No RPM under load.

[mtjbrown]

I have a used 75hp Mercuryapprox 97 model. I had it swapped it onto my 18 ft pontoon boat, took it out yesterday for the first time. Advance the throttle and motor will rev to 6K rpms, but under load max rpm is 1500 to 2000 rpms. I had a force 50 hp on the boat that used premix so there is some residual premix in the fuel tank, could this be my problem or is there more problems??
Thanks in advance for any suggestions.

 

[oldman570]

Re: No RPM under load.

If your reving to only 2000 under load I would check the pitch on the prop as it could be to much for the pontoon. A prop of 13.5 X14 should push it just fine. Any higher pitch will work the motor to much and lower the RPM's. Testing the plugs for spark using a timming light will tell if they all are fireing as one not fireing will also lower RPM's under load but will seem good when on muffs and will not have the back pressure on the motor. This simple test will tell the tale as to what is happening. Testing can be done on muffs at idle and NEVER rev motor while on muffs. Old premix should be mixed or replaced with new gas if the motor still has the oil injection system still working. The electrical system on those motors are very touchy if any testing is not done correctly.
Oldman570

 

[mtjbrown]

Re: No RPM under load.

Thanks, Oldman570. I do have a 21 pitch prop on the motor now but I have a 15 pitch coming as I was advised by my local dealer. We'll see how it goes.