Mercury 500 50 hp running hot???

I have a 1979 Mercury 500 that I have done a lot of work on myself. I replaced the impeller when I first acquired it. 3 of the fins were missing and it was running hot with the new impeller.I accessed the rear water jacket and removed the debris from the old impeller. Ran much cooler for 2 seasons. I recently had to replace the main switch block. All of a sudden it Over heating. Output water is very hot and I loose rpms as it warms. I then tore down the exhaust manifold to find more rubber from the original impeller. I now have a new impeller and clear passages throughout the engine and it still runs hot. The I output water is a very strong stream. Engine starts and runs flawlessly. Doesn't seem to be running lean. I don't think I and my running the correct spark plug. Could that and the new switch block have anything to do with overheating????
I am at a loss. Any advice would be greatly appreciated.

You might want to remove the powerhead.---------Then inspect the area where the water tube connects to the engine.

Even if I am getting a strong water water flow?

Good luck with this fine motor.

The 500 has no thermostat, so there's no reason for hot water at the telltale except a restriction. I agree with Racerone that there could be something blocking the water discharge from the powerhead.

I would also pull the rear cover on the block (over the spark plugs) and check for obstructions. I've worked on motors where trash/debris built up in there and blocked the cooling passages.

If the telltale is very strong (besides being hot), that's a sure indication that water is not getting out the normal discharge points. Likely if you pulled the hose off the telltale fitting, you'd have a strong flow of water which would eventually cool down as you idle the engine. But that's still not enough volume of discharge to cool the block while underway at power.

One thing you might try, is to see if the accessible bolts on the back cover will move with normal wrench force. If they come out easily, you might be able to get the cover off without pulling the powerhead. You'll have to remove the rear cowl support, then the cowling will drop down for better access to the cover.

Once the cover is removed, inspect for debris, blow out with air or flush with water. Before reinstalling with a new gasket, pull the lower unit, hook up a hose to the copper water supply tube, and flush out the block.

That might do it, if you don't have any debris trapped at the base of the powerhead. Reassemble and see if there's any improvement. If not, the base of the powerhead is the only other place to look for blockages.

One word of caution, the 1/4" screws on the back cover are easily broken if seized; use heat to release them. If you have a lot of reluctant bolts, especially at the bottom where it's really hard to get at with the powerhead installed, you'll need to pull the powerhead for better access.

HTH & let us know what you find...........ed

Thank you Ed,

I have already made sure that the rear water jacket and the exhaust manifold were clear of debris. I am a little intimidated by removing the powerhead. This just seems so random since it started overheating all of a sudden after I replaced the switch block. It has been running flawlessly all season.
Thanks again for your help with this. Please let me know if you think of anything else.
Adam

Have u tried saltaway, Has compression check been done?? Compression gases in cooling system causing cooling issues!!