How do I test non-firing cylinders?

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Missy - 27 Sep 2004 02:43 GMT

How do I test non-firing cylinders? I have a 1975 johnson 70hp engine
that is not firing on two of its 3 cylinders. The non-firing plugs
seem to be geting wet so there is fuel to them. Could it be two bad
coils and how do I test them? Any step down trouble shooting methods
would be greatly appreciated for a novice like myself.

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MikeG - 27 Sep 2004 22:21 GMT

> How do I test non-firing cylinders? I have a 1975 johnson 70hp engine
> that is not firing on two of its 3 cylinders. The non-firing plugs
> seem to be geting wet so there is fuel to them. Could it be two bad
> coils and how do I test them? Any step down trouble shooting methods
> would be greatly appreciated for a novice like myself.

The simplest way, if you can get hold of an inexpensive induction timing
light, is to hook it up to each wire and start the engine. If the plug
is firing the light will light.

The next best, read that simplest, is to, one at a time, remove the
spark plug wire, put a Phillips head screw driver into the plug wire and
hold it close too, almost touching, the engine block. Crank the engine.
If all is ok out of the coil you should see a spark jump the small gap
just as it does on the plug itself..

NOTE - Do not touch the metal of the screw driver when trying that last
method. If the coil is working the discharge can give you quite a jolt.
Assuming good health it won't kill you but it sure will straighten your
hair and stand you up on your tippy toes.

Signature

MikeG
Heirloom Woods
www.heirloom-woods.net
mikeg@heirloom-woods.net

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Boat Forum / Electronics / September 2004

How do I test non-firing cylinders?