kx250f jetting

i have an 06 kx250f only mod is a yoshi slip on exhaust the bike is popping real bad on decel

the jets are 40 pilot and 180 main stock clip and unsure of fuel screw. would jumping to a 45 pilot do the trick and adjust the fuel screw or go with a 42 pilot?

Put the stock muffler back on.


Fuel Screw/Pilot Jet
Fuel screw settings in the 'book' are recommended starting points. Every bike is different, as is the temp and altitude. Set the screw according to this method. Do it with the bike fully heated up.
Gently turn the screw all the way in. Now back it out two turns. Start the bike and fully warm it up, go for a 10 minute ride. Set the idle to speed to 1,500~1,800 RPM as best you can (I know, without a tach this is tough, just set it to were it idles relatively smoothly).  Once warmed, slow the idle to the lowest possible speed.
*** When turning the fuel screw, keep an accurate 'count' of the amount you are turning it and record it in case you have to reset it for some reason. Makes life easier when you can just set it from notes Vs. going through the procedure again.***
Turn the screw in until the idle becomes rough or the bike stalls.
if it stalled, open the screw about 1/4 more turn. Restart it and slowly screw it in till you can just perceive a change.
If the screw can be turned all the way in and the bike still idles perfectly and does not stall, then you need to go down a size in pilot jet.
Now very slowly, open the fuel screw till the idle is smooth. Blip the throttle, let the bike return to an idle, wait say ten seconds. Confirm it is the same smooth idle.
If the screw has to be opened more than 3 turns to get a smooth idle, you need to go up a size in pilot jet.
If you find it does not stall with the larger jet but has to be open more than three turns with the smaller pilot jet, put the larger one in and set the fuel screw at 1/2 turn.
If the idle speed increased, adjust the idle speed knob to return the bike to a real slow idle speed. You must then re-visit the fuel screw. Keep doing this till the fuel screw is opened just enough to provide a nice steady idle at the lowest possible RPM. Once this is done, increase the idle speed to the normal one for your bike, typically about 1,850 rpm, but go by the spec in your manual.

Don't have the stock pipe bought the bike as is

Not much you can do. You cannot 'jet away' a problem caused by something else. If you do, other problems will surface.


  • Make sure you have no exhaust leaks (Hold a wet towel over the muffler, listen for hissing)
  • Re-pack the slip on.
  • Do the fuel screw adjustments I outlined.

I had popping issues on my 2006 KX250F also on decel too!! what you need to do first is take off the header and pipe and put it back on and make sure the clamp that holds the header to the slip on is on properly . It could just be a exhaust leak like mine, just make sure everything is bolted on correctly and check the pipe clamps!

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