I just bought a full pipe a K&N and a jet kit. The dealer said I couldnt replace just the main jet your better off buying the whole kit which includes main, pilot, needles and springs.I didnt pick up the jet kit yet because it is on backorder. Does the kit come already setup for my system or does it come with lots of jets that I have to see which one is best.and also is it just unscrew the old and in with the new and be on your way. I need help

 

Not everybody agrees with your dealer on that jet kit issue. Your jet kit will likely come with an assortment of main jets, and an adjustable needle, maybe an extra pilot jet. Maybe somebody who owns a 300ex can give you specific info, but it may not work if their elevation or climate are different. I can tell you from long time shop experience that well over half the atv's and bikes I ride at work with jet kits run like POOH. I realize frequently it's because "the jetter not doing the homework" , but I don't like a lot of the needles that come with jet kits. Also they are (unless you forked over the special ones) aluminum and seem to wear really fast. I have seen too where they use main jets that fit, but are different calibration numbers than Keihin uses. I hate it when they do that. You aren't that far from bone stock. I would begin by turning your mixture screw out to about 3.5 turns and ride your machine. If your changes are really flowing more, you can assume that you need more gas. Go buy the next two sizes up main jet. Try each. You are looking for wide open perf. here. If it runs better with the richest jet you have, try the next size and so on. When you get that dialed in, assess your half throttle performance, roll ons at partial throttle settings. You might try changing the e-clip one notch lower (shortening the needle slightly to go richer) or if your needle isn't adjustable shim it by putting washers (Honda 3mm plain washers usually work for this) underneath. Your changes here should be about 20 thousandth of an inch at a time. Now go back and set your mixture screw and idle speed again. Adjust the mix screw at idle for the highest rpm, you should be adjusting your idle speed **** as you go along. If you find that it idles best at over 4 turns out, try the next size richer pilot jet, and reset your mixture screw of course. Resist the urge to make more than one change at a time. And always take your time and do good work. If you go thru this process you should have a VERY good running 300ex. Your cash outlay , maybe 20 or 30 bucks for jets, try to buy o.e. Keihin jets as they are most accurate. Keep in mind you will have to go through this same process with your jet kit.

 

Thanks for the great info but I have a few more questions. Does the carb have to come off each time you change the jets. How does the dealer do it in so little time? On my last 300 I didnt change the jets and it ran like crap I was to scared to change them people told me to stay away from the carb . I sold it and the motor went 2 weeks later. So I want to do this one right by the books. Thanks for the great info.

 

It doesn't take the dealer long cause it's easy to do, and they do it all the time. You need to remove the carb and float bowl to change the main or pilot jets, you can remove the needle by taking the top off the carb, mixture screw and idle speed adjustments can and should be done externally, while the machine is fully warmed and idling.