'95 Mariner 75hp struggles to get above 3800rpm

I've got a '95 Mariner 75hp (2-stroke) that is difficult to get to accelerate above 3800 rpm (it's on a 17' Aquasport) No one can figure out what's up. Carbs have been inspected/cleaned, new fuel pump, new plugs, spark is good, compression is good, oil sensor is fine, vents on nearly new fuel tank checked and new anti-syphon valve installed. all gas lines and filters replaced. lower unit dropped and exhaust inspected for obstructions. Tried running on external tank with same results. Engine seems to be running smoothly, just stops accelerating at around 3800rpm. I just discovered however that it WILL get up to around 4500 rpms if I give it long enough. After about 1000 yards in moderate seas it manages to reach planing speed and then rpms start to increase and will hold 'til I slow down, then process has to be repeated.

Mechanic suggested it may be a prop issue, but we checked and pitch on current prop is correct. It is however an older Pirhana composite prop and mechanic thinks it may be flexing.

Any thoughts before I go out and spend a bunch of money on a new prop?

I'd try a prop with a lot less pitch and see what happens. Load, a dirty bottom and other factors can really hurt a motor.


Jeff

Bottom is clean and it's just me in the boat. Weird thing is that I had no problems until after I replaced the fuel pump (had a pinhole in diaphragm but since it was old I went ahead and replaced the whole pump rather than do a rebuild) Mechanic has checked the new pump and says it's fine. Sounds like it may be worth trying the new prop option.

Flywheel key checked ?-----Cylinder sleeve position checked ?-----Crankcase compression checked ?-----Reed valves checked ?-------Tried removing plug wires one at a time to see if all cylinders are contributing ?

Plug wire test done. All cylinders seem to be contributing equally. I did read up about failing reed valves and the symptoms are similar to what I'm experiencing, but there's no backfiring... engine idles fine and accelerates smoothly up to 3800rpm. Just won't go above that until I get on a plane. Would this be the case with bad reed valves?

It sounds like you might have a partial block in one of your fuel lines. I've seen fuel lines get very brittle over time and actually start to fall apart. It is possible that something became dislogded during the work on your fuel pump causing a partial block in the fuel line starving the engine of fuel.

Already thought of that... I replaced all the fuel lines straight through from the tank to the carbs, as well as the check valve on the tank and the engine fuel filter... no effect. And this wouldn't explain the fact that IF I can get the boat up to planing speed, the engine does in fact go to full revs.

Try a less pitch prop. Most marinas can lend you one.

Jeff

Check your ignition I believe the 95 model still has high speed windings in the stator. Those are known to go bad www.cdielectronics.com has the diagnostic info for your motor.