All four cylinders are reading 50LBS. This is tested at full throttle. Boat runs like ****. Spark plugs are good. Possibly blown head gaskets or pistion failure? Thanks
Not too likely head gaskets both sides are blown. Maybe just worn out parts (rings/bores, etc.). Did you test with all plugs out? If not, you may have had too slow cranking speed.
Its a 2000 90HP Johnson 4 cyl 2 stroke. Yes I tried it with all spark plugs out. I first tried it without the throttle in WOT and it still read near 50lbs on each one. No more than a 2 or 3 lb difference. It was recently overheating and my alarm wasnt working so Im thinking maybe I cooked the piston before I found out? Thanks everyone
Ok I tried a new compression gauge. Guess what. The one I was using is no good. Starboard top cyl I got 128lbs bottom star. 120lb top port 100lb and bottom port 120lbs. I tried each cylinder 4 times exactly and got the same number each time. I'm going to try taking the head a part on the port side and hope it is a gasket maybe? I will be sure to look for any damage. Would putting a table spoon of oil in the top port side cyl thats low maybe work to show if its just a bad ring possibly? or not worth trying?
If you put a little oil in the cylinders and the compression comes up, it is usually a sign of a ring problem.
You might want to do a real good decarb job and see if it helps the compression come up.
Well this is interesting. Come to find out there isnt head gaskets on my motor the dealer is saying. It uses a o ring system around the end of the piston walls and then you pop the head back on. I will post pictures, so I'm hoping just those o-rings are bad possibly. I guess that explains why there was no signs of much of a head gasket there
yeah i just looked and youre right ... theyre only 5 bux each on marineengine.com though ... thats a new one on me but its not surprising that they went to that .... the thing i dont get is that they are made of rubber - id have thought theyd do em in copper like on a diesel engine ... doesnt seem like rubber would hold up to the heat and pressure
This is why it's always good to have the exact model # off the mtr when starting a thread. Those orings very rarely go bad. I think I've seen one mtr in the last ten yrs where an oring caused a prob.
Yah I'm going to see if thats the problem tomorrow but I'm doubting it as well. The piston is clean looking so it looks like water is definitely getting in there and washing it out but the cyl under it is scorched but still has great compression. This side of the motor was the side that had a bad thermostat. That's why I was hoping it would of been the o rings but I talked to a few people and they said maybe it's something to do with a cracked exhaust port or something like that. Where it pees out the back of the motor, the two holes on each side of that dumps out water every now and then when running. Is that normal by any chance? the model is J90PLSIF and the serial number is GO491854
