It could be vibration from an unbalanced prop foaming the fuel, bad fuel, bad plug, out of balance spinner, fuel lines kinked, muffler pressure blocked, holes in fuel line in the tank, switched tank lines, leaking head or backplate, loose motor mount, leaking needle valve, too much prop, too cold to run well (well that might just be here today a 5 degrees). You do need to run an engine a lot richer than just not dying when the nose is vertical.