What might cause a 10 mpg drop in my fuel mileage

[burkbuilds]

I know this is supposed to be for the GZ, but I'm in need of some advice on an issue with my Kawasaki Vulcan I'd like some input from you guys (and girls). I've put over 30,000 miles on my Vulcan 500 since I bought it in May of 09, and it has been a very trouble free bike for me. I changed my gears out to 17/40 not long after I purchased the bike and I've been getting 58-60 mpg consistently since then up until about a month ago. My mileage dropped off to about 55 mpg and now I'm down to 50 mpg. I thought it might just need to have the valves adjusted, so I did that today and got them all set to mid values (several were on the tight side of the spec and one was loose) and although I felt like the bike had a little more pep than it has had lately, I just drove down to Atlanta and checked my mileage when I got here and I'm dead on 50 mpg. Now I realize that a lot of guys riding the 500 get that or less, but I've been getting almost 60 for a long time and now it has really dropped off in a relatively short period of time. The only other thing I've noticed is that I've had to use the choke more lately. Until some time in January, I rarely if ever pulled it all the way out, usually the first detente was all it needed, but lately I've had to put it on full choke for about 30-45 seconds and then drop back to the first detente for the next minute or two. Otherwise, everything is running fine. No problems maintaining highway speeds and I really can't tell any performance difference at all other than my mileage drop. I just put new plugs in in January after I first noticed the fuel mileage drop, and that didn't seem to make any difference. I also replaced the air filter after I started noticing this drop in mileage and I change my oil every 3,000 miles and it's not due for another 1,000 miles and the oil level is fine.

Anybody got any ideas?

Thanks for any input.

Burkbuilds