I've been noticing in the last year a decline in power of my XX. A couple things stand out. When I bought it, I could release the clutch on a hill in first and it would idle up on its own. Starts from the light in 2nd were easy as well. The bike almost stalls doing this now. Generally speaking, the pickup on flicking the throttle is reduced as well. Here are the things I'm planning on checking, I'd appreciate it if if you guys could lend some advice on the order.
fuel filter
muffler (clogged??)
timing (though you can usually tell timing by a rough idle)
compression test
injectors
(D) Hookers