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80 model 70hp compression

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astroglass

New member
"okay, so i bought an older ba

"okay, so i bought an older bass boat this weekend that came with an 80 model 70hp mercury motor. the gentleman said he was using the boat last year and lost compression on the middle cylinder. he has already taken the carbs and some other things off to get to the powerhead, my question is(now that i cannot check compression) what would i need to look for or what would be my next step in trying to figure out the problem? thanks for all you guys help."
 
"If it's partially disasse

"If it's partially disassembled can you see the cylinders/pistons.

What does the middle cylinder look like? scored up, piston burnt? rings busted?

If you can not see them yet, remove the exhaust cover and take a look.

If the cylinder isn't too bad, you can go oversize a bit (.015") with an new piston and rings from Merc for about 100 bucks, plus the cost of having the cylinder cleaned up/bored.

If you can do some/most of the work yourself and it's a single piston I would personally be inclined to make it a winter project. Could be done for maybe 400 bucks or so in total.

But before I put one penny into it, I would make sure that the ignition is sound. The distributor is no longer available and if you can find one it will set you back many hundreds of dollars.

You can pick up a mid-80's (or newer) 90 horse (if your boat can handle it) for a couple of grand in the used market which is probably one of the best mid-range horsepower motors of all time (and still in production today so parts are not an issue).

Your particular motor was also rated in Brake Horsepower (BHP) so although it says 70, in todays terms, measured in Shaft Horsepower (SHP) it would sell today as a 60 horse, producing approx 62-63 horse in reality...

It's age and design plus lack of availability of some major parts leave this particular model right on the balance point of whether to work on it or scrap it - parting out what you can to sell to other guys trying to keep this model cobbled together."
 
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