mercury 115 low compression one cylinder - HELP

alvin · #1 February 16th, 2002, 06:50 PM

Just bought an '86 mercury 115hp. Wasn't running when I bought it ("just needs a new control box - everything else is perfect")

Post-purchase examination of the engine revealed a cylinder with a botched helicoil job and a nonfunctional spark plug. I retapped the plug hole for an 18mm spark plug and fired it up today. It starts almost instantly when warm but runs a little rough. The cylinder in question seemed weak - didn't seem to matter very much if the plug wire was connected or not. With the plug out and the waterjacket cover removed, I did my 'compression test': with the piston fairly close to the plug end of the cylinder, I put MY MOUTH over the plug hole and blew into the cylinder, holding pressure in the cylinder. I could definitely feel the pressure in the cylinder going down. I repeated this on one of the other cylinders and it definitely felt like the pressure was holding - a noticeable difference.

Here are the questions: How do I determine what the cause of the cylinder leakage is? Visual inspection of the top of the piston and the cylinder walls looks good (some carbon on the piston but no holes anywhere). Could the rings possibly be stuck from the cylinder being dead (for who knows how long)? I have a can of stuff called Power Tune. Can I use this to try to unstick the rings? Instructions for this stuff call for running the engine at full throttle for 5 minutes after a hot soak.

Any harm in running the engine with a weak cylinder? Whats the fix if the decarb doesn't work?

Fouled Plug · #2 February 16th, 2002, 09:08 PM

Oral compression test? Haven't heard of that since Clinton was in office. Beg, borrow, or steal a compression gauge and get a reading of all cylinders first. I wouldn't say your motor has been given the "kiss of death" until we see some solid data.

62_Kiwi · #3 February 16th, 2002, 09:16 PM

LOL

Good one Fouled Plug!

12Footer · #4 February 17th, 2002, 11:21 AM

LOL. Yea, all is not lost. It could be stuck rings or blown head gasket or something..U really need a gauge,or it will just depend on what your definition of "compression" is.

Sorry,had to add to this somehow.

evin300 · #5 February 17th, 2002, 12:04 PM

remember not rev the engine at full throttle, unless it is on a boat.

alvin · #6 February 17th, 2002, 01:31 PM

Thanks for all the input so far. Will try to get compression numbers in a day or two.

ajohnston · #7 February 17th, 2002, 03:38 PM

Had somewhat same problem and it was a blown head gasket. check plugs for water

alvin · #8 February 18th, 2002, 03:06 PM

Here's some 'real' compression data (taken with engine warm): 130, 125, 85, 125, 130, 130. A shot of 2-stroke oil in the low cylinder took the reading up to 115. What's the verdict? What's the fix (if any)?

Fouled Plug · #9 February 18th, 2002, 04:11 PM

Decarb the low cylinder first as a long shot. It may not help, but it's certainly worth a try. Could be scored cylinder walls, or bad rings. Maybe you'll get lucky and they're just stuck, though. Good luck!

alvin · #10 February 18th, 2002, 04:44 PM

As you're well aware, I'm kind of new to outboard motors. Could you outline a recommended 'decarb' procedure for me? Thanks a bunch!

Hooty · #11 February 18th, 2002, 06:23 PM

Hey alvin,

Not sure who's idea this was, maybe Schematic's, but tilt the engine up so's the cylinders are vertical. Remove spark plug and dump the de-carb in the low cylinder and let it set for a day or so, empty the stuff and follow de-card instructions. Keep us posted.

c/6

Hooty

alvin · #12 February 25th, 2002, 06:46 PM

Did the decarb. Took the motor out to the river to run the decarb stuff out. Motor seemed to be running a lot smoother. Ran it up the river about 2 miles - ran great. Then it cut off with kind of a clattering sound. I was able to get it restarted but it runs rough and clatters. We limped back to the dock. I pulled the plug out of the repaired cylinder and found aluminum deposits on it. When I first got the motor and pulled that plug I found the same stuff on the plug but figured it was some kind of 'liquid aluminum' stuff since the plug hole had a bad helicoil. Now I know why the helicoil was bad. Now I know why the hole was helicoiled in the first place - aluminum deposits! Hey Fouled Plug, I noticed in one of your other posts that you asked someone if there were aluminum deposits on the spark plug. What is this a sign of (other than major problems)?

I've rebuilt a couple car engines and dirt bike engines. Is dissassembly/reassembly of this motor possible for the home mechanic or are there a lot of special tool required? Is sleeving the bad cylinder a realistic fix for this motor?

Valv · #13 February 27th, 2002, 02:54 AM

Alvin,

I believe at this point you will have to rebuld motor, and/or if you are lucky just new pistons, rings, and cylinder hone. You will need ring compressors, which are very expensive or have somebody with them lend them to you. It's a 6 inline, and I've been there too, you won't put the pistons back in unless you compress rings. I spent 2 days broke 2 sets of rings (that means ca. 30 rings) and finally gave up and toke it to a shop.

I have a nice powerhead from a 1350 6 inline here, if you are interested I'll let you have it for just few bucks, but it costs a fortune to ship.

Val

Valv · #14 February 27th, 2002, 10:58 AM

On second thought, my powerhead won't work, this one has 3 side fuel pumps on starter side, yours has only one on throttle lever side.

Sorry.