I build custom high-performance Harley-Davidson motors (specializing in 106" and bigger motors). I have developed a carb kit for the stock Keihin (40 mm) carb. I have been trying to utilize the same principles for the Honda 400EX set-up, but on a smaller scale, of course. Here is what I want someone to try and to report back to this forum or e-mail me directly and tell me the results. First, you are going to have to get an aftermarket exhaust. Preferably, a non-tunable one like the FMF Powercore IV, Stroker SX-1 or Big Gun. Next, you need to purchase a K&N filter w/outerwear. Remove your airbox lid (do not drill it out, because you may need it on those wet days). Now for the carb. If you live in an area with an elevation of 3000 ft or less, try this. Get a few main jets starting with a #155 to #170. Get a #40 low speed jet. Remove the needle and set the clip down one notch to the 4th slot and replace (Good luck, the phillips head screws holding the needle are a real pain to remove). Gently bottom out the pilot screw and turn out 1.5 to 2.5 turns. Ideally, you probably want to run about a #160 main and have the pilot screw turned out about 2.5 turns. I have mine set-up similar but I am using an overpriced Dynojet kit and am looking to help guys out that can't afford this kit but want more performance. If we get enough guys to try this at different elevations, we can come up with some realistic specifications to help us all out. Now, go out there and get some jets, get her done, and get back here!


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a Fast Boy from Illinois -(FBI)- Riding a 2000 Honda 400EX