97 classic, rear cylinder won't fire
 

I bought my 97 vulcan 1500 classic off Craigslist. It had been sitting for a long time and had no spark. I've replaced the battery, coils, wires, plugs, voltage regulator, and ignition module. It fires beautifully, but will only run on the front cylinder. All wiring connections are good, cylinder has spark, compression, and fuel. Spark plugs are wet with fuel. Front cylinder plugs are nice and dry and virtually carbon free. Fuel in exhaust pipe from rear cylinderbackfires from fuel igniting in the exhaust pipe. Not sure what to troubleshoot at this point. Please help..