Is weak spark on one cylinder definitely a bad coil?

[jonathaninsc]

1985 V4 140 looper vro

My motor will surge to full power sometimes, but will usually stutter and refuse to exceed 3k rpm under load. I've rebuilt all of the carbs, new plugs and plug wires, new fuel lines under the cowl. I'm fairly certain I have no fuel leaks. My bulb stays firm, and squeezing during throttling up changes nothing. I've run Seafoam/premix from a portable tank with the same result.

I finally picked up a spark gap tester and found weak spark on my #4 cylinder. Using another plug wire yielded the same result on #4. Is this a bad coil? Or is it possibly something further "upstream" from the coil? Is there a procedure for testing just the coil? I've also noticed I lose signal to the tach occasionally. Can weak spark on one cylinder cause my engine to run so poorly under load?

Thanks in advance for all of your help.

 

[HighTrim]

Re: Is weak spark on one cylinder definitely a bad coil?

Quickest and easiest way to test the coil is to swap it with a known good one. If the spark issue goes away, replace the coil.

 

[jonathaninsc]

Re: Is weak spark on one cylinder definitely a bad coil?

Quickest and easiest way to test the coil is to swap it with a known good one. If the spark issue goes away, replace the coil.

Thanks Chris. I'll try that next. Do you think one bad coil would cause the motor to stutter under load?

 

[mikesea]

Re: Is weak spark on one cylinder definitely a bad coil?

yes it could,try the coil swap,be sure your ground is good on the coil and that the connectors are clean