I put an updated carb in my 99yz400 bike with a 99wr400 motor connected to a 06yz450 carb .That all went well until the carb started jumping out of the motor side boot so i forced it back in and held it their well tightening the clamp , it jumped out again and now i compare carbs more closely 970 stock carb 870 for the 06 450 carb lips on the carbs are different in the cut outs on the surface the 450 carb has a mid rib i got lazy and cut my collar on my clamp so i could get more bite on the neck of the new carb-it still pops out . I need to get this sorted out may be my old intake boot has seen better days and has hard'nd up and wont take shape on the new carb neck? Would a new boot from an 06 yz450 fit on to the 400 head? maybe its longer? Air box is forward -stretching mounting holes could help Maybe a better clamp on the boot? Cut that middle rib off the neck of the carb and it would fit the boot a lot better but its all stretched quite a bit as is when connected? lets hear some good fixes guys, this was supposed to be a simple swap and an improvement.