i dont doubt for a minute that the kehin is the better carb but the bike should run properly with the mikuni if its jetted right. remember that if you cant jet then trying to switch to another brand carb will be hit and miss also. you say that your problem only occurs in first and second gear with full throttle. well in lower gears there is less load on a motor but it can rev out so much quicker than the higher gears so you are definitly using the main jet circuit in the carb, not the needle or its position. try leaning the main jet one size at a time and see if the problem lessens. do a plug check (see jetting links for this) to help determine jet size also. from memory the stock main is a 380, i have a 390 in mine and it runs great but jetting depends greatly on the condtions the bike is ridden in. so what will be right for your bike will take some trial and error to find, good luck.