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Where to begin troubleshooting

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B. Joseph Davis

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Hi all, I just purchased a 1991 quantum bass boat with a 150 hp Mercury black max. The seller did not know much about boats, had recently acquired it, took it out one time and did not work properly and so he had it checked out and was advised that the oil line to the outboard motor was not working and that the motor had a "blown piston". This guy then went and bought a new bass boat. When I asked him about if it was just one blown piston or if there were others, he acted as if he thought the motor only had one piston. Again, he knew very little about boats.

The purchase price I paid was so low, $660, that I had no fear in obtaining this boat and motor; which included trolling motor and fish-finder and trailer with spare tire. I bought the boat anticipating that I would need a new powerhead and would need to pay to have that replaced and then the motor remounted.

After this guy's comment however, indicating that he thought the motor only had one piston, I am now interested and starting the troubleshooting process process from scratch. I am fairly skilled a small engine repair but just have never worked on an outboard motor. I have rebuilt a Chevy 305 engine from the block up and rebuilt a Volkswagen rabbit diesel engine essentially from the block up, in addition to many other complicated automotive repairs.

My instinct tells me to remove all spark plugs put some oil into each cylinder and try to just crank the engine to see what happens. Assuming that the engine turns over without any unusual noises it seems that I would then fuel the engine with premixed gasoline and should try to start the engine using muffs of course.

What I do not understand is how some engines have automatic shut off mechanisms such that when the oil injection system is not functioning properly that the engine will not start. The seller advised me that the oil line was broken.

Can anyone point me to a post on how to bypass the oil injection system if that is necessary to attempt to start the engine. Or on the other hand if I have made a total fool of myself by my ignorance so far in this post can somebody point me to an appropriate beginners post for outboards.

thanks,

joey
 
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Disable the ignition by tripping the lanyard kill switch, set the throttle wide open and then run a compression test to determine which and how many cylinders were affected. The engine is 22 yrs. old and will need a thorough inspection and rework. There are inspection plates on the side of the powerhead you can remove to inspect the pistons and rings. Use a lot of penetrating oil and time w/an impact wrench on the lowest setting and your hand on the socket to loosen the bolts because they will break off. Good luck.

You can reroute the oil pump output back into itself w/a "T" and then add TCW-3 oil to the main tank unless it is full...it won't mix well w/out a several mile trip to mix it well or just hook up a small portable tank w/50:50 premix for testing.

There are many YOUTUBE videos with good and BAD rebuild information. Your experience will enable you to rebuild the powerhead w/little trouble. Get a factory manual for the engine...eBay should have one or ck w/Merc. to see if one is available for about $85. Good luck.
 
Thanks to all whom have replied so far!!! I now have a plan of attack.

Tomorrow I will disable via lanyard safety kill switch, check compression on all cylinders, and report back here the results.

I will order service manual tomorrow as well.

Maybe those posted test results will help guide my next steps.

Thanks,

Joey
 
As others have already said, the very first thing I'd do is a compression test. You should see around 110-115 PSI in each cylinder (plus or minus 5-10 pounds.) But anything significantly low (like 30 psi, or even ZERO) is an immediate failure of the test. And to be honest, if you do find low compression in any of the cylinders, it's cheaper to just buy a replacement powerhead than it is to even attempt to rebuild the existing one.

If it does pass the compression test, I'd just try to run it. Forget about the "bad" oil line he told you about for the mean time and just run 50:1 straight to the engine, just for the sake of finding out of it actually runs or not. It should also have an onboard oil tank (mounted underneath the cowling on the powerhead itself) and you should see a level of two stroke oil in it. If there is none, it's a bad indication, because he may have ran the engine out of oil, hence his "blown piston" comments. Fill this tank with some TCW3 two stroke oil and don't worry about disabling the oiling system. Running 50:1 fuel to the engine, plus the oil injection, will give the engine extra oil, which will not hurt, since it hasn't ran in a long time.

The above is all based on the fact that this engine actually has good compression though. Again, if it fails the compression test, stop right there. Any other efforts to salvage the engine will be a waste of time.
 
Hi again to all. Had a partial knee replacement and so will be down a couple of weeks. I REALLY greatly appreciate how helpful and enthusiastic everyone has been. I'll keep saying my prayers that the compression test--once completed-- will be a positive indicator and I'll be able to proceed with a good tune up. If a new powerhead is needed I'm not sure Ill proceed with this 91 Merc. I don't need that much HP around here (east central Indiana) and I may be better off with a reasonably priced 50 HP on the 20 foot 91' Quantum.

thanks again to all! I will follow up so this thread can be useful to somebody in the future.

best,

joey
 
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