Dirt Rider Magazine

I am not so sure you are ready for a 250 with the RWHP of around 45. I think you might do better on a 125 with around 30 RWHP to build your confidence. That may piss you off, but a 125 is a good jump from a 100 in terms of RWHP. My son is your size, roughly, and a little older and needs nothing more than the YZ125 he rides, and he rides at a level you do. He rode my 300 and wrecked, pretty bad, fairly quick because it was too much for him. If you want more power, go up a step to the 125 and not two steps to the 250.

i tried my buddys 125 yz and it feels like you have to have it pinned to make the most out of the power, but i may even consider a 250f because i dont have to spend time mixing oil and gas i can just fill up with gas and take off but i want something i can have for years till i grad so i can buy a sled eventually. my bud is riding a lil drz 125 lol.

Is a 125 as fast as a 250f?

When you stated you want to ride and go tells me you do not need a 4t at all. They require more maintenace in the way of being spot on with your tolerances in far more aspects than a 2t. Get the 125 2t as what EEE299 said is true, all 2t bikes need to stay pinned to be ridden properly.

They're all good when they come out of the crate, so you'll need to judge each used bike on an individual basis. Some people take care of their bikes, and some don't. When you go to look at a bike, don't be in a rush, don't be pressured by the seller, and ask lots of questions.  It helps to make up a list of questions ahead of time so you don't forget to ask something and this way you're asking each seller the same thing.

Valve clearance, valve shims, cam chain specs, cams, valve spring height/length, cam timing, cam wear, timing overall, and a good deal of other aspects you never see in a 2t bike due to less moving parts. Plus, the bike is slow due to a rider. A good rider pulls similar times on the same track using any 125 or 250F, the 250F just has more torque and revs longer, but does not always produce more RWHP. If the 125 seemed slow compared to a 250F, it was probably due to the rider. I am sure there are riders on this board who could go much faster than I could on a 125, and me on my 300. The rider makes the bike, not the other way around.

Rookie question, whats RWHP

how much does a kx100 have

The KX100, stock, makes around 23-26 RWHP. A 125cc 2t bike makes around 32-35 RWHP, stock, while a 250 2t produces around 45-50 RWHP in stock form. These numbers also depend on the make, model, and year also. All of these numbers come from a quick Google search.