Supposedly the 03's and up have the camshaft thing fixed so engines are bomber till 50k + but thats just what I hear.
There are a few minor tweaks required of earlier LC4's (640E and 640Adv). But they are just a bulletproof as the later models. So, I would not shy away from an earlier one. Creeper (on AdvRider) was very helpful to me in my purchase of a used '01 LC4E. It is the most fun and versatile bike I have ever owned. I currently have a set of SM wheels/tires in addition to the Enduro setup. I keep D606's on the the enduro wheels and can swap them out in 20 minutes.
The Adv is an entirely different animal. I have ridden them back to back. Perhaps due to its weight, it feels much more smooth than the E. My Enduro blurs my vision at certain RPMs.
So, I cannot offer you any help if you really want to be talked out of buying one. My suggestion, however, would be to buy an older bike. I have seen them trading just north of $3k. Save a few beans that you can put into accessorizing the bike to suit your riding.
There were some performance upgrades to the head in '02 or '03. Forget the details, but a bike of that vintage with the added power might be a good buy.
This is an old pic of my Enduro. I have made many changes since this was taken. Now has all orange plastics, except for the tank which is now all black (just peeled the sticker off and stuck it to my garage wall). Removed the heavier racks and installed a billet aluminum rack and wings for soft bags.
I have ridden many long trips on this bike and hope to ride many more. It is not as erelaxing as riding my BMW R1150GS was, but it was really exhilarating.
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