for me when the compression is poor the engine will be hard to idle when it is warmed up, which will also be hard to start when it is warm. a fresh piston/cylinder is needed.

where are the bubbles coming from? can you see them coming from the engine thru the fuel line?

i haven't seen crankcase compression causing backpressure in the fuel line, but maybe the top end is leaking enough to produce backpressure in the crankcase? and this is pushing vapor back into the fuel feed line?