Sounds like the same thing my SV90 was doing. Ran great on the ground, pulled a 35% Giles 202 (27lbs) in the air like a truck but had a strange rich sounding 4 stroke burble in the air. It kept pulling great, never dead sticked and i got tired of messing with it to get the burble out. After about 5 gallons it got better and better and a few small tweeks and it runs great. If it doesn't dead stick and pulls good try just flying it. Good Luck