Cooling system blockage

[Mojo700]

Hey guys, I have to say thanks for all the help provided here. The volume of questions this time of year is amazing, and I appreciate the help I've been given, as I'm sure others do.

I have a 1977 Evinrude 35 that appears to have been pretty well maintained. I disassembled/cleaned the carb and it was pretty clean. As you guys recommended, I pulled the impeller before ordering one, only to find it was brand new! Still had a coat of grease around the cup. I drained the lower unit and the lube was brand new there too, so the previous owner (who passed away earlier this year) seems to have maintained it well.

The issue I have is the cooling system appears to be plugged between the tell tale elbow and where the water tube connects from the lower unit. I know there is a thermostat, but I didn't think it would block it off 100%. I put the air gun from my compressor on both ends with 150 psi and couldn't get air out either way. Could the thermostat possibly be that restrictive? I thought they usually have a bypass hole so some water can always pass?
If it's not the thermostat, what would cause that kind of restriction? Is there a check valve that could be stuck solid? I did attempt to blow air both ways, but if there is a check valve it could be corroded in, motor was sitting for around 7 years according to the son of the previous owner. Also, I did check compression in both cylinders to make sure this motor was worth fixing, and I got just over 100 psi on both, so I know if the system was plugged last time it ran, at least no serious damage appears to have been done.

I was planning to pull the water plate off and remove the thermostat and attempt to blow air again to see if that's where the blockage is. But I wanted to check with you guys for ideas in case I'm missing something. I definitely don't want to remove the powerhead if I don't have to, but obviously the cooling system needs to be open before I can run it.

Thanks again for the help everyone.

 

[jimmbo]

Have you tried running it in a barrel, to see if it is cooling properly?

 

[Mojo700]

No, motor is not running. It needed new wiring harness and ignition, and I wanted to clean the carb, replace the fuel pump and check the impeller before ever starting it. While it was all apart working on those parts, is when I discovered the blockage, .

UPDATE: I blew air through several more times and I started to be able to hear air coming from the upper part of the water tube, so it's coming through, but with a lot of resistance.
I took off the water plate, and removed the thermostat and it was completely gummed up. So I checked with air pressure again, and I have a tiny bit of air flow through the cooling passages. But I am getting more air out from the water tube, and barely a hint of air at the water plate, it's barely enough to move the corner of a piece of tissue held over the thermostat opening with the thermostat removed. I am pushing pressure backwards through the cooling system, could there be a check valve somewhere preventing air from going to the top end?

 

[jimmbo]

You will need to pull the Powerhead off the exhaust Housing and inspect the passages. Also you might want to pull the Cylinder head off, and the Water Jacket covers off the Head and the Exhaust Cover on the side of the block

 

[Mojo700]

I got it figured out. Ran a piece of 18 ga stranded wire from the pee hole to the elbow, and when I pulled it out there was black gunk, pulled the outlet hose off the elbow (not at all easy since I definitely didn't want to break the elbow) and blew out a ball of crud. Now I get good air flow when I put the air gun up to the water tube and blow. I tried pushing air in from the tell tale outlet and it is actually quite restrictive even though the tube is completely clear. So now I know that isn't the best method for checking system flow.

 

[jakedaawg]

Instead of air I flush water in the tstat housing and out the water tube with the LU removed. This backflushes any old impeller bits out. I do this on outboards and inboards when ever I find a missing chunk off an impeller. On inboards it's not the thermostat housing, just the supply hose so that you back flush the heat exchangers. You kind of have to follow the path and figure where the pieces will plug regardless of the motor you are working on.

 

[oldboat1]

+1 on water flushing up and down. Run the motor in a plastic trash can full of water to continue flushing. Monitor temp. at the top of the cylinder block -- IR gun is a good tool to have on hand.