2.0 liter overheating

[chevy355mark]

So I've got a 1990 mercury 135 black max we just picked up and hung on a 24ft pontoon. I know, not very exciting stuff here. The motor overheats at low speeds or after 10 minutes pr so of idling. If you get the boat up on plane and into the 3000rpm and up area it wont overheat. Go for full throttle and it will once again overheat, steady pee stream all the time, no steam, no froth or air pockets.

I've been searching this forum and reading up on the 2.0/2.5 cooling system and I'm at a loss here. I've done a new water pump with a new base plate and gaskets, thermostats, poppet valve. I have inspected the water tube and put a garden hose to it looking for leaks there. Overall this outboard seems to be corrosion free, it has been sitting a very long time I suspect. I've ran the motor in a flush bucket with 5 gallons of vinegar, then again with new water and a 5 gallon pale of purple power.

2 major things to note. The first time we ran this motor it idled alarmingly quiet in the water, like a honda 90 4 stroke would be louder. After the degreaser flush it now sounds more like a 2 stroke outboard should. I'm thinking maybe I have some blockage in the exhaust chest area? The second thing is how horrible the water quality an engine gets on a pontoon boat, the prop will cavitate at any trim yet water still runs over the cav plate, this tells me the motor gets some pretty frothy water. I have the engine as high as it can go, I'm trying to get the water line under the cav plate but maybe that's not possible and it needs to go deeper?


Heres the boat if anyone is looking for a good laugh

Idk why it wants to show up sideways, sorry.

[Baker343]

Sounds like you've changed all the normal and safe parts. Does it set off the over heat alarm if so have you felt the heads with your hands. That sensor I believe grounds at 195ish.
You may have to pull your heads, before you pull the power head and inspect water passages never know what kind of grud or surprises you might find.
Plus you'll get a good idea of the condition of your motor that's been in hibernation

[chevy355mark]

So I went ahead and checked the carbs, thinking lean=heat, they were clean. Pulled the exhaust divider, looks nice and clean, all the ports are good and the tuner is free and clear. Cooling jackets look super clean, very little corrosion considering it's a Florida motor.

Yes it does set the heat horn/indicator I rigged up. The temp sensor was bad, I actually used a waverunner temp probe I had laying around, tested it in hot water and it kicks at 190. Pretty close to a direct fit actually.

The motor is over propped, getting 4200 at wide open with a 15x15 4 blade. Hoping we can pitch it down enough to keep it, it's going to be tough finding a lower pitch prop for a full size gearcase. Could this be creating that much extra heat? I knocked the timing back to 18 degrees and feed it premium while we are playing with props considering the extra strain.