'69' 85 Hp gradual everheats - starts to miss

[pugetsoundboater]

Hi, my 1969 85 HP motor starts to miss after about 15 minutes at full throttle due an overheat condition. After letting engine cool - runs good for another 15 minutes (suspect amplifier is getting hot) Have changed thermostat & impeller - checked water impeller -is OK. Recently changed head & water jacket gaskets- did notice some dried deposits at edges of head gasket when running in saltwater. Can a leaking head gasket cause an overheat? What are the next steps in diagnosing this problem? Thanks in advance

 

[Mas]

Re: '69' 85 Hp gradual everheats - starts to miss

Are you running the correct spark plugs?

MAS

 

[pugetsoundboater]

Re: '69' 85 Hp gradual everheats - starts to miss

I have QL77 JC4 plugs installed - bougth them from an OMC servicer.

 

[jonesg]

Re: '69' 85 Hp gradual everheats - starts to miss

Got a good pee.stream, is it warm or cold ?

but yeh, a leaking head gasket can cause lean overheat condition.
A loose sparkplug can do that also.

 

[pugetsoundboater]

Re: '69' 85 Hp gradual everheats - starts to miss

Hi,

This motor predates Johnson having a "pee stream". The "hot" light does not go on at all in my situation - the motor just starts to "miss" progressively worse. I felt the cylinder banks and they were hotter than normal as could not even begin to put my hands on them for any length of time and even the top of flywheel was very hot to the touch.

After looking at the head gaskets again, I now see that really the water jacket gaskets have some deposits on/near the gasket edge. I have torqued the bolts on the water jacket to beyond specs to try and stop the suspected leakage.

Is there a cleaning solution that I can inject into the cooling system to remove possible corrosion that could be restricting water flow? When I put on the new head gaskets, there visually appeared to be very little corrosion around the cylinder walls. Unsure where the narrowest channels are that could be partially obstructed. History - last Spring I had the right bank of my V-4 motor overheating - the repair shop used compressed air to blow it out. Nothing obvious blew out but it ran normal after that.