1978 70hp merc rebuild?

[aerotow1]

So I have a 1978 70hp merc outboard. It is in need of a rebuild and i am courious if i can upgrade this motor to make any more power. Is this a common powerhead for the 80,85,90? If I am rebuilding it I thought I would get somemore out of it if possible.
I aquired the engine in a trade and was told that it got hot and the number 3 cylinder was shot. I have not pulled this engine apart to do any diagnostics. I do have the factory service manual and 35 yrs of engine building experience however I have not done any outboards so this is new to me. The engine looks to be in very good condition very clean and no corrosion. Oil in lower unit clean and no water. Thanks for any help!

 

[Fuzzytbay]

Re: 1978 70hp merc rebuild?

HP in general is increased by inputting more fuel/air into the compression stroke, causing a "bigger bang" on the power stroke. Unfortunatly 2 stroke engines must do this in a different manner than 4 strokes. So as a general rule, increasing HP is not as simple as putting on a carb/fuel pump from an 80 hp to a 70 hp. It usually involves changes to the ehaust port, reed blocks, stroke, etc, and even then, upgrading the performence is still not guarentied.
IMHO the best, cheapest way to increase HP is to simply buy a large HP motor.

 

[Faztbullet]

Re: 1978 70hp merc rebuild?

The 80/85 are 4 cylinders and the 90 is a 6 cylinder. Just bore it and use it or find a small 500lb boat(Allison/Baby Checkmate) and make a toy with it!!