I have friend that has a ison .12, when the motor is cold it is easy to start, and very stable and smooth.... but when it is run, and after it is very difficult to start the engine again, I noticed when the engine is running, it makes bubbles in the tank a lot of them, but we do not see where they came from... and the engine as I say it is very stable when running.... What can do bubbles??

When it is hot and it will hardly start it is most of the time an end of life of the P/S set.

When the engine is hot, spill some fuel on the head so it will cool down a bit, then try to start again, in a race that should help.

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?