1970 Evinrude 4hp overheating after water pump change

[AJ168]

I decided to change the water pump on this engine after I felt that it was running hot (could only hold my hand on cylinder head for 5 sec.) and that not enough water was coming from the exhaust. The old impeller was in bad shape and the new one went in without a problem. Upon testing the motor in a bucket of water, much more water was coming from the exhaust and it was coming out hot. But when I went to touch the cylinder head, I burned my hand. I'm stumped as to what may be happening. I would say a thermostat, but this motor doesn't seem to have one.

 

[Paul Moir]

Re: 1970 Evinrude 4hp overheating after water pump change

The cylinder head gets freakin' hot on those 4hp engines, as they have little by the way of water jacketing in them. Try this:<br /><br />With the engine running and spurting water out the back nicely, put your hand on the top cylinder bulge just aft of the fuel pump. Does that feel hot or luke-warm?<br />Put a drop of oil on the top of the cylinder head. If it sizzles it's too hot. If it simply evaporates, it's likely just fine.<br /><br />It's possible for the water jacket around the cylinders to plug (causing overheating) but to still get excellent water flow through the block. If the block outside of the head feels relatively cool, that's not likely a problem.<br /><br />Avoid the flat plate on the port side of the engine - that gets very hot as well.<br /><br />PS - I noticed your previous posts: although it's officially sanctioned by OMC, I would never run one of these engines at 50:1. I run mine at 24:1 and sometimes at 32:1. Check out the oil mix FAQ in the FAQ forum for details.