1990 70 hp Johnson - Tired powerhead, worth a rebuild?

[ShoestringMariner]

So I’ve done it again. I’ve gone and dragged home another motor that is less than ideal. It runs but just OK.
lumpy idle, lazy top end. I’m assuming it’s due to the fact it has low compression.

I would’ve walked away from it but these motors are so hard to find in my area. It came with a boat and trailer that I’m going to clean up and turn back out for sale with my Merc 650 bluestripe.

The lower unit was apparently rebuilt through insurance with all OMC parts, though that’s just what I’m told.

So I guess my question is twofold;

1) can I go to a thinner head gasket to boost the compression a little bit.? (Currently 95-100) I would like to use this motor for the next year if I don’t have time to rebuild it this winter. Any other compression boosting tips? ( I can’t think of any aside from a full rebuild)

2) are these power heads worth rebuilding? I figure I can do it with the rebuild kit and machining for under a grand if I do it myself. everything else on this motor seems to be great

 

[racerone]

No a thinner head gasket is NOT available !----You take the cylinder head off now to inspect what you have.-------Yes they are worthwhile to rebuild in my opinion.-----You take it all apart / inspect / then decide on spending the coins.----Do not forget to inspect / reseal lower unit as repairs there can shock your wallet too.

 

[Faztbullet]

These motors are bad about scuff exhaust side of piston due to bridgeport design. They also have a decompression port drilled into sleeve so compression at cranking will only be around 100-105psi as it builds compression as RPM builds. Pull head and inspect cylinders.

 

[ShoestringMariner]

These motors are bad about scuff exhaust side of piston due to bridgeport design. They also have a decompression port drilled into sleeve so compression at cranking will only be around 100-105psi as it builds compression as RPM builds. Pull head and inspect cylinders.

Oh that’s interesting. So they aren’t like the 1970s OMC’s? I’ve got a 1977 75hp Evinrude on the stand right now with 135 psi readings across-the-board.