Yeah, what he said! But really, take off the endcap via the 3 allen screws, and there will be just a little hole where the exhaust comes out. If you drill more holes around that, you will get much better flow and will be louder, as KdxChode said.
Another thing to do is get a "EKP" or "EMM" needle for the carb. About 12 bucks each. They are yamaha parts, yet work in the DRZ carb as well. The EKP gives a bit more power in the mid-top end power (and is fatter, so the main jet must be reduced 2 sizes or so than with a regular needle).
The EMM needle is the same exact profile as the EKP from 1/4 throttle to full, but from 0-1/4 throttle it is richer, giving even better performance at lower throttle postitions.
Contact sudco.com or carbparts.com and tell them you want a EKP or EMM needle for a Kehein FCR39, they'll hook you up. :D
Another thing to do that I have done is the "Taffy Mod". What this does is reduce the duration of the accelerator pump, as the stock duration is far too long. As a result of being too long, there is usually a pretty bad off-idle bog.
I made a page describing how to do the taffy mod, click
HERE .
Well if you need any further explanation or mod ideas just ask! :)