Desert Racing
I'm with Chris, my money is on the tune as well. Maybe your carb needs to be cleaned out and jetted correctly. With your set-up you should easily be in the 80mph range and above. Get that carb fixed ASAP!! and check the valves as well.
As an example I went out and test drove a fully built Mr. Horsepower 730 that a guy was selling here locally. It had the works, HPR4 cams, Lectron 48 carb, K&N, 730 BB, Vortex CDI, Ported head, etc etc. It had everthing done and it felt quite a bit slower than my mild stock bore. It had everything to do with the tune. If it was tuned right it should have been twice as quick and powerful as mine. So I can't stress enough that you probably have a bad tune. Give us your elevation and maybe we can steer you the right direction a little better on jetting and such.
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01' DS650
A Few Goodies 
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