Just bought a second hand Nexstar plane kit and am having trouble getting the engine to run right. I have a hunch the engine is toast, but here goes...

The plane and engine look new and reported to have been ran a couple of times but never flown. It does look this way. Got it running and got the mixtures to where idle and throttle response is good. Anytime I run it wide open for more than 20 or so seconds, it comes to a stop. I watched the fuel line and it was full while the engine died. Filled tank, changed plug. Ricthened mixture. Will not hot restart by hand. Compression seems weak to me, especially when hot. When I hold my finger over venturi, open throttle and turn prop over by hand, cannot get fuel to pull into engine. Can barely get it to happen when cold. That seems very strange to me.

I'm going to pull the head tonight to see what the cylinder looks like.

Any ideas?