Mariner Magnum III 150 hp losing top end

[Rowroy]

Usually I can figure out odd issues on my own, but this one has me stumped:

I have a 1994 Mag III 150 hp outboard with a 21" pitch stainless prop mounted on my 18' Ebbtide bowrider. At WOT, I can usually see ~6200 RPM, but I back it off to ~5800 RPM since 6200 is too much. However, when I took the boat out two weeks ago, at WOT I was only able to achieve 5800 RPM. Then yesterday, at WOT, I could only get 5500 RPM.

For whatever reason, my top end is becoming weaker. I also noticed that it takes longer to get to the upper RPM range. However, the motor runs great . . . no hesitation and smooth running throughout the RPM band with no surging.

This motor was rebuilt in December 2008 by a very reputable outboard guru and may have ~40 hours on it.

So far, I have tested compression - all cylinders between 120 - 130 psi, replaced fuel filter, and ran a can of SeaFoam through it.

The plugs look brand new, but they are just a little oily, so I will be replacing them this week.

Does anyone else have an inkling of what might be happening here???

 

[Rowroy]

Re: Mariner Magnum III 150 hp losing top end

Well, I just checked for spark on all cylinders and it appears to be good on all 6 plugs.

Any ideas?

 

[Rowroy]

Re: Mariner Magnum III 150 hp losing top end

Well, I didn't find any fuel delivery issues, either.

Anyone?

 

[j_martin]

Re: Mariner Magnum III 150 hp losing top end

the xr/magnum series has an "advance" module on it that is supposed to advance the timing about 4 degrees (from 19 to 23) at 5000 rpm, then retard the timing 6 degrees (to 17) at 5500 rpm. Maybe it wasn't working and now is, or vice versa.

Regardles they are no longer available because they are basically useless. Lose it and time the engine to 23 degrees WOT.

When they fail, usually by frayed wiring, they drag the bias way down and over advance the timing, grenading the engine.

If you want to see how it works, hook a voltmeter between the bias terminals and ground and take it for a ride.

hope it helps
John

 

[Rowroy]

Re: Mariner Magnum III 150 hp losing top end

the xr/magnum series has an "advance" module on it that is supposed to advance the timing about 4 degrees (from 19 to 23) at 5000 rpm, then retard the timing 6 degrees (to 17) at 5500 rpm. Maybe it wasn't working and now is, or vice versa.

Regardles they are no longer available because they are basically useless. Lose it and time the engine to 23 degrees WOT.

When they fail, usually by frayed wiring, they drag the bias way down and over advance the timing, grenading the engine.

If you want to see how it works, hook a voltmeter between the bias terminals and ground and take it for a ride.

hope it helps
John

Thanks for the info. I'll check it out.