I had the same problem with a Thunder Tiger Pro 21. I found the lower unit halves were so poorly fitted together that the muffler pressure wasn't sufficient to keep fuel flowing to the motor (at speed). It would run on the bench all day, but put it in the water and it would quit in about 30 seconds. I figured it out by blowing into the exhaust while plugging the pressure fitting and having the piston up enough to cover the port. On mine there was almost no resistance. I took the lower unit apart and filed down the mold flashing they left. Then I threw away the paper gasket. I sealed the area where the paper gasket was with blue RTV gasket. BE CAREFUL not to get the RTV where it doesn't belong! It only takes a little bit. Some parts should not be sealed off so the oil from the exhaust can lubricate the drive components. After doing this there is now significant resistance to air blowed into the outlet. That motor runs great now. Very reliable. Good Luck