RE: trx 2.5
As for an alternative motor for the 2.5.....I still stick with the O.S. .15 CV-RX with a hardcore racing head. Not quite as much power but reliability like no other. The most easy to tune motors I've ever owned!
As for the air bubbles. Do you use a fuel filter? If so take it off your problems should dissappear. Especially if it's a 2 piece filter with foam in the middle!
Good Luck,
CJK