Re: 1978 Johnson 75 hp Stinger stalls when throttled up

[Don]

Well, these high performance engines are best suited for light high performance boats where they can wind up and breath. Unfortunately, many were used on regular rigs and underpropped, causing detonation/preignition. The engine was probably running a little hot due to a worn pump/leaky exhaust cover gaskets also. Let’s not forget this 1978 engine is almost 40 yrs old also.
In any event, I’m sure you won’t get away with just replacing the piston rings (and one piston). Yes, the engine will run, the compression might be OK also, but the piston cylinder fit will be poor, so the engine won’t idle for beans. These loop scavenged engines are very fussy about piston fit, and the cylinders get worn more due to lack of support in the cylinder (all those crazy ports). I’m sure all three holes are oversize and will need to be bored out. The other problem I’m sure you will discover is damaged/marginal bearings/journals due to running the engine with water in it. I’m sure it was run this way before you got it!
So, the question is how far do you want to go with this engine? Is it in nice cosmetic condition, fresh or salt water engine?
Maybe you can find a nice used powerhead for it.
If you are not looking to get in too deep, or spend too much, I would just put a new head gasket on it, and "send it on down the road", kinda like what the last owner did.
Sorry, sure want to be wrong about my assessment, but don’t think I am. D