I've been screwing around trying to get my 426 to run smoothly on the road (supermoto) for four years. Finally just broke down and bought a 03 WR450 carb on ebay put it on and bingo. The problem was at 1/4 to 1/2 throttle cruising on a flat road doing 45 or faster and having a surge/hesitation that was really anoying, especially going through turns on the street. The old carb had excellent acceleration and top end. When dirt riding I never noticed the problem. I ripped the two carbs appart to compare and the only major difference I noticed was that the starter jet on the old carb is 65 and 78 on the new. This is a Canadian carb. I think it's 72 on US models. Does any one know what the starter jet is doing?