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This won’t be the last time you hear from me. Hope I’m in the right area. 99KDX 220r. eats fuel like a drunken sailor. Smokes like a freight train. Stock main and pilot. Oil oozing from cylinder back. Starts easy. Low end good. Mid is a little sketchy. No top end unless ran hard, still struggling. This is my boys bike. He is 14. Decent rider. I have not opened the motor up yet. Willing to do whatever it takes. Has an fmf gnarley pipe and fmf silencer. Having trouble figuring which model. Does not foul plugs not sure how. To idle correctly the fuel air screw all the way in. This bike is a teaching experience for my son. I dont care what has to be done but he is paying. Keep in mind what a 14 year olds budget looks like. Looking for solid advice thanks.

My suggestion is to disassemble and clean the carb thoroughly (do NOT dip or soak) noting what main and pilot jets are in it now. While the carb is off, remove the reed cage and check that reeds are good. Clean the air filter and oil it properly. Buy two new spark plugs and replace old gas with new premix. Then go out and check the jetting. The fact that you have to close down the air screw indicated it is not currently jetting correctly or the carb is dirty. Search the jetting procedure on these forums and do not take any short cuts. If after this it still runs poorly, come back and we will get into other possibilities.

Take off the carb, inspect and clean.

How's the air filter?

Which spark plug?

What is the fuel and oil ratio and brand?

Are the reeds OK?

I have cleaned the carb well. Twice. B Fuel mix,started at 40:1 went to 50:1 couldn’t tell little difference. Used klotz benol 40-45:1. With the 50:1 used an injection oil for my beta 300 motul not for sure the exact oil at this time. I will check. The air filter in new pre oiled w oem lid and snorkel. Stock plug cant remember the number at this time but it is new. The plug google siad to run. I have yet to inspect reeds

OK, sounds good so far.

NGK BR8ES is the stock plug.

Really first thing would be to make sure everything is mechanically sound. Leak down check to assess top end and crank seal health. Then pick a mixture ratio and don't change it and jet to that.

Oh, and since its a learning bike for a future mechanic...service manual and a torque wrench.

Just,
I have same model year 220. I rebuit top end & re-jetted carb. Yup, these are thirsty motos

Justlooki, is this problem fixed? These KDX 220"s get only 20 to 25 mpg. I was getting less than 20 mpg before replacing carb jet block gasket ("o-ring").

just reread this. If you have to run the air screw all the way in, the carb is not jetted correctly. Start with the basics.
(A good lesson for a new mechanic).

If your crank seals are leaking the jetting will not matter. If the crank seals have not been replaced, they need to be as they are over 20 years old.