Stock carb on a LT250r is a 34mm Mikuni TM. If that's what you are getting, its a pretty decent carb. Is your engine's top end ported? If so, it will love that larger Mikuni carb. If it isn't, it will still work fine since you are running a pipe, improved airbox, and stroker crank.
I run this same carb on my white Blaster with a 35 pilot and a 290 main. A 5FP17 needle will work for you, but you'll need to lower it one notch from centered for the engine to transition off of idle correctly. I'm experimenting with other needles in mine, and I'll let you know when I find the right one for the Blaster application of this carb. As of now, the 5FP17 needle is too rich right off idle and a bit lean at 1/2-3/4 throttle.
Your existing throttle cable (provided it was used with the Keihin PWK28) will work fine on the Mikuni carb. You may want to cut a couple coils out of your throttle return spring to ease the stress on your thumb.
Another note about a 34mm Mikuni TM carb: They bolt right up to your engine, but you'll need a reed spacer to get the large float bowl to clear the clutch arm. Also, you may have to grind a little bit of the carburetor casting down to get it to clear the lower left intake boot bolt. Its not hard to do, but be prepared to do it. Just FYI.
Give Ken O Connor a PM about this carburetor. Last I heard, he runs an older TM carburetor on one of his Blasters, so he has experience with tuning them for best performance.
