How do I find the best idle?

I have a 2002 yz250f and when I first get it started, ill hit the gas and it bogs then revs up like it should.. I have my fuel/air mix at factory setting and when I mess with the idle screw it's either too high and races when I click the bike in gear or too low when I put it in neutral to let it idle... Idk how to go about this problem and how do I get rid of the bog problem??? Please help!

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.


It can be a fine line of adjustment. !/16th of a turn can sometimes be all it takes.


Bog is something else (and not the fuel screw). If it only does it in the first fifteen minutes of runtime, then your bike is fine. Remember you have a race bike, not a trail plonker.

Ok so when I first start adjusting the fuel/air screw, how should I have to idle screw set to begin with?

Re-read what I wrote., Ten times.

Slow idle speed. Hot engine. Adjust fuel screw. Slow engine speed, adjust fuel screw.........................

Ok gotcha thanks

Would you be able to tell me why my bike back fires and sputters when I let of the throttle? My valves are in spec, could it also be the fuel/air mix causing that? It blows white smoke when I first start to warm the bike up if that helps any?

White 'smoke' is condensation in the exhaust, assuming your do not have a head gasket problem (leak down test to check, or a cooling system pressure test).


Decel backfire is somewhat normal. If you have a non-stock exhaust, it will be even more pronounced. Confirm the packing is good and that you have no leaks. Otherwise, ride like a race bike and do not putt around.

There are no leaks, I made sure of that when I first got the bike and I re packed the exhaust last month so I'm guessing it's normal then

There are no leaks, I made sure of that when I first got the bike and I re packed the exhaust last month so I'm guessing it's normal then

3-5 decel pops are normal and shows you're close to optimum. More than that is a lean problem or an air leak.

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