Motorcycle Safety & Driveability: Buying and maintaining a motorcycle, gearbox oil, japanese bikes

Question
I am in the market to buy a used 125cc motorcross bike. I'm not perticular on any type but i would like to get one from 2000 to 2003. Is there any perticular model that stands out? How do you adjust the forks and shock for a rider of about 145lbs? What do i need to know about maintaining a bike like this? eg. 2stroke oil/gas ratio, air filter, does the gear box oil need to be changed regularly?
What are some deffinite signs of abuse to look for?
 Thanks in advance for helping me I'm a little worried of looking dumb and getting taken advantage of so any info you could give to a rookie whould be just great!
                   Bryan, Canada

Answer
Bryan,

I've always liked the Yamaha and Suzuki 125's myself, but truth be told, all the Japanese bikes are very close in terms of performance, durability, and price. I'd just pick the bike you like best.

As for adjusting the suspension, that's not something I can really explain here (whole books have been written about it). Basically, start riding the bike the way you get it, and make adjustments as you see fit. That's how most people do it. There is no "perfect setup" for a 145lb rider- every rider is different. Ride it and play with the settings until it's riding the way you like.

For maintenance, I would change the oil and filter at LEAST once a year, maybe more if you are riding the bike a lot (every weekend or more). Always check the oil level after each ride once the bike is cool. Same with tire pressure. Always clean dirt and debris from the chain after each ride, and oil it after each ride with chain lube (not WD40 or engine oil). If the air filter is cleanable, clean it after each ride. Typcially, there is no "gearbox oil" in dirtbikes, the transmission used engine oil, so when you change engine oil, it all gets done at the same time.

Signs of abuse include bent rims, bent bars, dry chain, warped brakes, or just obvious poor physical appearance. It should start on first or second kick and not smoke when warmed up. If you have a friend who knows a lot about bikes,  you might want to bring them with you, they may be able to spot some problems you miss.

Best of luck, enjoy your new bike!

Bill Roberson