TOPIC: 350 40hhp 1961 Mercury overheating

2 years ago I tuned this up and put new impeller in it. Ran it in the barrel a couple of times. It would run hot at slow to fast idle. Crank it up to 2,000, it would start cooling down. Never ran it on a boat. Got it mounted on my 14' Klamath with added forward control. Same problem. Pulled the lower unit off, Impeller is still perfect. Blew all the lines out. Made sure the upper water tube went into the grommet. Still runs hot at slow speeds. Gets up to 180 - 200 and I shut it down. Running out of summer and luck too.

Any ideas? Thanks Hugh

Did you replace either of the port side exhaust gaskets - or the powerhead base gasket at some point??

I don't recall which, but there are similar, but wrong gaskets that restrict water flow resulting in ober-heat.
Seems like I recall it also produces higher pressure at the tell-tail??
Been a while.
doc

No. I just did the mag, carbs and fuel pump 2 years ago. Ran it in the barrel and noticed it ran too warm at idle. I don't have the time to pull the powerhead right now as that lead to a lot of other things I should take care of. That would be winter work. I didn't think of the exhaust gaskets on the port side. When I do pull the power head, I will replace all those gaskets too.
Thanks, Hugh

When you pull the powerhead, be sure to remove the cyl block water jacket cover (over the spark plug holes) and check the interior water passages. I've seen 'em build up silt that covered the bottom 2 cylinders and then they'd overhead, even though water was pumping thru the block.

Hugh,
I like Master Ed's suggestion, and encourage that if needed. Kinda like back-flushing a radiator, but probably needs removal of the "top cover" (water-jacket cover around the spark-plugs) to resolve the problem - if it ends up being there.
If you haven't messed with the port side gaskets, it "probably" isn't the problem.
With that in mind, I would double check the rubber sleeves in the gear case.
Specifically the one at the base of the water tube.
It can easily get pushed out of place as the water tube is slid into the lower unit as the lower unit is pushed up in place subsequently
blocking the water flow to the engine.
Sounds like you have considered this, so after double-checking this possibility, I think you will have to consider exhaust gasses are getting into the water-jackets on the port side - calling for new gaskets.
Keep us posted Hugh,
doc

This will be a winter project much as I need to get it in the water, I do have another boat.
This is some good food for thought. I would normally not open up the exhaust or the block water jacket where the plugs go in. I will open it all up and go over all of it. I plan on running this motor for a while and need to replace seals and all while it is opened up. My 500 on another boat is a keeper and I have a strong 400 also if I need it one day.

When I do get into it, I will let you know what I find.
Hugh