TOPIC: Looking for a nice 70's 4 cyl. 85 HP Mercury

WTB: 1974-1979 4cyl longshaft 85 HP Mercury. Freshwater, never apart. Weak paint and decals OK, but extra clean under the hood. Gary 954 443-9031 or This email address is being protected from spambots. You need JavaScript enabled to view it.

Any one of these you look at, make damn sure you do a leakdown check on that engine, a compression test will tell you nothing. These engines overcool at low RPM (ie trolling) and the bores suffer excessive wear. The engine will run well at top end but it will not idle worth a damn, this is a very common issue with these engines unfortunately, and a rebore plus oversize pistons will be required if you get a bad one.

You know, I here that here on F/G but have never seen one with that issue, but then again I live in centeral Indiana.....who trolls for hrs on end? I guess if you live near BIG water it might be an issue to look into. I have ownd several 850s with great success, but I dont Idle much!

Lucky! I have rebuilt tons of these and every one that has come into my shop with low speed running issues has needed a rebore. But you raise a good point , don't drive them slow and they will last... if you can afford the gas!! Good power for the size however, and they are easy on fuel for the horsepower.

where do you live

It sure is amazing how much difference there is internally between the early '70s 850 4 cyls and the later '70s ones.
I have had several that had 150+ compressions on all 4 cyls, but were total junk inside due to carbon build up under the rings that caused the pistons to rub their sleeves on one side. The leak-down tests were between 3-4 percent too, so that doesn't necessarily tell the truth as to internal condition either.
Ripping them open is the only true way to tell.
I find that they do real well rebuilt with fresh reed blocks and bleed restrictors.
Like George said, an aweful lot of them need reboring to be worthy of rebuild.
I'm hoping that with use of the newer synthetic lubes after thorough break-in has been completed, and better educated operators, my rebuilds will give folks years of service.
They sure are a lot smoother idling than the evil 3 cyl loopers that have replaced them.
Thom

True enough, the only true way to see if they are toast or not is to tear them down and measure, but most sellers I have met would be a little weak about agreeing to that I think. However as you say with modern lubes and right operating procedures, who knows??? Might be a renaissance for these old warriors!