1988 150 overheat alarm when over 4,000 RPM

[UncleGrady]

Hello,

I have a 1988 150 2-stroke that sounds the overheat alarm when cruising but only if the engine is rev'ed over 4,000 for a minute or so. You can run all day below 4k and no alarm. With the T-stat covers off, I do get water to them, strong tell-tail at all times.

Here is what I have done:

Run the engine w/o the thermostats, still alarms.
Replaced the thermo sensors (both), still alarms.

Next I will:

Replace the water pressure valve (poppet)
Flush the engine with saltx

If these don't get it, I am down to two ideas:
Running lean at high RPM (clogged jet(s))?
Massive scale build up?

Anyone have any other ideas?

Thanks,

UG