A good rule of thumb is if the compression is under 85 the motor probably needs new piston rings, but those old OMC motors from the 70's will start and run at 78-80 psi. Great to see you back working on the small outboards again.
Pull both plugs when doing a compression test, especially a hand cranked outboard. Also should have the throttle wide open. I love these old motors, my favorite is my 58 4hp evinrude.
Great point! Thanks for the tips. If you noticed both spark plugs were removed, but I did not have wide-open throttle. You’re right these are fantastic little motors.
On those small OMC engines, I've seen many start and run fine with as little as 60 # compression. Having both cylinders within 15% of each other is important though. By the way, the top crank seal possible leak is from crankcase blow by and that is in no way related to cylinder compression. To check that seal, remove the flywheel, spray soapy water on the seal, put in the flywheel nut until it buttoms out and turn the motor over with a wrench (removing the spark plugs will facilitate this). Bubbles at the seal base confirms leakage. Regards !
@danielrobert7181… if the upper crankcase seal leaks wouldn’t it cause running issues, like poor idling or power loss at higher rpm, or ignition problems with oil contamination on points? This motor seems to run very well without these symptoms. I will let the owner know of this potential problem and suggest next steps if he still has symptoms. Thanks for your input and taking the time to watch and comment. Much appreciated.
Of course there would be running effects but with those low compression engines, unless the seal is very badly leaking, it might be unnoticeable to most... like adding one crank pull on start-up or introducing a slight miss. Regards!
