my yfz 450 wont start
 

my yfz 450 wont start. it cranks for like a half a second and then it stops for half a second and then it starts again. i have no idea what could be wrong. maybe the starter? it has a big bore in it if that matters.

Could be the battery if you are still running the stocker. They don't have a lot in reserve power to start a big bore in the cold. Have you tried a new battery or at least jump starting it? I modify the battery section of the rear fenders to hold a larger batt, I think it is the one for a 350 Warrior/CBR600 that fits with a little "plastic sculpting." If it is the battery you might want to look into the new lithium types. Spendy but smaller and more powerful so you might not have to modify much.

I just put in a new battery.

In that case I would check the basics(plug, fuel, etc.) and then check the timing/valve clearance. What year YFZ do you have? How many hours since the BBK was installed?

It's an 04 and I don't think very many hours are on it. What do you meen by the timing?

By timing I mean you have to open the timing covers on the gear shift side of the engine and put it at TDC. Then you have to remove the valve cover and check the alignment marks on the cams. This needs to be done anyways before you can check valve clearance, which is a likely culprit. I had retarded the exhaust cam one tooth on my brothers '04 for him(popular for a bit more mid/top end power) He eventually had a shop rebuild his top end and install aftermarket cams. They also retarded the aftermarket exhaust cam one tooth and it acted just like yours until I put it back to stock timing. There are other possibilities, but valves are one of the first places I would look on a 450.