Topic: 1978 Johnson 75 hp Stinger stalls when throttled up – Page 3

This topic has 32 replies, 6 voices, and was last updated 9 years, 8 months ago by fleetwin.

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August 24, 2015 at 8:20 pm #22597
mgsolakis
Participant
Ok I’ve finished the oiling process and pulled the cyl head. It looks like a blown gasket. I’ll take a picture and post it a bit later to confirm. But the bottom cyl part of the gasket on the inner surface is badly damaged. I have to move and disconnect a lot of wires to completely pull the cyl head off and inspect the cyl. As well as take a picture of the head gasket.
August 24, 2015 at 9:06 pm #22603
mgsolakis
Participant
I took the head right off. No doubt a blown head gasket. However that might not be the end of my problems. The cyl has a small amount of rust in it already. No doubt from the water leak. But the even more worrying thing is when I pushed on each piston I noticed a lot of play in the one on the bottom cyl. It moves in and out like the others but if I push on either side of the piston it can shift in the cyl quite a bit.
August 24, 2015 at 10:26 pm #22609
fleetwin
Participant
US Member
"But the bottom cyl part of the gasket on the inner surface is badly damaged."…..Do you mean that the cylinder deck sealing surface under the blown head gasket is damaged? I fear there is more damage as well, hopefully I’m wrong. Take some emery to the lower cylinder to remove rust/glaze to get a better look. Take a wire brush/wheel to the piston dome and inside the cylinder head as well, look for little dimples that might be from broken ring chunks banging around in the combustion chamber.
August 24, 2015 at 11:12 pm #22613
mgsolakis
Participant
Fleetwin, I don’t see any visable damage to the piston. The rust does not look bad on the cyl wall. The gasket is blown completely though. I don’t know how to post a picture. But a part of the metal ring is missing. The cyl head is undamaged. However as I said there is a lot of play in that piston.
August 24, 2015 at 11:15 pm #22614
fleetwin
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US Member
Clean up the piston, use the emery on the cylinder wall and have a closer look. You can email me the pictures if you want and I will post them. Don
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August 25, 2015 at 1:18 am #22624
fleetwin
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US Member
OK, well the head gasket is definitely blown:

The head looks OK, don’t see any obvious dimples:

This next picture is most telling. I don’t see any obvious dimples from rings banging around, but have a look at the exhaust side of the piston, looks worn away. There is a large gap between the cylinder wall and piston on the opposite side which would explain the excessive side slop. I’m thinking the rings just slipped out of the exhaust ports without leaving the obvious dimples in the head/piston dome:

The next pictures don’t show much, block deck surface looks OK though:

So, I really fear this piston is toast, even though I don’t see the usual dimples from rings mashing around in the combustion chamber. I suppose it is possible that the broken ring pieces just snuck out the exhaust ports. You have little to lose (except the cost of a new head gasket) if you want to try replacing the head gasket to see what happens. Again, I sure hope I’m wrong. Don
August 25, 2015 at 2:35 am #22630
mgsolakis
Participant
What would cause the rings to break off like that? I do think your right Don, the piston moves too much for everything to be ok. The question is- is the motor shot? Would it have ruined the cylinder or could the piston be re-ringed?
August 25, 2015 at 9:10 am #22636
fleetwin
Participant
US Member
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
August 25, 2015 at 2:42 pm #22639
mgsolakis
Participant
No I think your right Don. The motor is likely done for. The only question: is it even worth putting a new head gasket on it. With that much play the rings might be gone and if they are the bottom cyl would have bad compression regardless of the head gasket.
August 26, 2015 at 11:13 am #22689
fleetwin
Participant
US Member
You are right, unfortunately I don’t think you will see any improvement with a new head gasket. Oil up those cylinders and just reinstall the old head gasket and snug up the head bolts to keep moisture out of the cylinders. You could sell it "as is", or put it aside, I’m sure an affordable decent used powerhead will turn up over the winter time.
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