I recently installed a Bison Pitss and the engine finally runs normally. I think its the muffler causing the air bubbles since they are now gone...

lots of people have the same problem with this engine, the problem is the o ring in the needle valve,os will sell you two o rings for around 8 bucks or you can ship the engine to them and they will replace the rings free,that is what I ran into last summer. I bought the rings and installed them myself, cured the problem, but felt like I was seduced on paying to repair there mistake!!
John