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Author Topic:   1974 70hp Johnson head gasket

blueboat posted 04-25-2003 10:57 AM ET (US)
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.
Scott

Florida15 posted 04-25-2003 11:46 AM ET (US)
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)
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.
Sal
Steve Leone posted 04-27-2003 01:06 AM ET (US)
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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Author	Topic: 1974 70hp Johnson head gasket
blueboat	posted 04-25-2003 10:57 AM ET (US) 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. Scott
Florida15	posted 04-25-2003 11:46 AM ET (US) 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) 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. Sal
Steve Leone	posted 04-27-2003 01:06 AM ET (US) 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