I've been helping a friend troubleshoot a vexing problem with low speed bogging on a '99 900 STX. He bought it used with this condition and 1 bad piston. Here's what we've experienced and done>
#1 piston @ 50psi = Honed cyls replaced rings, gaskets - all pistons now at 100-110psi
Boat would only start if fuel directly injected in intake - could coax to moderate speed but boggs/cuts-out at low - couldn't restart without fuel "prime"
Rebuilt all 3 carbs with new seals/gasket/membrane kits (inc clear flapper valves), cleaned jets, added primer kit. Gasket kit for main pump only included seal/membrane between pump assy and rear carb. (Not the seal/membrane inside the pump itself)
Boat now starts with one-two shots of primer but still boggs/cuts-out at low speed. If, when it boggs, we depress the throttle and give it a shot with the primer engine will go to full speed where it will run fine with plenty of power in 3/4 - full throttle range. If we drop below that, engine boggs/dies.
It seems like we still have a fuel delivery problem that mostly impacts low speed performance. Could this be a leaky/faulty main pump (the membrane that we couldn't replace)? Should I just replace the pump? Or could it possibly be a bad line or pick-up at the tank (I've heard these may corrode and obstruct flow). This doesn't seem as likely due to the good flow/performance at high speed. Hopefully, someone out there has seen this kind of thing before and can point us in the right direction. Thanks.
#1 piston @ 50psi = Honed cyls replaced rings, gaskets - all pistons now at 100-110psi
Boat would only start if fuel directly injected in intake - could coax to moderate speed but boggs/cuts-out at low - couldn't restart without fuel "prime"
Rebuilt all 3 carbs with new seals/gasket/membrane kits (inc clear flapper valves), cleaned jets, added primer kit. Gasket kit for main pump only included seal/membrane between pump assy and rear carb. (Not the seal/membrane inside the pump itself)
Boat now starts with one-two shots of primer but still boggs/cuts-out at low speed. If, when it boggs, we depress the throttle and give it a shot with the primer engine will go to full speed where it will run fine with plenty of power in 3/4 - full throttle range. If we drop below that, engine boggs/dies.
It seems like we still have a fuel delivery problem that mostly impacts low speed performance. Could this be a leaky/faulty main pump (the membrane that we couldn't replace)? Should I just replace the pump? Or could it possibly be a bad line or pick-up at the tank (I've heard these may corrode and obstruct flow). This doesn't seem as likely due to the good flow/performance at high speed. Hopefully, someone out there has seen this kind of thing before and can point us in the right direction. Thanks.
