I have a '99 Drifter that I love. Although I live in PA and bought it from a guy in Maine, I've found that it is a California-spec bike (purchased in Arizona when new). My only problem with it (aside from not enough time to ride) is that the filler cap on the gas tank leaks when the tank is full or near full. I'm wondering if there is a problem with the emission controls on this bike, whether because it is California-spec or not, that is causing this. I have two other Vulcan 1500s (2003 Nomad, 2008 Classic), both fuel-injected like my Drifter, that do not have this problem. By the way, the Drifter has about 16000 miles on it. To my knowledge, there has never been any damage to it, and the only modification I've made (so far) is to put Vance & Hines straight shots on it.
Any advice appreciated.
The fuel cap assembly is a ridiculously complex design. Could be one of the seals in the cap. Maybe you can check and see if the drain to the ground (overflow drain tube) is open.
The problem that I have had with the filler cap is the o-ring and the gasket that mates with the filler opening. The o-ring is easily replaceable but the gasket that seals the filler is not. I looked for an OEM gasket but could not find it. So, I cut a new gasket from a heavy inner tube that I just place over the original gasket and hold in place when I close the cap and lock down. I keep several gaskets in my saddlebags because after about 6-8 fillings the gasket gets stretched out of shape. It is kind of hard to cut it in a uniform manner because it stretches, but it works for me.
May be of help:
https://www.vulcandrifterriders.com/gascap.html
Thanks, everyone. I'll try all these suggestions (or ask my service guy to look into it). With all your help it seems that if I can't figure it out and fix it, it's my own damn fault.