1984 johnson 75 hp loss of compression

[JoshKeller84]

J75ECRD

got this motor with a boat i bought to use with my current motor.. its a 75/50 1984 3 cylinder johnson jet. top two cylinders are 135 psi, bottom is 30 psi. pulled the head and discovered this. The bore looks surprisingly good. I havent torn it down yet (will happen this winter after fishing), but am I looking at a ring broke and bounced around the cylinder, or possibly a reed? Something way worse?

Cylinder bore looks surprisingly good from what I can see moving the flywheel. Just a spot of rust on the bottom. Also is there a way to look at the piston and see if its oversized without pulling it? Would it say it on the crown? All I see is "75 UP"

I have rebuild a 1984 115 johnson v4 from the ground up, will this motor be of similar difficulty? thanks!

 

[BigB9000]

Im no expert or anything, but try cleaning off the top of the piston. Mine say .030 on them.

 

[emdsapmgr]

Looks like you've lost the top ring on the piston. Pcs of it have beaten the crown of the piston repeatedly. The head for that piston probably looks just the same. Before you get involved in a rebuild, you might want to determine just why the cyl ringset let go. Some type of overheat/lean condition. You may want to pull the carb apart that feeds that cyl and inspect it. Esp the high speed jet. It could be clogged/restricted.

 

[JoshKeller84]

will definitely do that. are the heads cleanable, or should I look for a replacement? It looks to be pretty carboned up on that cylinder.

 

[bonz_d]

May be wrong but as I believe the 1984 is a Looper engine and the pistons should all have flat domes on them. The 2 you picture with the raised domes appear to be from a crossflow engine. I think you should look into it closer!

 

[racerone]

All the 3 cylinder OMC motors were " looper " style.----Measure the cylinders to see if it has been bored before.

 

[JoshKeller84]

are these motors pretty reliable? Is it worth rebuilding? Are they pretty thirsty?