I have a klx 250 with a big bore 350 kit and a keihin pumper carb on it. It runs well and has lots of power around town, but when I get on the highway, and try to do sustained higher rpm's, the carb will run dry and the motor will sputter. I can pull in the clutch, and it seems the bowl refills and then I go on my way. It seems I can go approx 70 MPH, but any faster and my fuel burn exceeds the fill rate of the carb. Before the big bore I could do about 90 highway with no issues. Any ideas on what the problem may be? Float bowl height(I'm guessing probably not), vacuum issue? I'm stumped. Haven't run into this type of a problem with a carb before.
Clogged/blocked/kinked float bowl vent tubes? I would check the hoses first (easiest), then pull the carb and check the float height.
I had a similar issue with my SV superbike at the end of straightaways. I switched from the vacuum fuel pump to an electric fuel pump. Don't know much about a KLX, but maybe something to consider?
first make sure primary is clean. are you using an inline fuel filter? cause it might not flow enough. adjust the bowl height or put a couple steps bigger primary in it. make sure the tanks vent hose isn't blocked creating a suction effect. ...or richin it up and see $.02
