Can i use a bigger pilot jet with factory main jet

2 stroke or 4? if 2 stroke definitely jetted wrong choking makes it rich tells me your main jet is too lean (smaller hole) you need to pull carb check jets for plugage clean jets write down numbers check dealer for correct jet size for your elevation if 4 stroke back fire thru carb indicates timing is off or bad intake valves or cam lobe worn

You may have to check your plugs , I suggest getting a few new ones , and watch for color of plug ,it should be a nice tan color ..if not you may need to jet up or down depending on color , but for bigger valve's u will likely b going up, to take ADVANTAGE of these new valves you may wanna consider a nice carb a touch bigger in conjunction with an exhaust , mind u this is the most horsepower combination for the money and may not meet your needs . :)

The number 1 problem with ATVs is the carburetor. This could be a mixture problem... too rich will cause backfiring and difficulty starting as the carb needs more air. Try leaning it down a 1/8 turn. The choke is always intended to be used when starting; if you don't need to use the choke, the card is definitely running too rich. If you can't adjust to make it behave properly, then rebuild the carb, It's not difficult.. get a new throttle spring and float too. Ensure you place the keeper clip on the correct groove of the needle valve... where the throttle cable connects. Replace all the jets and gaskets... don;t get them mixed up. One larger jet is the primary jet and the smaller one, is the secondary. Get new screws, if the screws are a bit dog eared. Write down your previous mixture settings and start from there to adjust the mixture for starting and assure a slightly skipped idle. (Don't worry as you add throttle it will run smoothly.)

Go up 2 sizes on the pilot jet that's the little jet next to the main
but before you change sizes make sure that it's not plugged up hope this helps you

If you need further help, I’m available over the phone at https://www.6ya.com/expert/brandon_79b580f5a99c4386

sounds like you didn't get the pilot jet /air screw / idle circuit clean.
the symptoms sound like the pilot jet needs cleaning.. the jet has a VERY tiny hole that's easily blocked or dirty

Your pilot jet in the carb is plugged up or to small you will have to pull the carb out and clean it if you have a wire brush you can use 1 of the needles off it that will fit in the tiny hole in the pilot jet push it through and then spay carb cleaner through it and the main jet put back together and it should run fine

If you need further help, I’m available over the phone at https://www.6ya.com/expert/brandon_79b580f5a99c4386

Dirt may have clogged the main pilot jet try taking the carb appart and blow carb cleaner thew both jets while your at it

Hey Dave,
thought I answered this already,,,??? Classic "brass kit" symptom! >>> You need a "brass kit".. this is the needle jet and the jet needle in the center of the carb. The jet needle is the thin needle that hangs down from the center of the slide... The needle jet is the "hole" that the jet needle slides into...Remove the carb, take the bowl off, remove the main jet, the needle jet holder should be able to be gently tapped out, the needle jet is a short brasspiece,, it comes out through the hole where the slide goes,,, reinstall the NEW needle jet, needle jet holder...(it may have a groove in it for correct location). once its seated, you can reinstall the main jet..NOW >> to install a new jet needle,,, remove the slide from the carb.. remove the spring and seat from the slide that holds the needle jet in place.. NOTE which groove the clip is in on the old needle.... (should be the middle one). re install the new needle, reinstall everything else and your atv WILL run fine for a long time..This is a common problem on ATV's..
you need part #'s
1990 Kawasaki BAYOU 300 4X4 KLF300 C2 Carburetor

http://www.kawasakipartshouse.com/oemparts/p/kawasaki/16009-1503/needle-jet-n36w

http://www.kawasakipartshouse.com/oemparts/p/kawasaki/16017-1270/jet-needle-6

sounds like its running rich. check your float height and then lower your main jet needle a notch