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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 as well as 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 speed - 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.

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. Checked plugs - seem fine (brown).

Replaced "common" pump (+ new seal) and all fuel lines.

Engine still hard to start and bogs at low speed.

It seems like we still have a fuel delivery problem that mostly impacts low speed performance.

Thinking it may be a bad line or pick-up at the tank (I've heard these may corrode and obstruct or introduce air in flow)???? This doesn't seem likely due to the good flow/performance at high speed.

Possible low end vacuum leak (affecting pump function)???

We're running out of ideas.

Hopefully, someone out there has seen this kind of thing before and can point us in the right direction. Thanks.
 
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