35 HP Johnson 1977 with blown head gasket

[flintcreek]

1977 Johnson with blown head gasket, what else do I need to check. I looked at the cylinder walls and piston skirts they appear to be good. I guess I need to change the thermostat. What causes these to blow the head gasket. Put on a new head gasket and compression is 130 on both cylinders. I would like to get to the root cause and determine what caused the head gasket to blow?
Any ideals.

Flintcreek

 

[tashasdaddy]

Re: 35 HP Johnson 1977 with blown head gasket

running hot, new impeller and tstat. flush system while you have them out.

 

[Cofe]

Re: 35 HP Johnson 1977 with blown head gasket

Make sure the head is flat by checking it with a piece of glass. If it is dished you can stone it flat, or bring it to a machine shop. TD is right on about overheating.

 

[flintcreek]

Re: 35 HP Johnson 1977 with blown head gasket

Thanks, guys I did check the head and it was good and flat. I had planned on changing the impeller and Thermostat. How would you flush it out? Stick hose to the thermostat hole?

Thanks for the help.
Flintcreek

 

[Cofe]

Re: 35 HP Johnson 1977 with blown head gasket

When you drop the lower unit to work on the water pump. Attach a garden hose to the copper water pump tube and flush it out.

Here is a pic of a water flush tube I made to run the motor with the lower unit off. It is just an old toilet water supply pipe with a brass hose adapter on one end, and an extra water pump grommet.

 

[flintcreek]

Re: 35 HP Johnson 1977 with blown head gasket

Ok, I see how you built up the flush tube, I should be able to do something similar to flush mine out....Thanks, Flintcreek