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1970 Johnson 9.9 hp fouling plugs

T

trinjboro

New member
I have a 1970 Johnson 9.9 that has a 15 hp carburator. It was running slow so I checked the plugs and they were fouled. Changed the plugs and it then ran great for about two hours and then did the same thing. Checked the new plugs and they were fouled. I'm mixing the fuel at 50:1 (2.56 oz. oil to a gallon of gas). What might be causing this?
 
"no 9.9 in 70...74 first yr fo

"no 9.9 in 70...74 first yr for both 9.9 & 15,..ck ur spark,compression and fuel,for water..."
 
"the prescribed plug,.. up to

"the prescribed plug,.. up to 77,..is champion,UL81C,..i have had great luck,w/j6c,..gap all at .030"
 
"I would first ensure that you

"I would first ensure that your low speed carburetor adjust is properly set. If it was too rich, you will get fouling. The next thing is to ensure that your T-stat is working properly. If it is failed open you will also get fouled plugs, although I am not sure it would happen in 2 hours but it depends on how cold the water is. On that motor it is a pain to get at so I might try to measure the temperature of the powerhead first. If you can hold your hand on the powerhead when the motor has been running for 5 minutes or more, without any discomfort at all, then I will assume that the T-stat is probably failed open. If it feels like you can touch it but you really would rather not, then it is working fine.

I have a 1975 9.9Hp (same motor) and found that it really doesn't like the UL81C plugs from Champion and therefore I have been using NGK B7HS with great results."
 
"There was an issue back in197

"There was an issue back in1974 with these engines fouling plugs when trolled for long hours. I personally know a few guys who sold off their mega hours no issues 9.5 sporttwins and bought new 1974 9.9's and wished they hadn't. I remember they had a different ignition system that didn't stay around long. That seemed to solve the problem, maybe even by 1975."
 
The 74 9.9 & 15 used auxil

The 74 9.9 & 15 used auxillery gapped plugs the spark jumped a gap inside the insulator to make the system produce a greater spark.

The 15hp also had a different megaphone on the exhaust sytem if my memory serves me correctly and this may also be a problem if you didnt change it with the carby.
 
"Here is a link to the ult

"Here is a link to the ultimate source of information on these motors:"

Hmm some good and some crap advise amounst his ramblings
 
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