90 hp 2 stroke will not go above 5000 RPM

[LynP]

1989 90 2 stroke mounted on 750 # 17' center console bay boat, motor weight 675 plus 25 gallons fuel, and my weight of 250 # total weight of +/- 1830 #. I am at 3000 feet of elevation. I have two props one 13.8 x 15 pitch that was to be used at 6000 feet and above. One 13.3 x 17 pitch for 3000 feet. However both props at 3000 feet turn at the same 5000 RPM at 80% of WOT. When I try to go above 80% of throttle both props act like to much air or lack of fuel. Not sure the Tachometer is correct. 90 2 stroke has rev-limiter of 6000 RPM. So if the Tachometer is off, I don't believe the motor can turn 17 pitch prop at 6000 RPM at 3000 feet.
LynP

 

[jimmbo]

"@ 80% of WOT"
What do they turn at WOT? That is the Number that matters. 80% of Throttle Lever Movement is meaningless, as the Throttle Lever is not Linear to Rpm.
Since you have doubts about the Tachometer Accuracy, then that should be the Priority. Verify that it is set correctly, if it is, then compare it to another tach, known to be accurate. Until you have a Verified Rpm, any speculation on anyone's part, as to what might be wrong, may not be valid, and might send you down various Rabbit Holes, wasting everyones Time

 

[racerone]

There is more to it , like condition of motor.----Post the compression numbers here first.

 

[LynP]

There is more to it , like condition of motor.----Post the compression numbers here first.

All at 130 PSI. Also if the tach is off it does not explain why 13 pitch and a 17 pitch start acting like they are starving for fuel and/or air at the same RPM. I would think a good strong 90 hp would not turn a 17 pitch at 3000 ft more than 5300 to 5500 RPM. That would put 13 pitch very near rev limiter at 6000 RPM.

 

[racerone]

Hurry-----Find a shop with a dynamometer to check power output on this motor.-----Need to establish this before juggling / guessing with props.

 

[roscoe]

You are not likely to fix this situation with just prop changes.

Has the motor had new jets installed to allow it to run properly at the higher elevations?
The higher the elevation, the worse it will run unless it is rejetted.
The higher you go, the less dense the air, so smaller jets are required.

 

[jimmbo]

If the tach is reading low, and both props are triggering the rev limiter, it will happen at the same Rpm, regardless of the prop

 

[saltchuckmatt]

1989 90 2 stroke mounted on 750 # 17' center console bay boat, motor weight 675 plus 25 gallons fuel, and my weight of 250 # total weight of +/- 1830 #. I am at 3000 feet of elevation. I have two props one 13.8 x 15 pitch that was to be used at 6000 feet and above. One 13.3 x 17 pitch for 3000 feet. However both props at 3000 feet turn at the same 5000 RPM at 80% of WOT. When I try to go above 80% of throttle both props act like to much air or lack of fuel. Not sure the Tachometer is correct. 90 2 stroke has rev-limiter of 6000 RPM. So if the Tachometer is off, I don't believe the motor can turn 17 pitch prop at 6000 RPM at 3000 feet.
LynP

675 lbs for a 90 HP 2 stroke?