Dash lith

needle valves are not interchangeable if one is longer than the other

If low performance was the original reason you took it to a repairer, it is clear they have failed to discharge their contract with you and you have grounds for complaint and to expect the repair completed for little or nothing extra.

There are a number of possible reasons why an engine that runs fine would be down on power and if the fuel system really needed that big clean out, it could still be fuel related. A carb and fuel system that was in a bad state often needs cleaning a couple of times, especially if water was a problem.

Other possibilities are a strangled intake, a strangled exhaust, low compressions especially due to bad intake valves or bad valve timing.
Another (unlikely) possibility is that of ignition timing which will likely be electronic with programmed advance characteristics. It is unlikely but it is possible the programme could be damaged leaving the ignition timing too retarded at speed to produce proper power.

It is also worth considering things like tyre pressures, binding brakes, wheel alignment...

Float metering needle is sticking causing it to overflow back into the block has dirt causing a needle to stay open when you shut it off

It needs to be calibrated, you can't do that at home.

Hey Heather,,,

Sounds like the throttle position sensor is bad. It won't let the engine start while it is "sensing" wide open throttle. This is something for the repair shop... 70 miles.. Warranty???

Take the float bowl off the carb. Pull the pin that holds the float on. The needle is attached to the float. The needle just came out of the needle seat. The needle seat can now be pulled out. This is the fun part. Are you really sure it need replaced? If it is needing a good cleaning, take some Mother's aluminum polish and a cotton swab and put it in the seat and spin the heck out of it. It will polish up nice. Be sure to clean with carb cleaner to get any extra polish out before use.. reassemble.

ok.. clean around the spark plug really well.. you do not want crud falling into your engine when you install the compression gage..
Make sure your spark plug wire is out of the way..
Once you have the gage screwed in..(MAKE SURE THE GAGE THREADS ARE THE SAME LENGTH AS YOUR SPAK PLUG!!).. hand tight! Hold the throttle wide open and turn the engine over for about 3-4 seconds.. it should read about 120psi+.. I have seen a single cylinder engine run with 88 lbs. before..but if you have less than 100psi.. its time for some investigation.. it could be the valves not seating (warped - too tight)..or just normal wear for a 20 year old atv.
New piston/rings.. complete valve job,. carbon clean out.... you should be ready for another 20 years.. KEEP the air filter clean and the box sealed!!

Next step is inspect the float needle valve assembly (needle connected to float, and shuts off fuel flow to float bowl when float rises high enough).
Some needle valve have a rubber tip on them that goes bad over time.
Or, the needle or it's seat are just worn out, not fully shutting off the flow of fuel.

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

The float needle is stuck open. Typically the needle can get a speck of dirt or the float adjustment arm is not properly aligned. The arm should be able to close the needle at approximately 90 degrees. Remove the float bowl and look at the carb from the side and adjust the arm to seat the needle when the floats are about where they would be as if the bowl were full of fuel.
Good luck.