I got to ride my 2001 Katoom 400sx today and it rocks. The gearing is way tall so i installed a 13 tooth front sprocket and that helped a bunch. I think 14-52 would be the best set-up. It helps use 2nd and 3rd more but lets your powerband be plenty broad.
Mine is bone stock except for a WER stabilizer, the gearing, and some Acerbis handguards.
Most fun bike i've ever owned.
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2001 KTM 400SX #529
2000 SVCC 125cc H.S. #1
SPONSORS: MA'S CYCLE, SIDI, WD-40, ACERBIS, SMITH, SPROCKET SPECIALISTS, N-STYLE, WER, DEVOL.