A little clarification...

So let's make sure I have this straight:
- Plugs 1&4 are on the same coil
- Plugs 2&3 are on the same coil
- if 2 doesn't fire, 3 wouldn't either if it was a bad coil
- if 3 fires, check HT wire, cap and plug on 2
This is my understanding. I can get the bike to start but it's running really rough and I can smell unburnt gas. Right hand bank gives a strong steady beat, head warms normally, seems to be fine. Left hand bank gives an uneven beat, isn't warming properly and seems to have a dead cylinder. No. 2 plug is not showing spark. I can't see any damage to the coil, wire or cap.
I posted this as a reply to an earlier thread about 3 of 4 cylinders working, but then I thought I should post it on its own.
Thanks for any help.
Chuck

 

That depends on the nature of the coil failure. SQLguy explained how one plug can fire if the coil fails in that other thread, but might not have elaborated enough. Coil failure can be complete or partial (my term).

The short story, as I was taught:

A bad connection at the coil end of the lead to one plug would be considered a partial coil failure. A short in the secondary coil would also be a partial failure. In both cases only one plug will spark (depending on where the short is in the secondary coil).

A short in, or failure of, the primary coil would be a complete failure and would cause neither plug to spark.

 

Thanks k-moe. So I can rule out a failure of the primary coil at least. I'll swap a couple of plugs and see if I can get a spark with a different plug, that should clarify if it's a plug issue. If there is no change I'll see about testing the HT wire and the cap.

 

Cap should be 5K ohms that’s the cheapest think to replace