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I was given a Coleman 2250 watt generator with a Briggs model 135212 engine. Carb was gummed up from old gas. Got it running. Shut it off and restarted it 5 or 6 times. Last time I shut it off, I let it sit and cool a while. Went back to start it and it felt like it lost compression. Wouldn't start and spun over way to easy. It made no strange noises on the last run. Piston is going up and down. Should I just pull the head and check the gasket? I am thinking this engine may have plastic cam gears.

Search web for Briggs 135212 and you can see the IPL.
I just had a newer flathead - Briggs 126M02 with 0 compression.
The exhaust valve seat had come out and valve was stuck open on it

I would pull the head and check the valves.