I have a 70HP 1975 Johnson Outboard. After pulling plug wires one at a
time while it is runnign i found out that only one cyclinder is
actually firing. It appears that the two plugs not firing are wet, so
they should be getting fuel. Can it be two bad coils and how do i test
a coil? What is the best top down troubleshooting i should apply to
isolate the problem for a novice like myself? I replaced the
powerpack. Thanks

Missy wrote:
I have a 70HP 1975 Johnson Outboard. After pulling plug wires one at a
time while it is runnign i found out that only one cyclinder is
actually firing. It appears that the two plugs not firing are wet, so
they should be getting fuel. Can it be two bad coils and how do i test
a coil? What is the best top down troubleshooting i should apply to
isolate the problem for a novice like myself? I replaced the
powerpack. Thanks

Take out a suspect plug, rest it on the engine block, use a piece of
string or tape to hold it against the block if you must. With the
plug wire attached, crank the engine. If the ignition is good, you
will see a strong spark (as strong as the working cylinder.) If that
is good, and you have compression in the cylinder (a compression
guage is about 20 bucks), your carb seems good if it feeds one
cylinder ok, the timing is ok, if one cyl is running.

If you have weak or no spark, you may need a new coil, or two, I
don't know the way your engine is put together.

Otherwise with low compression cylinders as compared to one that is
firing ok, you may have ring problems, which you can test by putting
a few drops of oil in the cylinder and testing compression again.
If the pressure comes up significantly, you have worn rings. If not,
you may have a bad head gasket.

The cheapest and most common fix is to change the plugs. You should
awlays carry spares, antway. If they get fouled often, your mixture
might be borderline, and if you smell gas while trying to start, it
is probably getting too much gas, usually as a result of a leaky
needle and seat in the carb bowl, an item easily replaced.

You can check roughly for mixture by looking at the plug. If it is
all black and dry, you may have a fouling problem caused by too cool
operating habits, and it may be good to run the engine hard regularly.

The exact oil ratio may cause problems, if you have too much oil,
the engine burns lean and hot, if too little, it may run well, but
rings and main bearings will wear out quickly. You need to be in the
middle ground.

A skilled mechanic may be your best bet.

Terry K