I have had this issue now on 3 Vulcan 750s. I believe I now know why it happens. I have been starting the engine up and letting it idle for a while every once in a while on my '97, even though it is not yet rideable. I notice that if I start it every day, it fires right up and runs smooth. But if I let it sit for several days before starting it, it takes a lot of cranking to get it started, it will often backfire a couple of times, and often starts up on the front cylinder only. I have had this issue since 1993, for almost 200,000 miles. It finally dawned on me what may be causing it. While sitting, the gas evaporates out of the float bowls (ethanol gas evaporates even faster than real gas) and because it has a vacuum petcock, it can take some time to fill the float bowls back up again, as gas only comes out while the engine is being cranked. And apparently the front float bowl fills up first. It starts up on the front cylinder, then about 10 seconds later the rear cylinder kicks in. Recently after it had been sitting for about a week, I put a small amount of gas into the carbs through the float bowl vent hose. It fired right up and ran fine. I am definitely getting rid of that vacuum petcock (it's too bad they removed the "prime" position, it should have been obvious it was there for a reason) and going to a TW200 petcock.