25 HP 4 stroke Bigfoot Overheats at Full Throttle

[Mr. Florida]

Troubleshot all the usual suspects. Replaced impeller, (old one looked fine) bottom pump wear plate. It's peeing fine. Replaced t-stat. Did a reverse flush through thermostat hole and did get some black oily carbon deposits in the bucket, with lower unit pulled. This makes me think exhaust is leaking into water jacket.

Does not over heat unless I run it full wide open, over 4500 RPM.

Spark plugs are white, got me thinking it was running lean. Pulled carb and cleaned. It was pretty clean to begin with. I also replaced ECM unit, made no difference.

I am starting to think blown head gasket or adapter plate gasket. Maybe I should do a compression check?

 

[Mr. Florida]

Re: 25 HP 4 stroke Bigfoot Overheats at Full Throttle

I did a compression check this evening and had close to 180 PSI on both cylinders, which is what factory spec is.

As I said before, I replaced ECM thinking it was a timing issue. Also the only water temp sensor, as well. And as I said before it is peeing fine. Have not checked water pressure.

Some of the local folks seem to think it may be head gasket as one plug is white on one side.

Your thoughts?