loss #4 cylinder on 1999 135hp optimax

[scooter89]

Last spring i bought a 1999 135 opti-max with 302 hours, they told me to use quikleen on every fill (no ethanol)which I have. Last Sunday I had a 15 mile trip back to the boat ramp before dark, and i ran it at 5400RPMs and about 1/2 way back to the ramp i loss about 500-600rpms, After getting home i found that I fouled number 4 plug (Black), if i unplugged the coil or injector the miss didn't change on number 4, checked compression all cylinders the same, reinstalled the plugs and put the fowled in a different cylinder and installed timing light on all cylinders which i had spark on all, and i moved the fowled plug to a different cylinder and the problem stayed at #4, Injector problem? Reed valve? What should i do next?

 

[tommarvin]

I don't have a lot of experience with this, but some thoughts from a amateur
I have read that a timing light is not the best way to check spark.
Theres something wrong with #4
I would clean the fuel injector, get a spark checker to check spark on #4 clean the fouled plug with a nylon brush/toothbrush and cleaner
You could test the plug wire with a multi meter, its on you tube.
That's a long way from home at a high rpm I would test the coil before you buy a new coil.

 

[Chris1956]

Anyway to tell if it is fouled with oil or gasoline? Since they each take a unique path (in Optiimax) to the combustion chamber it might narrow down the issue.

If oil fouled, likely an issue with oil pump. If gasoline fouled, likely an issue with injector, fuel pump or air compressor.

Optimax is a complicated beastie. Hopefully you have service manual and pressure testers for air and fuel pressure.