All the explained above is great advice, things that you could verify yourself.

I would add: check the nipple of the muffler, where the silicon tube is plugged to, sometimes the little hole is clogged and the tank does not receive proper pressure.

Set ups that are sensitive to non-level positions, normally have problems with low muffler pressure reaching the tank.

Last week I had a similar problem with one engine that had been working well.
It was hard for me to find the problem: a little crack in the silicone tube, just after the muffler nipple, was leaking out the exhaust pressure.

Best luck with the engine.
If you quit on it, just send it to me in Florida!