edit up front: Wow the site I found is the greatest thing since sliced bread for engines like you have and my son has. I'm going to keep it in my good stuff!
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I found wiring diagrams on the net, which is for your engine?
http://www.maxrules.com/JOwireindex.html
I figure it may be this diagram since you said the 70's and I figure you meant 140hp so it must be a 4 cylinder, but I'm not sure.
http://www.maxrules.com/graphics/omc/wiring/V4_1973.jpg
Does the engine run ok?
If it's running ok then the spark is jumping both the plug gap and the gap between the boot and the base of the plug at the same time.
The fix will be to either make it easier to jump the gap of the plug, harder to jump the gap at the boot or some of both.
The most likely reason is the boot is old and has become a better conductor of electricity. The reason you don't want is the fuel mixture is somewhat fouled and is making it harder for the spark to jump the gap in the plug, so the coil puts out more power and it is enough to jump both gaps. The wire between the coil and the plug could also be bad and cause the problem.
Which leads back to the question I asked about if it runs ok. The question really doesn't need answered and even if it does run ok, if you fix the problem it will probably run better. ...
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copied from the site:
10. I have great spark at the plugs, but I have a bad miss. top
Here I usually find a case of a weak coil or dirty breaker points, leaky condensers, etc. What looks good on a set of plugs laid on the block is not indicative of a healthy ignition system. The spark from a modern electronic system should consistently jump a 3/8ths to 1/2" gap. The older Battery CD motors are especially hard on coils, so much so that you should gap the plugs down to .030 or less and replace the coil whenever there is the least misfire.
11. I have great spark cranking my motor with the plugs out, but it goes away when they are installed. What's wrong? top
The first question, is the motor cranking fast enough to properly energize the ignition system! the average magneto energized CD ignition needs at least 600 rpm cranking to work. Check the BATTERY, the CABLES, the SOLENOID, the STARTER. If they are all OK, look at the ignition STATOR, SENSOR and the COIL(S) OMC V4 motors from 1973 thru 1977 have a problem with weak sensor coils. New ones are EX-PENSIVE! Look around at the local O/B junk shop for used ones, old 1.5 volt coils are white,the improved 3 volt ones are reddish in color.
12. I get a shock from my control box! top
You have a leak in the high voltage leads in your wiring system, allowing the primary ignition voltage to leak to ground.(You, in This instance!) These systems can create over 300 Volts AC, more than your house wiring. Disconnect the shorting lead(S) at the powerpack, and if the shock disappears, identify the bad wiring or switch, and replace. This can also cause the problems in Qs #10 & 11.
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wow again, the more I read the mans site the more I appreciate it. Below is something else I copied from the site:
There is an apocryphal story about "photographer-to-the great" Yosuf Karsh (may have misspelled his name) who used to charge $10,000 a portrait against an old army blanket. He would place his subjects there, fiddle a minute with the lights, and shoot off a few photos (believe he used an 8 x 10 so had to change the plates) but in all, about 5 to 10 minutes. He would then tell his clients he had finished.
What? They would say, $10,000 and you just took 5 minutes?
Ah, came the answer, but it has taken me 40 (or maybe it was 50) years to be able to take a beautiful portrait in 5 minutes....
Same as the famous surgeon called into the operating room, who examines the patient carefully, reviews the x-rays and steps up to the table and makes his incision. He then turns to the other surgeons, says "You may continue" and walks out. The patient, aghast at the $10,000 invoice for 5 minutes and one cut, complains. The surgeon replies, "The $10,000 is for knowing where to cut."
And you're right, Bunky. This does apply to many of us. Most of us are quite dedicated to our profession, craft or trade. We work very hard to stay up-to-date, remain knowledgeable and creative. We do a great deal 'behind the scenes' to be able to make our work look effortless.
I suppose that's why Doctors, Clergy and CPAs all worked hard to keep their professions cloaked in mysticism. The more obscure their work, the more they could charge.
So be glad you have the MASTERTECH'S expert advice available here, and please support my effort with a parts order!!
THANKS! GOOD LUCK!