Johnson 50/35 Jet 5000 RPM at WOT

[Shinyboat]

I have a 91 Johnson 50/35 on a 14’ river runner. It takes longer to get on a plane than I would like. At WOT it runs 5000 rpm whether accelerating or on a full plane. The throttle stop is set so the intake is wide open. I think the holeshot would be faster if I could get rpm’s up to 5300-5500 since there would be more thrust. Jets have fixed impellers so I can’t change the pitch.

Thoughts on how I can increase rpm’s?

 

[racerone]

I would start with checking the health of the motor.-----A compression test first.-----Post your numbers here.

 

[Shinyboat]

I would start with checking the health of the motor.-----A compression test first.-----Post your numbers here.

Tested tonight. Started up and idled with foot in a 20 gallon bucket with water running until the head didn’t get any warmer. Head never got super warm so I’m not sure if the test was good enough.

85psi both cylinders tested twice. Pressures didn’t spike on starting and didn’t bleed off so i am thinking low pressure for isnt from rings not sealing from not being hot. Seemed low so I checked against my brand new rigid air compressor and it was within 5psi.

 

[saltchuckmatt]

Tested tonight. Started up and idled with foot in a 20 gallon bucket with water running until the head didn’t get any warmer. Head never got super warm so I’m not sure if the test was good enough.

85psi both cylinders tested twice. Pressures didn’t spike on starting and didn’t bleed off so i am thinking low pressure for isnt from rings not sealing from not being hot. Seemed low so I checked against my brand new rigid air compressor and it was within 5psi.

That's low for a 2 stroke outboard. Might want to try another tester. Otherwise I don't know much about jet pumps.

 

[Shinyboat]

That's low for a 2 stroke outboard. Might want to try another tester. Otherwise I don't know much about jet pumps.

That’s a good idea. I’ll see if I can borrow one to test mine.

I think the thermostat is also acting a bit stuck which may have contributed to the engine running cold.

What should be the difference in psi on a hot and cold engine?

 

[saltchuckmatt]

That’s a good idea. I’ll see if I can borrow one to test mine.

I think the thermostat is also acting a bit stuck which may have contributed to the engine running cold.

What should be the difference in psi on a hot and cold engine?

Shouldn't matter much. By the way, you want your motor running somewhat cool....shoot the top of the head with an IR gun.