1986 150 Black Max overheat on WOT

[crazyedd]

I am having a problem with the engine running hot at WOT within 1 minute but when I return to idle it drops within 30 seconds. I have changed the sending unit, gauge, wiring, checked timing, replaced a diapragm in the poppet valve, new thermostats, and impeller. Water pressure gauge shows the pressure goes up as throttle increases. The tell tale spits out plenty of water once the thermostats open and on land, it seems to be too hot to touch. I revved to 1500 RPM's on land and measured each cylinder at the head with a laser thermometer. Port side ranged 155 to 165. Starboard 175 to 195. Any ideas what to look at next.

Thank you

 

[studlymandingo]

Re: 1986 150 Black Max overheat on WOT

Sounds like you may have some blockage in the Starboard side water passages.​

 

[crazyedd]

Re: 1986 150 Black Max overheat on WOT

I had removed both thermostats and covers that go with them and turned the engine on. There was plenty of water flowing out of the heads. Is it still possible that something is blocked inside? Someone told me to check if its running lean. The spark plugs were a very light brown color. Could a lean mixture cause this problem?

Thanks

 

[j_martin]

Re: 1986 150 Black Max overheat on WOT

What does the water pressure do as engine speed increases.

It should advance to about 10-12 lbs, stutter or drop a little, then advance a little more, but ususally not more than 20 psi. If you don't get the bump as the poppit opens, it could be that the passage going out of the bottom of the poppit (triangular shaped) is plugged. If a little water leaks out of the bleed hole on the poppit cover, the diaphragm is bad. If pressure runs too high, the poppit or it's passages are plugged. In addition there are several other small passages that let water into and out of the heads that could be plugged.

The other thing is that lean fuel mixture, or timing off can also cause an overheat. These things burn 15 GPH at wot, and the fuel filter is the size of my thumb. It's pretty easy to lean out. Does it back off a little on power also?

This is important because it can quickly do very serious damage to the guts of our engine, making the overheating problem a mute point.


hope it helps
John

 

[crazyedd]

Re: 1986 150 Black Max overheat on WOT

The water pressure is working how j_martin describes. The diaphragm was leaking but I replaced it. The temperature does drop at low speeds. What puzzles me is that on the gauge it increases to 210 within a minute at WOT and maintains there and down to 150 at low speed or idle. I am afraid to crack or warp a head. Has anyone ever used "salt away" to clear passages? Or should I remove the heads? Does anyone also know the proper running temperature for this engine? I was trying to find it in the manual but haven't found it yet.

Thanks,
Eddie

 

[j_martin]

Re: 1986 150 Black Max overheat on WOT

The overtemp switch on that engine is set at 240*. That might be a clue.

If it were mine, I would check the fuel pressure at WOT, just to be sure it isn't running low and lean.

On another note. Some folks overly concerned for their engines add too much oil to the fuel. What it does is lean out the mixture, causing it to run hotter or even causing detonation. Exactly the opposite effect than what was expected. Engines designed to run richer oil mix's also are jetted a little fatter.

What is the compression of your engine? That engine, with a little trueing work on the heads, will require 92 octane gas to run cool. The line is somewhere near 130 - 135 psi compression.

hope it helps
John