1989 Merc Black Max 150 rough on the high end

[marineman16]

I just bought this boat this year and have no clue what may or may not have been done to cause this. Basically, the motor starts with ease, runs great up to a speed of about 55 for a couple minutes, then "surges" a bit while dropping down to running around 40 over the next minute or two. I can feel and hear the bogging occur, but can't find anything on here that matches my problems. Once shut off or even slowed down for a few it will repeat the cycle.

I'm not very mechanical, but I wonder about:
-Fuel pump / fuel filter
-Carbs
-Spark plugs

I haven't checked anything fuel-related yet, but have run carb cleaner through to try to clean everything out a bit. No luck.

The cooling stream (water pump release) is a steady stream but not a hard stream. I have no clue how strong it should be. Either way, it doesn't seem like overheating to me, but I could be wrong.

Does anyone have any input on where to start?

MANY THANKS!!!!!!!

 

[WillyBWright]

Re: 1989 Merc Black Max 150 rough on the high end

Start with the fuel pump and stop using gas additives. Many mess with the oil's ability to stick and lubricate.

 

[marineman16]

Re: 1989 Merc Black Max 150 rough on the high end

I checked my fuel pump output and it is blasting it out. I've heard it may be a water pump or thermostat issue? That just doesn't seem to be what the problem would be to me...

Also, I gave up on the additives!

 

[DamianJP]

Re: 1989 Merc Black Max 150 rough on the high end

Mabey the high speed jets in your carbs. My Idle speed jets were off and my idle sucked and I constantlly had to keep here alive by nursing the thottle.
So mabey the High's are off and letting in too much fuel at High Speed. There are Idle and High speed jets on the Port and Starboard side of each Carb. The Idles are on top and have a brass cap then the jet. The High speed jet's are on the bottom and the Jet actaully sit's inside the brass High speed plug then is screwed into the carb. They have diffrent sizes printed on the tiny jet (example .074).My advice would be to have someone look at it if you decided that you wanted to go this route.Shoudn't cost much to inspect those jet's. I did all my work but learned as I went and broke a High speed plug ($22.10) over tightenin' it. Hope this helps or narrows it down for you.


Damian

 

[j_martin]

Re: 1989 Merc Black Max 150 rough on the high end

If you have an aftermarket primer bulb and hose, replace it with Mercury. Also the bayonet fitting on the engine. Mercury was metal, aftermarket is plastic, and does not flow enough fuel for a 150.

The little thumb sized fuel filter clogs easily, and will starve it at WOT. If you change it and it temporarily helps, you have a fuel contamination problem. I did also, so I cleaned everything out as best I could,and installed a 60 GPH Racor fuel filter and discarded the filter on the engine. No more problems.

The surge you feel is the engine leaning out and actually increasing output, then starving enough to lose power. If you run it there very long, (as in a few seconds or minutes) you can easily melt a piston.

To definitely prove that fuel delivery is the problem, replace the end plug (probably lower carb, starboard side) with a barb and temporarily install a fuel pressure gauge (10 - 15 lb, steal the gauge of grandma's pressure cooker in a pinch) and install it so you can see it when running. (a length of hose, and in your lap works.) You'll see fuel pressure dive from about 5 to 7 lbs to 0 when the problem occurrs.

hope it helps. That's a great engine.
John