I have a 400 scrambler that seems to be running rich at full throttle. If i run it at full throttle for about a half mile the motor will start to skip and cut out. My plug was a light tan color when i started out. I dropped my main jet down one size at a time but it's still cutting out after about a half mile. However when i checked the plug after dropping the main jet down it was almost white. Could it be an air leak? I'm have a keihin 39 pwk carb, hotseat pipe and delta v-force reed cage.

 

Albino,
Sounds like a running out of gas problem to me, by what you explained.
I too had a problem of it cutting out after about a 1/2 mile of wide open throttle, but would run all day without a skip as long as I didn't run wide open. I found that the clear breather hose was rubbing against the exhaust and melted shut.
I always disconnected it at the carb and never looked at the other end when rejetting and adjusting the float to try and fix the problem.
Just something to look at.

 

I had the same problem about a week ago, I changed my pilot jet to a 35, and it ran awesome. Now it flies!!

hope i helped

 

my jetting is a 230 main, and a 35 pilot

 

Check the fuel filter also. If there's a little bit of trash in it, it may be enough restriction to allow the engine to lean out on a long full throttle blast.

 

I think it has something to do with my fuel petcock because i unhooked the fuel line before it gets to the fuel filter and the gas wasn't flowing very fast. I let all the fuel drain out of the tank (reserve too)but when i unhooked the short hose going from the tank to the fuel petcock about a cup or so of gas flowed out really fast.