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10R74S 9.9 Johnson 1974 Spark Plug L82C or L86C

P

patiolatern

Regular Contributor
The parts breakout lists the 2 spark plugs.
Does this indicate both are acceptable? I use this motor as a backup and for trolling.
I have searched the forum and websites but no clear answer.
Which is the right or preferred one?
Thanks!
 
You will probably find many answers/opinions on that topic. Fact is, that motor was notorious for fouling plugs, even when brand new. I had several customers that couldn't get them to run one weekend. The factory came out with several "fixes", none of which really worked. Good luck.
 
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Originally the choice was between UL4J and L7J.</p>
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The latest recommendation that I am aware of is for UL81C, gapped at 0.030.</p>
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I doubt if you will notice any difference between L82C and L86C but the latter may be the better choice for trolling.</p>
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The U prefix denotes an auxillary gap within the plug. The theory is that this allows the voltage to build higher before the plugs spark and that this in turn helps clean away any fouling. UL81C may therefore be an even better choice for trolling.</p>
 
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That motor is probably the worst one you could find for trolling. The 1974-1976 9.9/15Hp motor were notorious for fouling spark plugs. Eventually OMC had to change out the entire ignition system to fix the problem. In any case, I don't think there are any spark plugs that will allow that motor to troll for more then 4 or 5 hours without eventually fouling a spark plug. If you want to extend its spark plug life you should switch to the NGK equivalent. NGK B7HS plugs. Again, no plug will turn that motor into a good trolling motor but the NGK seem to resist fouling on that motor much better then the Champion plugs.
 
Did a complete tune-up / check over on one about 8 years ago.----Idled perfect.-----Owner loves the motor.-----There are holes in the exhaust tuner that plug on these.-----Need to be cleaned out.-----Know of a respected dealer that took one apart to see what goes on.-----He and his mechanics figured it out , due to little help / support from factory.
 
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