Symptoms of a Failing Ignition Coil

I have a 1977 KZ750B Twin

Its been acting funny lately. First of all, the starter clutch gear is shot, so fixing that.

I am now noticing, after changing the spark plugs, that one plug is dark but not fouled, and the other is fine. Carbs are in good shape, but I notice the idle racing somewhat, but not often.

A friend said that coils on Twins go fail often because of vibration. For giggles, Im going to change the coil this weekend as I had a brand new spare.

Is poor running and one plug darker than the other, a symptom of failing coils?

Fouled how? Black carbon build up can be a sign of weak spark, over rich mixture or burning oil and/or low compression. 

If it's an original/stock coil, then it could be that the windings are bad or the wires going into the coil have bad connections, especially the plug wires which might not be easy to fix since they're molded into the coils & not intended to be removed.

Thanks for responding. I changed the coil about 4 years ago.

If the carbs are good the difference could be due to hardened manifold rubbers leaking air - you could try spraying WD40 etc round the rubbers with it idling and listen for a change in engine note 

Was on it today. Its backfiring upon deceleration as well. I think its the coil. This happened 4 years ago

Just a thought, one big cause for backfiring upon deceleration is that of too small of pilots, doh just noticed user is blocked

It's easy to check a coil.  Measure your primary and secondary resistance.  If they are in spec then the coil is fine.  It may be your voltage to the coil is low.  Check and clean all of the contacts or maybe do the Wired George coil wiring change.