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Author Topic:   1974 70hp Johnson head gasket
blueboat posted 04-25-2003 10:57 AM ET (US)   Profile for blueboat   Send Email to blueboat  
I picked up a 70hp 1974 Johnson. When I ran it, it started fine, idled fine but just seemed to lack power. Switched from a 14X17 to a 14x13 prop and it picked up a bit. Last night did some closer looking and ended up with the head gasket on my work bench. The middle cylinder had burnt the gasket so it was letting water foul the plug. Here's my question. What are the main causes of blown head gaskets, could it be from running to lean?
Thanks again.
Florida15 posted 04-25-2003 11:46 AM ET (US)     Profile for Florida15  Send Email to Florida15     
Scott, I don't know if there are other causes or not but I had a 1982 35hp Evinrude
on my 13' blow a head gasket when it ran hot. My dad gave me the boat and I ran it without checking the impellor/water pump and thermostat.
It quit pumping water and got hot before I noticed it. I had to get the head shaved and replaced the gasket and it runs fine now.
Sal DiMercurio posted 04-26-2003 01:17 AM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Blue, the engine is 29 years old, sometimes they just start to leak from corroding away, others, to much heat.
If it was heat, you may need to resurface the block or head by using a piece of glass as a base & some 1200 or finer grit emery or wet sand paper taped on top of it then nice & smoothly [ new word ] in big circles, sand it down.
Those gaskets in time do sometimes leak on their own.
Steve Leone posted 04-27-2003 01:06 AM ET (US)     Profile for Steve Leone  Send Email to Steve Leone     
Blueboat: Go down to your local auto machine shop. They will re-surface your head properly for a minimal cost. They use professional equipment and you can`t beat the results. When you are getting ready to put it together apply a generous coating of Aviation Form-a-Gasket to each surface. Torque the head to specs using the right torque sequence. The book will say put the heads on dry but at O.M.C. school all the head techs (pun intended) used "cheeze whiz". You will not have a problem with it bowing again unless you have an overheating issue. Steve

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