Evinrude 4hp Compression Testing

[willyboy]

I recently picked up a 1975 4 hp evinrude at a yard sale for $20,missing the carb-which I have from a scrap motor. It needs some tlc but before I get into buying parts I tried a compression test. Dry I got 30lbs each cylinder after a 4 pull average per hole,after that I gave them each a shot of pb blast and tried again and pulled 50lbs average each hole. Just getting into this hobby and this is only the 2nd engine I’ve really got hands on with,not sure if I tested it correctly. No carb attached,both plugs out,and lower unit has been taken off to check the impellar.Should I test it with the carb and lower unit on? Should I be looking for 70lbs. or better?

[fleetwin]

Yeah, that is pretty low for a 4hp, these engines have higher compression than the older 3hps. You might try a different compression gage, readings can be inaccurate on these small combustion chambers.
Once the reading is verified to be only 50PSI, I would pull the head. Hopefully just a blown head gasket.

[frankr]

Don’t count the pulls–pull it until the highest reading is obtained.

[willyboy]

Thanks fleetwin and Frank,in either case with a spare guage and multiple pulls I have 50lbs. pressure in each cylinder after I gave each a short shot of pb blaster,dry they read 30lbs.. Looking at this one as another learning experience as this one has a few issues. Decided to pull the head and check replace the gasket,of course 1 of the 6 head bolts is stuck fast. Tried some heat and no go so sprayed it up with some pb blaster and letting it soak. In the mean time I’ve pulled the flywheel and found a cracked coil and dirty points to go along with the low compression.

[fleetwin]

The stuck bolt might be stuck in the head, not the block, oftentimes the corrosion builds up around the shank of the bolt, not just the threads. Try gently tapping the head (with a plastic/rubber mallet) in a direction that will loosen the stuck bolt, the bolt might just move with the head. I realize you won’t be able to move the head much, but maybe enough to determine whether the bolt is stuck in the head, or the threads are stuck in the block.
Don’t mess with the ignition or much else until the head is off and you figure out what is causing the low compression. Like I say, hopefully just the head gasket.

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