and now it runs with the exhaust on! Never would have guessed replacing a working plug would make a difference, but it did. Thanks to Papa hogg scoot.

Thanks for the suggestion Papa hogg scoot. I am also facing the same issue and you made my day