1973 20' SF - Engine Trouble?

[StrangeBrew]

All,

I posted a few weeks ago regarding my engine compression issue. I have a 1996 Johnson 115 HP Outboard (new to me this season). Compression reads 110 psi on 3 and only 60 psi on 1. I tried to decarb (PowerTune in the low cylinder)through the plug hole. I also used a bit of Mystery Oil in the low hole. Yesterday I ran the boat for about an hour...Sounds great and runs like a champ...I planed at about 3800-4000 RPM no problem...ran it to 5500 (WOT)no problem...motor ran like a champ...I ran her for just over an hour at 4000+ RPM with no problems at all. I got back to the dock and the compression on the low hole still read 60 PSI. Given that she runs fine and I only paid $800 for the motor, do you think I should just run her for the season or should I pull the head and do the rings if necessary? I guess the question is, if it's running fine with the known compression problem, should I just ignore the problem? Everything I've read indicates that all cylinders should be within 10% of each other..but there does not appear to be any performance problems. Only thing that did happen was at idle on the dock she did sputter and shut down after about 10 mins of running time, but underway there were no issues.

[76Red18]

Does the color of the spark plug electrode in the 60# hole look the same as the others? Check it after a period of idle then again after a period of WOT. I dont think 60# of compression is enough to create combustion. That number can rise as your rpm increases which would give you combustion at high rpm's but not at idle. At minimum I would pull the heads and inspect the cylinders for scoring. If theres noticeable scoring I'd rebuild, If not, replace the gaskets and recheck compression. Maybe its just a blown head gasket. . Just an opinion from a back yard mechanic.

[StrangeBrew]

Thanks. The plug does look the same as the others as far as i can see. This model engine, strangely enough, does not have head gaskets (60 degree V4). Instead this model uses o-rings beween the block and the head...I guess there are semi-circular notches in the head and block which the "o-ring/s" are seated in. Either way, it does sound like I should pull the head and inspect. Perhaps the o-rings are shot? I find it odd that it does sound like it's firing fine on all 4 at idle and underway...I will pull the head and inspect. Could running the motor as is "kill" it?

[NoBones]

I agree with Red, The motor will more then likely give
out before you get a season out of it..
You could have a stuck or broken ring.

See ya, Ken

[76Red18]

Could running the motor as is "kill" it?

Yes

[Bushwacker]

Could running the motor as is "kill" it?

It might not kill it, but it could end up costing a lot more to fix it! Here's an example . . .

I had a 1975 115 90 deg V-4 that I ran for over 30 years. They had higher compression back then and required 89 octane, which again was no big deal at that time. However I used to run in the Bahamas a lot, and gas over there is always questionable - you're lucky if someone hasn't dumped some diesel in the tank, let alone get adequate octane! Instead of installing lower compression heads, I decided to just pull the heads and exhaust cover to clean out the carbon from head, pistons and exhaust ports. (This was before I knew about the various decarb products, but as Fr. Frank said, gaskets are cheap, and it's easy to pull the heads and exhaust cover on those motors!) This typically would drop compression from 130 to about 110, but I never lost any top end rpm, so evidently the benefit of clean ports offset hp loss from lower compression.

One year I found a light scratch in one cylinder and there were top ring cross-section impressions in top of piston and head! Turns out the anti-rotation pin in top ring land had come out, allowing ring to rotate into port area and break pieces off end of ring, which had also taken out a small secton of top piston land. Fortunately the scratch was light enough that honing removed it. If I had continued to run it, the scratch would have probably gotten deep enough to require boring. To avoid screwing up engine balance, if you have to bore one cylinder, you bore 'em all and install oversize pistons, so what was a $500 repair for one cylinder would have probably been over $2000 to bore/hone and install 4 new pistons if I had continued to run it! (And these were 1980 prices!)

One other thing I learned about that motor: unlike the 135/140 hp versions of same 100 cu. in. block, the 115 is not a "souped up" highly tuned temperamental engine. Consequently it always idled very well and would never die when maneuvering around the dock, etc. On a cold start, I could always move the fast idle lever all the way down IMMEDIATELY after starting. Turned out to be a pretty good engine test, because if it EVER died when I did that, there was ALWAYS something wrong, like a weak ignition coil or bad power pack, or a broken ring! It's like the engine was trying to tell me something! This may no longer apply to the new electronically controlled engines, but if you have a carbureted engine, it's important to learn what's "normal" operation and check out any changes from "normal"!

[gofastsandman]

Compression should be measured w/ motor cold, not hot, and all plugs out for 2 strokes.
Just a thought, how are your threads in the head? Someone may have galled them or semi - stripped them. Can you feel any blow by the plug while someone else cranks the motor?


How much power tune did you use and how long was the soak?
Was the motor tilted all the way up and the trailer tongue down as low as possible? Do you have a hill where you can move it to and chock the tires . Get the motor as horizontal as possible. I`d try again. Carbon is strong enough to blow rings and landings and I`d give it some time again. Get a strong small flashlight and check the cylinder for scoring at night while rotating the flywheel so the piston hits bottom dead center and leave it there for the decarb. All wires off and plugs out please.
GFS

[Bigshrimpin]

Don't run the motor anymore . . . it's time to crack it open and see what's wrong with the bad hole. Right now . . . you will probably get away with a hone and new piston. Keep running it and you will need an entire powerhead.

[Fr. Frank]

After your previous attempt to decarb with Powertune, I have to agree with Big. Pull it down and re-ring the cylinder, with a new piston if necessary, and at least carefully inspect the others. A little work now can save you a lot of money later.

[gofastsandman]

Wisdom is learning from others. Follow the sequence for the head bolts.
GFS