66 Johnson 3hp

[Burkeke]

I have a 66 Johnson 3hp that runs great at all speeds. My problem is it overheats after about 10 min. For the first 10 min water is flowing good out of lower unit holes. But them it just seems to stop. I suspect the exhause gas is leaking into water passage and building up pressure and when its hot enough stoping the flow of water. Pump impeller is about 2years old and the water pump casing has some scouring. I would think if it was not getting enough water it would heat up right away. I have had to exhaust cover gasket off and the head. I cleaned up all the passage ways I could see and put it all back together with new gaskets.

The reason I suspect the exhaust cover is the cover had some corosion on the plate but I thought a new gasket would take care of it.

Its a great little motor and I would had to cook it with heat.

Any suggestion would be appreciated.

Thanks

 

[bktheking]

Re: 66 Johnson 3hp

Have you tested the thermostat? It is under the stat cover at the top of the head, pull it out and inspect/test it in boiling water.

 

[Burkeke]

Re: 66 Johnson 3hp

This particular model has no thermostat. I did check compression and it was 78 and 75 lbs in each cylinder.

 

[Daviet]

Re: 66 Johnson 3hp

I would install a new water pump kit in it.

 

[bktheking]

Re: 66 Johnson 3hp

What is the model number on the motor? According to parts lookup it does.

 

[Burkeke]

Re: 66 Johnson 3hp

Model is JH 21E it was made in Belgium.

 

[F_R]

Re: 66 Johnson 3hp

No thermostat on a '66 3hp. First things first, always check the water pump before moving on to something more exotic.

Exotic possibilities:

1. Water tube bent and rubbing on driveshaft, wore a hole in the water tube.
2. Water tube grommets, both lower, AND UPPER.
3. Something floating around in the cooling system, causing intermittant blockage.

Both #1 and 2 will cause water to spill out before it ever gets to the powerhead, then it blows out the exhaust relief holes, fooling you into thinking it is cooling.

If the water pump doesn't cure it, you are going to have to do an exploritory. Pull the powerhead.

 

[bktheking]

Re: 66 Johnson 3hp

My bad, thought it was a 6, I read 6....hmmm must be going crazy.

 

[Burkeke]

Re: 66 Johnson 3hp

Thanks for the tip on the water tube. I did try connecting the copper tube at the pump to a plastic tube and then to a garden hose but it was hard to tell if I was getting enough pressure to move the water through. Anyone know of any tricks to test the flow from the water pump through the block?

 

[Burkeke]

Re: 66 Johnson 3hp

I installed a new water pump kit and also found that there was something obstructing the water tube. Problems seems to be solved. Ran for 20 min in a barrel with no problems.

I noticed that the drawings call for a screen between the pump and the water intake. There was none in this motor.
Anyone know if this is required and would it have prevented the problem?

Thanks