1976 Merc 500 thunderbolt oil leak?

New guy here...live in Missouri, active duty Air Force, soon to be Army. I am restoring a 1961 Glasspar, it currently has a 1976 500 thunderbolt with tilt/trim on it. I have rewired the engine, compression is good, never had it in the water and actually ran it...It does run good in the drive-way though. Pumps water, idles fine, transmission is good. Very surprised with it, since it sat in a field from 1991 to 2011 unprotected.

Been lurking a while, decided to join up. I dont think that i have much to offer(yet) due to my experience level, but looks like I can learn ALOT! New to boats all together, but very mechanically inclined. I ran into a problem with my restoration project. On the port side of the powerhead, close to the bottom, beneath the CDI box, there is a valve of some sort. Can anyone tell me what this is, and if it should lead to something? It is misting oil out of it while running.

My diagnosis is it is a oil overflow/blow off valve. We are running 25:1 mix due to the motor sitting so long and wanting to ensure it is lubed properly. running this ratio, is causing too much oil to the point that it is coming out of the valve because the engine cant burn it off. Am I even in the right ball park???

Re: 1976 Merc 500 thunderbolt oil leak?

I think a couple photos will help with this troubleshooting. You don't need to run 25:1 go to 50:1 to keep the engine clean and running properly.

Re: 1976 Merc 500 thunderbolt oil leak?

I think you are talking about the little brass barb on the bottom of the powerhead under the switch box?

If so then there should be a thin pipe on it and the pipe travel up behind the switch box and on to a similar brass barb just under the flywheel.

The barb at the bottom has a one way valve on it, any unused fuel/oil mix goes out this barb and up pipe to lube the top end of the crankcase.