Yamaha XT250 Exhaust Uncork Mod by Ms Lil Mynx on 4Strokes.com

Here's how to uncork or derestrict the stock exhaust system of a 2008 Yamaha XT250 for better performance. This modification, along with any air intake mods, will help the engine breathe better. Here are the steps to uncork the XT exhaust:

Remove the screw from the bottom of the baffle and pull it out of the muffler.
Measure from the end of the muffler to the restrictor plate inside. Mine was 7", so I went with a drill bit, that when chucked up, was about 9" long.
With either a drill bit or hole saw, drill a 1-1/2" hole in the restrictor plate. The only drill bit I could find to fit my 1/2" chuck was 3/4" so I had to do some additional reaming. The restrictor plate is extremely tough so use caution when drilling. I left the muffler on the bike because it was easier to work on, then removed it to shake out the metal shavings. You can start the bike without the muffler and run it for a few seconds to blow out any metal shavings that may have gotten up in the head-pipe.
Install the muffler and baffle back on the bike.
After uncorking the exhaust, and/or air intake, you may also want to richen up the idle mixture. With the exhaust uncorked, my XT runs best with the idle mixture screw 2-1/2 turns out, which is exactly what Yamaha recommends. However, after removing the factory plug (XT250 Mixture Screw Plug Removal), I found the idle mixture screw 2 turns out from seated. This my have helped with emissions but caused a poor throttle response and a longer warm-up time.

Credits: Yamaha related tech tip by Ms. Lil' Mynx. Edited and published by 4Strokes.com.