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Johnson 15hp - Drops when gets to half throttle

P

PJ2075

New member
Hi All,
Hoping I can get some advise on diagnosing my outboard problem.
Firstly, I think my outboard is a 90's model - Built in Belguim - Model: BJ15BAENM - Doesn't seem to match Models listed.

I bought the motor from a guy & he told me it had a new water pump & impella.
Also mentioned the telltale does not put out much water - I've only seen a few drops of water.

Problem:
Motor starts first time - no problem,
first time I took it out, got it up to top speed for about 5 mins, then it just dropped to idle.
tried to accelerate again, gets to about half throttle, then drops to idle again.

Puttered home, checked fuel connections, replaced the fuel connector on motor in case it was sucking air.
Tested motor on trolley, seemed OK.

Second time out, putter out, then accelerate - same problem. Gets to about half throttle, then drops to idle speed.

I'm thinking its either a fuel/carb problem or possibly overheating?
Would the motor restict to low throttle if its overheating (thermostat)?
(Although when I got it home and flushed(engine coldish), it drops also).
Possible the water pump/impella wasnt done or badly done?

Thanks in advance.
Cheers.

PJ
 
BJ15BAENM
B=Belgium
J=Johnson
A=Don't remember this one. (rope start, electric start, long shaft, short shaft, something)
EN=92 (1992)
M=Factory run number, of no consequence.

S/Plugs should be Champion QL77JC4 plugs, gaped to either .030 (long life) or .040 (super strong spark), your choice.

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Basic Troubleshooting:

Check compression w/both s/plugs removed. What are the actual psi readings of both cylinders?

Check spark w/both spark plugs removed. Tester must be the type that incorporates a adjustable "Air Gap".
The spark should jump a 7/16" air gap with a strong blue lightning like flame... a real SNAP! Does it? Explain.
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Check that the throttle butterfly starts to open when the scribe mark on the cam is dead center with the throttle roller.

Check that the throttle roller is not damaged and that the throttle butterfly opens gradually as the throttle is applied and that it opens fully to stay in that position at full throttle (Engine NOT running, in forward gear)
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Question: Are you saying that as you approach the full throttle position, the engine actually returns to its idle rpm... or... that the engine has a tendency to bog, to die out. It does make a difference.

The most common problem with outboards is that the engine sits quite a bit and due to this sitting, the carburetor fouls, gums up, clogs... to which the cure is always to remove the carburetor for disassembly, cleanings, and as needed rebuilding.
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Air Gap spark testers can be purchased at any automotive parts store.
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(Spark Tester - Home Made)
(J. Reeves)

You can use a medium size philips screwdriver (#2 I believe) inserted into the spark plug boot spring connector, then hold the screwdriver shank approximately 7/16" away from the block to check the spark or build the following:

A spark tester can be made with a piece of 1x4 or 1x6, drive a few finishing nails through it, then bend the pointed ends at a right angle. You can then adjust the gap by simply twisting the nail(s). Solder a spark plug wire to one which you can connect to the spark plug boots, and a ground wire of some kind to the other to connect to the powerhead somewhere. Use small alligator clips on the other end of the wires to connect to ground and to the spark plug connector that exists inside of the rubber plug boot.

Using the above, one could easily build a spark tester whereas they could connect 2, 4, 6, or 8 cylinders all at one time. The ground nail being straight up, the others being bent, aimed at the ground nail. A typical 4 cylinder tester follows:

..........X1..........X2

.................X..(grd)

..........X3..........X4

This can obviously be modified to a 6 or 8 cylinder setup tester.
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Joe, I wish this resource was around when I was starting my trades as a youth......in particular, my outboard repair. Our techs here are outstanding. There is really no excuse to not be able to diagnose and repair your motor with the use of this forum. I thank all my fellow techs for their time and experience.
 
There are no temperature sensors on it to shut it down on overheat.----Test spark as suggested.---Test compression as suggested.------Check for water in the fuel tank.------Test run with timing light to perhaps pinpoint the cylinder that is loosing spark.-----Test run with the cover off.-----Test run with another tank and hose out on the lake.
 
Last edited:
Thanks Guys,
Found the problem, the throttle Cam has broken.
Found the broken off piece under the block.

Looks like I need to take off the flywheel & plate to replace the Cam.

Any reason this may have broken?
Just failed plastic?

IMG_3116.JPG

Thanks for your assistance.
 
Common problem.-----Just junk plastic.------Plastic on newer motors is going to be a huge problem in the future.-----Yes you have to take flywheel off.----Then loosen about 5/6 screws to replace that part.
 
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