1996 Johnson 15 H.P. Will Not Reach Max RPM While In Gear

[fstbttms]

Well, most of time, that is. Saltwater engine, used almost daily. Will always rev very high in neutral but when in forward, usually will not. Historically would get my loaded 13' inflatable out of the hole and up on a plane quickly and easily. Now, most times will barely get to a plane and frequently will not even do that. By sound, the engine is not revving as high (in gear) as it used to. Although every once in a great while (like Thursday morning), leaving the dock she seemed like her old self. But once shut down and restarted, the new, weaker version reared her ugly head again. Any thoughts?

 

[gm280]

fstbttms, I would do a compression test first. If those numbers are good, then look at the fuel delivery system starting with the fuel pump and then carb. May be time to disassemble the carb, clean if thoroughly and install a carb kit. JMHO!

 

[fstbttms]

You know it's funny, just prior to leaving the dock Thursday, I took off the fuel pump cover just to check the screen (something I haven't done in a long time.) Then I launched the boat and the engine got me up on a plane quick as anything and sounded great. Just like old times! Of course, then I stopped to tow a disabled boat back to the ramp and when I was done doing that, the motor ran weakly at WOT, again. I was going to go look at a 2004 model today for $1100, but I think maybe I'll mess around with the fuel pump and carb before I drop the spank on a replacement motor. Thanks!

 

[OptsyEagle]

Usually when you have a good running motor except for reduced power at the top end WOT, you have a motor running on only one cylinder. I would first do a cylinder drop test.

Start up the motor and let it warm up. Bring it to a fast idle and with a pair of insulated pliers, pull one spark plug boot at a time. These motors will run remarkably well on one cylinder, albiet with significantly reduced power at the top end. So if the motor dies when a plug boot is pulled, you now know that the other cylinder was never firing or is very weak.

 

[fstbttms]

So if the motor dies when a plug boot is pulled, you now know that the other cylinder was never firing or is very weak.

Thanks for that, I'll definitely do that first thing. Assuming it does die during the test, what is the cure?

 

[oldboat1]

A self powered timing light might be great to initially check for firing. Just clamping on to the plug wire should do the trick (and intensity of the strobe might indicate intensity of spark). Lifting the idea here from another lister -- don't remember who to credit. I used an old fashioned 12 volt light yesterday for one running in a barrel -- works fine to see if system is producing a signal. Could still have a failing plug/bad spring connection, of course. If problem persists, would go to a spark tester to better rule out ignition issue.

See it as a simple initial check -- seems preferable to reaching back and pulling off plug wires (pliers, leather gloves....), which I usually do. Excuse for another gadget, too.

 

[OptsyEagle]

Thanks for that, I'll definitely do that first thing. Assuming it does die during the test, what is the cure?

I would want to do a spark test. With an external spark tester, can the ignition provide a spark over a 7/16" gap. Also, pull the spark plugs and take a close look at them. Is the bad cylinder plug all oily or shinny clean or a regular tan color. I might even put in a new set of plugs to see if I get lucky and a bad plug was the problem in the first place. If it is spark, see if you can switch the coils and see if the problem moves to the other cylinder. If it does, you now know it is a bad coil. If you have good spark, good spark plugs, tan colored plugs or oily plugs (reducing the probability of water in the cylinder, usually providing shinny clean plugs) then it is definitely time for a compression test to see if something more mechanical is the problem.

Since it is only one cylinder (if that is the case) you should be able to eliminate a fuel pump, carburetor and sheered flywheel keys, since those items service both cylinders.

Anyway, do the cylinder drop test and let us know how that turns out. Also, you may have an intermittent cylinder loss so if you do you may need to wait until the loss of power is observed and do the test then.

 

[fstbttms]

Thanks very much, again. The compression is good, I believe. At least it was when the engine was in the shop last year (and the engine did not run "right" coming out of the shop.) In any event, I will follow your recommendations and post back the results.

 

[fstbttms]

OK- I did a drop test. The engine died when I pulled the lower plug boot. That said, I also started it with only one plug boot attached and it ran regardless of which plug was working. Thoughts?

 

[oldboat1]

If both cylinders firing, would likely have kept running on the top cylinder when you pulled the lower boot. Test both boots/coils with a spark tester (top one probably not producing spark).

Not sure, but think you might have used the same boot when doing that starting test (probably the lower one, tested on each plug.) guess that would indicate both plugs are good....

Do the spark tester..... adjustable type. Look for about a 1/4 inch gap, then bump it up to about a half inch gap (considered a strong spark -- easily produced with that ignition system. Easily get a pretty powerful shock, too, so be careful.)

 

[fstbttms]

Roger that. Thanks. I'll check back in after.