Question About Overheating OMC 2 Stroke Outboard - Overheating?

Braintrust:

I have a line on a low price used late 80s Evinrude 70hp 2 Stroker, that seller says "has no compression, but the yard thinks its just head gasket."  

Is there any way to tell if this size outboard has over heated (deformed the head) or has terminal overheating related problems without running it? Seller has it off his boat, so we cant even crank it to see if it turns over. I will be able look down the spark plug port but thats about.

Thanks for any wisdom you can share! I feel like I only post on here about boats as of late.

Probably ran it without oil, melted piston

You can still check to be sure it turns over, bring a piece of 1/4” rope with you.   Pull the cover and you’ll see there are notches in the top of the flywheel for this purpose.  Tie a small knot in the rope to grab in one of the notches and wrap it a couple of times in a clockwise (been a while but pretty sure) direction, loop the other end around a wrench or screwdriver to make a handle and pull away.    Pulling the plugs makes this even easier.  

As far as overheating, that could be any number of issues.  Worn or disintegrated water impeller, stuck thermostat, water passages full of mud or salt, etc.   

Being a 2-stroke, the head is just a chunk of aluminum with some water passages.  It's only about 2" thick.  Any warpage is easily fixed by the 65 ft-lbs of torque you put on it.  I never even check the heads for straight, because they never were after about the first 20 heat cycles.

Usually, one of two things happens - the head gasket blows, they stop running it, you replace the gasket, and you're good for another 15 years.  OR - the head gasket blows and they keep running it which hopelessly pits the head/block, seizes the pistons, and makes the engine a waste.

Definitely turn it over.  If you forget a rope you can usually turn it a little by hand.  It's hard but it can be done.  If it turns over and you buy it, make sure you get it cheap, then you can do head gasket, water pump, thermostat, and make sure the water passages are clear.

Oil injection pump fails slicks down cylinder walls or burns hole in piston, no compression. Son in law just went through this on a 150. New power head was only fix. Kept getting harder and harder to start but didn't check for the reason until it was too late.

Bring your compression tester and crank the engine over by hand. You won't be able to tell whether it meets the upper or lower compression specification, but you can find a "dead hole"

What Curtis said. I will add the low oil or no oil and overheating will also smear the pistons and stick the rings in their grooves. They aren't particularly hard to rebuild as long as you can get them apart. I have spent days pulling down a salt outboard for a rebuild. 

I wouldn't pay more than scrap price for it unless I could see it running. 

Thanks everyone!  I've been struggling a bit with what to offer here, since I don't want to be greedy.  This engine is 3 years newer than the one on my boat, but in much better cosmetic condition.  There is 98% parts interchange between the two, so I figure that if its bricked I will at least have the carbs, tilt/trim unit, cowl, remote contrl and lower unit either as spares or to sell off, and can dig into this second engine without worrying too much. 

I guess I can see if it spins/holds any compression, and then it will take about 10 min after I get home to pull the head and see what the pistons look like.

Asking price is mid 3 digits, and that includes the remote control...

I don't think they did oil injection on the 70hp until 89 unless it's a VRO, then it got an economixer (bad) from about 82-84.

ManhattanM (fka NY535iManual) said:

Thanks everyone!  I've been struggling a bit with what to offer here, since I don't want to be greedy.  This engine is 3 years newer than the one on my boat, but in much better cosmetic condition.  There is 98% parts interchange between the two, so I figure that if its bricked I will at least have the carbs, tilt/trim unit, cowl, remote contrl and lower unit either as spares or to sell off, and can dig into this second engine without worrying too much. 

I guess I can see if it spins/holds any compression, and then it will take about 10 min after I get home to pull the head and see what the pistons look like.

Asking price is mid 3 digits, and that includes the remote control...

The remote is worth $150 easy if the cables all look good.  If the engine makes compression but just needs a head gasket, I would give $350 for it.  If it's missing compression on a cylinder or turns over hard, I would give him $50.  That's easy for me to say since I'm not in the market for a 70 horse OMC.

That being said, I wouldn't hesitate to rebuild it.  There is a reason they were mostly unchanged for a couple decades.  They were great powerheads.

Fresh water or salt water use?

OK, engine acquired.  It spins freely, only the middle cylinder is missing compression.  Overall mechanical condition seems good. Plugs do not seem oily or carboned up.   According to seller, the carbs were just rebuilt and the impeller just changed.  Will check when I tear into it.  Sitting in a BK parking lot about to drive home, looking forward to popping the head cover when I get home  to find out what I've really got!

Sounds like a winner.  Even if that cylinder has some other issue, it's a motor worthy of fixing.

For the price that you apparently paid, it's a winner even if you just use it for parts and make one out of two.

I had a 50 that lost all compression on one cylinder. It took a set of pistons, rings, and a gasket set to put it back to rights. I lightly honed the cylinders and stuck it back together. It ran another 10 years before I sold it. The bottom end runs roller bearings and they are almost impossible to destroy. 

Zero progress.  Kids sports mean it's still lying on the garage floor.  Hope to at least have it on a stand by Sunday afternoon.