1981 Merc 70 hp outboard compression test numbers not right

[boaterinsd]

I had by boat out for the first time this weekend, sat all winter after it didn't run well last year. Anyway it was starting kinda hard, didn't wanna run all that well , kinda like it was dragging the dock. I have a new switch-box in it since it only have spark on the #1 cyl. I did a compression check on it and have 25psi on #1, 145psi on #2 and 150psi on #3. It did get hot last year from a rotted water pump impeller. But it did run good after switching it. This weekend there was a good telltale stream. What can or is the cause of the 25psi compression, the motor doesn't knock at all. Do i have to tear it down to find out and are they fixable? TIA

 

[emckelvy]

Re: 1981 Merc 70 hp outboard compression test numbers not right

These are marginal motors at best, and 25 psi is terminal. Likely it got hurt when it overheated and has just deteriorated since then.

Short of a full teardown, you can remove the exhaust manifold cover and inner baffle and you'll be able to see directly into the exhaust ports. Likely you're gonna find a badly scored piston/rings. Maybe even melted aluminum across the piston lands.

There's a slight possibility that the cylinder isn't damaged, and any melted-on aluminum can be removed with very careful application of muratic acid followed by a clean-up with a ball hone. Worst case you'd need to have the damaged cylinder bored-out, and install an oversized piston.

But, hey, it could be worse, many times what happens when you overheat these I-3's is that the block cracks and then it's a boat anchor! At least you have some compression which indicates the block isn't likely cracked. And providing you've no indication of water being pumped into the cylinder. Water or rust on the spark plug would tell you that right away.

Teardown requires only your basic hand tools, plus a flywheel puller. You can pick up a serviceable flywheel puller at the iBoats Store or places like eBay. Mercury dlrs sell these as well.

http://motors.shop.ebay.com/i.html?...ler&_osacat=0&bkBtn=&_trksid=p4506.m270.l1313

HTH & G'luck with the motor..........ed

 

[boaterinsd]

Re: 1981 Merc 70 hp outboard compression test numbers not right

do u have a possible cost to this , just a guess? I am stuck between fixing this or the volvo aq170 cylinder head , thats cracked of the Larson boat of mine.