Hey guys! I just started messing w/ a 4t 139qmb. First thing was a 50mm kit w/ head. The stock vacuum carb should be tossed right away. I am using a 19mm delorto slide carb from an old 2t, and it works like a champ. The ONLY draw back is if you "snap" the throttle full open there is a split second bog due to no accelerator pump. If you ride smooth you will never know this. Adding a pipe will look/sound good, but no real help there. Ohhh the intake from a 125 qmb engine will work, or you can burn off the rubber on your stock intake. The rubber will flake off when toasted, and you will have a new aluminum intake to adapt any carb you want to it. Bottom line is a slide type carb 19mm-24mm will be the ticket. Jetting is anyones guess. I have a 40 pilot, and 95 main in mine. 53-54mph top. 6gr rollers/stock clutch setup. A good long term help for the kit is to drill oliers in the wrist pin bosses, and mirror polish the dome. The rough cast dome will hold carbon until it becomes a problem. ps. I am using Castrol Syntec straight 30 weight for this engine. It spins up like a sewing machine!