Sportjet 90 problem that I can't solve

[jworth21]

Thank you for letting me join the forum. I've been searching fruitlessly for a solution to my engine problem. It's a 95 sportjet 90. I can't get it to rev out. I can get it to rev to about 3500 rpm which is enough to get her on plane and 30 mph with 2 people. Runs smoothly, but no more RPM than that. Here's what I've done so far:
1 Replaced all of the ignition including stator, trigger, cdi ignition, coils, plugs, and wires. I freshened the ground while I was at it. Magnets in the flywheel are intact.
2 Complete rebuild of the engine as I had lost the #3 cylinder. I now get good compression on all 3 cylinders ~150 psi
3. Rebuilt the carburetors
4. I've run it with the rev limiter disconnected. No charge. It'll rev out under no load as with wave jumping.
5. I've run it both with the oil injection system in place and also with that defeated but on premix 50:1
6. Don't see how it would be related but I rebuilt the wear ring as well as changed bearings and seals in the lower unit.
7. I've checked the idle and WOT as per the manual.

I'm going to verify timing while running on the lake today, but I think I'll just confirm that it's right. The reeds appear to be in good shape as best as I can tell. I'm kinda at wits end with this. Any suggestions would be appreciated.

Thank you
JW

 

[Cobra Jet Steering LLC]

I had issues with dirt in the fuel lines between the carbs , it would run good for a minute or less and then drop way down on top speed because it would block off the fuel supply to the 2 top carbs. I also added an electric fuel pump as I always did on 2 stroke power heads. Old stale fuel will also make it go slow weak fuel pump or any leak in the fuel lines on the suction side will starve it for fuel if you can run it with the engine exposed and spray starting fluid in a short burst in each carb and see if it immediately picks up speed you will know what carb is causing the issue and know it is fuel related if it does nothing you need to look elsewhere for the issue including replacing the main ground to the engine block directly to the battery and check the ground wires to the coils and brain.

 

[jworth21]

Thank you for the reply. As I had the carbs off rebuilding them, I also replaced the lines between the carbs. When I was out yesterday I did adjust the timing. While it was more than I expected, it only helped marginally. It's interesting that it runs so smoothly even at full throttle, just doesn't reach the RPM and thus power that it should. These aren't especially potent engines to start with. When they aren't performing optimally you really notice it. I had a third person in the boat with me yesterday. It truly struggled to get on plane and about 24 mph was all she could manage with the 3rd person. And we aren't talking big people here either.

 

[Cobra Jet Steering LLC]

It has basically an old force outboard engine in it I have had similar situations that scrubbed top speed due to exhaust leaks in the engine compartment but other than what I mentioned that I am out of suggestions

 

[Greg M]

if it had a propeller, would instantly tell you way wrong pitch (been thru that before)... with jet drive, it will usually give you indicators, but maybe check the impellor pitch because you've literally replaced everything else

 

[Sbrown]

I have seen reeds cause exactly what you are experiencing, but only once.

 

[Cobra Jet Steering LLC]

One more thing that most people do not even know about "POWER HOOK " on older boats the beams supporting the center of the hull get rotten and the faster you try to go the more the hull develops a hook or cavity and that will slow you down immensely older fiberglass hulls are known to develop power hooks , often if it is on the trailer you can actually see the bottom of the hull curved upward near the back of the boat. A straight edge should show it and often just pushing on the bottom of the boat and feeling it flex will let you know you have one.