I recently found that the reason my bike was backfiring was because the rear coil was producing a low spark. At idle the rear cyclinder will not run when I checked spark it was very low. When I rev it up it kicks in and runs great. When I pulled the plugs it is obvious there is unburnt gas on them. I have replaced the coil and is a little better but the rear cyclinder still won't fire at idle. The battery seems to be OK although I will be getting a AGM very soon. If there is anyone out there with some advice it would be greatly appreciated. What should I check next. Thanks in advance
First things first. Welcome to the Vulcan madness.
Now we can get down to business.
Did you replace the plug wires too? or just the coil itself?
The impedance, or resistance, of the coil secondary circuit measured through the spark plug wires at each cap should be from 19K to 29K ohms.
In a recent thread a problem similar to yours was being discussed. I suggested to take the plug wires off and measure the resistance of the coil and each wire separetely. Each of his coils measured about 20K ohms.
One set of wires measured about 5K ohms each, which is about right because plugs wires are often designed with ~5k Ohms resistance per foot.
His other set of wires had infinite resistance, meaning there was a break in them. He replaced that set of plug wires on one coil, and put the original used good set back on the other coil and the bike runs well now.
That is a quick, easy no cost test (assuming you have a multimeter already
Be sure to test the front coil and wires too so that you can compare a known good coil wire combination against the rear set that are not working properly.
If the coils are in spec and the wires are not, new wires may cure the problem.
If you still need more spark, look at fergy`s coil mod supplying 12v directly to the coils from the battery through a relay: