We had a similar problem with a Tbagger at Topeka, turned out to be fuel valve, but that was FI which is why I let it slip by...but wouldn't both cylinders be huffin black...?
check the breather line to the valve for obstruction...
I know i may get stoned to death by this post....but have you tried to run a hotter plug on the rear cylinder? just a thought? chad.
Really not a good suggestion to mask a problem with a hotter plug.:s
Replace the coil.