Coil problem??

[Bryan]

1989 25sf with rectifier installed for charging batt.
Recently the engine will drop the bottom cylinder (checked by pulling off plug wires while running)after 1-2 minutes of sustained 3/4-WOT condition.
It cold starts fine and if you shut if off and wiggle the bottom coil wire it will often start up on both cylinders again but the problem re-occurs after a minute or so under the same conditions.If you allow it to just idle it seems to run on both cylinders fine for hours at a time.
Yesterday when it acted up, I tried a simple thing of swapping plugs from top to bottom.
Strangely, it then ran fine for an extended WOT run(10-15min).
Any help appreciated, as it seems like a coil issue to me but the plug swap thing is confusing me.

[jb123]

firstly i would check your grounds, secondly I would swap coils to see if the prob follows a coil. thirdly, 1`d run it at night to see if theres and leaking spark from your wires.

[Ken]

Bryan,
Sounds like a bad spark plug wire.
When you swapped plugs, you had to pull the wire/cap, which may not have been making good contact. If you can replace just the wire, try that. If not, then replace the coil and wire assy.
Good luck [img]smile.gif[/img] ,
Ken K