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Old 06-18-2019, 11:05 AM   #7
Dave in MN
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Join Date: May 2010
Location: Jordan, MN
Posts: 1,202
Default Re: Mystery Missfire

John,
I am still puzzled... You are verifying the condition exactly as I would
Given that you have spark when you short #4 plug to the head with a screwdriver it proves the energy is getting to the terminal. You changed the plugs around to rule out a bad plug. Try cleaning the base and threads of the plug hole to improve conductivity (long shot here)
Based on what you are describing, the only thing I can think of is because the compression is slightly higher in #4, the spark may not have enough energy to fire due to too much gap. Try slightly decreasing the gap at plug #4.
Double check that at the low idle, you still have a strong spark when you short the plug to the head....

Likely this is not the cause but it is easy to lessen the gap.
Keep posting what you try and someone will come in with the correct cause.
I'll keep thinking...
Good Day
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