I would install a fresh spark plug thats gapped correctly, and clean your air filter, and drain your fuel tank, check your fuel flow, and put in some fresh premix with a good quality fuel, and good two cycle oil, and mix it accordingly.
I really think it has to do with carb wear, or being dirty somewhere, and/or needing rejetting. To the OP, you can measure the inside diameter of the intake side of your carb, not the air filter side, and find out what size it is. Good luck, and get that thing running right, as they are too much fun. Coming from alaska i would say it was jetted quite fat for the cold air, and the elevation changes would make it different also. If your choking out at high speed 3/4 and up it is the MJ being to large. If you do have the 38mm maybe you could contact CT, and see what jetting specs they recommend for a starting place for that set-up, and they might have a carb rebuild kit for it also. I would double check the carb, and make sure what it is, and check for air leaks in the boots, and also check the reed valves for wear, and do a compression test just for maintenance sake. Heres the link to the CT site that shows what came with that kit, and the carb listed is a Keihin 38mm pwk carb. Regardless of whether you choose the Keihin or the Mikuni take the time to keep notes on your jetting until it becomes second nature.I bought the fourtrax 250r new back in 86 and it came stock with a 34mm keihin carb, so if your saying that 310 kit has a 48mm on it, (I'm not sure if they even made one of those)that has to be the issue as that is way to big for that set up. The needle 676BBH173 was spot on in the center clip position. It comes with a 420 main and a 35 pilot both of what were rich in my ESR 330. If you do decide on the Mikuni carb, the one to get according to Eddie Sanders is the 2001 CR 250 one directly from Honda. I have always taken notes on the jetting of both our quads with the Keihin for all 4 seasons of the year that we have here in Wisconsin. I ended up recently switching to the 2001 CR 250 Mikuni TMX and just love it. It did take a little bit to get it dialed in but I found the TMX Mikuni did help with a better low end power with my particular setup. Over the past couple of years a friend of mine suggested I try his Mikuni 38 TMX carb. Quote from: flynbyu 68075 I have ran a 38 Keihin A/S on both my sons 250R and also on my 330R for quite a few years and have always been satisfied with that carb. Regardless of whether you choose the Keihin or the Mikuni take the time to keep notes on your jetting until it becomes second nature.
I have ran a 38 Keihin A/S on both my sons 250R and also on my 330R for quite a few years and have always been satisfied with that carb.