TOPIC: 1961 Merc 400 Operating Temp

I got my 1961 400 running in a tank but felt it may not be cooling correctly. Hard to tell if the new water pump is pumping the volume it should when the water/exhaust exits under the cavitation plate. I was getting very little water out the the exhaust relief tube on the back of the drive shaft housing. I also noticed there is no exhaust release baffle in the drive shaft housing. I am hoping my year/model may not have come with an exhaust relief baffle and if it came with one, do I really need it to make the motor run/cool properly?
I wanted to monitor the temp of the block while the motor runs to make sure it's cooling correctly. What are the temp readings I would get at the cylinder head using a touchless temp gun?

what are you temps. run a tell tail from the top of the block and use a hose to exit the cowl. hand warm on the exhaust cover.

There is no thermostat in the 400-4 cyl, so the temperatures shouldn't get over 100ish, and it will probably run a lot cooler - say 70* +/-.
There is a tell-tail "pee stream' that should be barely warm to the touch. It exits the engine pan on the port side as shown in the attached picture.
When running, you can sometimes clear blockages of corrosion flakes, etc by blowing compressed air up the tell-tail.
doc

Feel the exhaust manifold cover while it's running in the bucket. If it's cold or at most lukewarm, you've got enough water going through the powerhead. If it's hot, there's a circulation problem.

Good advice. I learned was not pumping water out of the water pump housing. I solved the problem by removing the impeller, and re-installing it after bending the the blades in the opposite direction. Has anyone had this experience or, is it more likely I cleared an obstruction and only think the bend direction of the impeller blades makes a difference?

It's never a good practice to reverse the rotation of a used impeller. You're just begging for the rubber blades to break and the shards to get sent into the cooling system where they can restrict water flow.

I'd highly recommend you install a new impeller before taking your boat out for a test run, you could burn up the powerhead if the impeller goes. You can go to www.marineengine.com and look up the part number for your engine's impeller.......ed