I cut open a silencer off of a 1974 DT 250 to remove the inlet pipe for another project. The internals should be the same for any Yamaha with a separate  silencer/ spark arrestor marked KRIZMAN. This is the model that is completely welded internally and cannot be "repacked". Only the end cap is removable. I did not want to burn up data space on the site so if you would like to see these photos, please send a cell # and I will send you a text or I can E mail. I can also send additional photos if you need any. I also cut out the section with the 2 perfectly good mounting studs if any one needs them.        I am trying various chemicals for cleaning. Full strength alcohol and EvapoRust make the outside nice but do nothing to the carbon build up inside. Next, high heat application. Followed by various drill mounted stiff wire brushes. This works best.