Posted September 12, 2011 - 10:46 AM
Compliments of Thumperfaq.com for setting the fuel screw. I would set your fuel screw to 2 and a half turns out, then get your idle screw set properly, then start over with tuning the fuel screw again following these steps:
(You may want to put a fan directing air to your radiator when doing this to keep your bike from overheating)
When the pilot circuit is jetted properly, starting the bike should not be a problem. You will need the choke to start cold and it won't run well off of choke for 30 seconds or so. When hot you will probably need the hot start. The bike should idles smoothly and have no hesitation of idle and minimal backfiring on deceleration. For more info about the pilot screw, see this article that recently appeared in Motocross Action Magazine.
Setting the Fuel Screw / Pilot Jet "by Ear"
Adjust the idle with the black knob until it is too fast. Then adjust it back down until it is around 1900-2000 RPM or if you don't have a tachometer (see below) until it sounds just a little high.
Before you start adjusting, count the turns required to tighten it up lightly.
Then start the bike with the slightly elevated idle and turn it out 1/4 turn, 1/2 turn, 3/4 turn and so on until you get to 2 turns. Listen for best RPM and best response to a quick 1/4 turn tweak of the throttle at each position of the fuel screw.
Now turn back in 1/4 turn at a time doing the same thing. By now you should have been able to distinguish the speed of the idle and the responsiveness to tweaking the throttle.
If it gets better between 3/4 and 2 turns out, set it at the best location and leave the rest of the pilot circuit alone.
If it is getting better turning it in or is best less than 3/4 turns out, replace the pilot jet with a smaller one and go through this procedure again.
If it is getting better as you turn it out or best at more than 2 turns out, replace the pilot jet with a larger one and go through this procedure again.
Setting Fuel Screw / Pilot Jet with Tachometer
Warm the bike up by riding about 10 minutes. Place it on a stand, have it idling. If you have a fan, direct it into the radiators (A YZF will start to boil out if you take too long to do this, WRFs have a nice catch tank).
Turn the fuel screw 1 1/2 turns out. Read the RPM for about 10 seconds (on my tachometer, cause it bounces around). If the avg RPM is not between 1700 and 1900, adjust to about 1800 with idle screw knob on carburetor Write down average RPM.
Turn the fuel screw 1/2 turn out. Write down the average RPM.
Turn the screw 3 turns out. Write down the average RPM.
If #2 is greater than #3 or #4, you have the right pilot jet. Usually the difference will only be 50 to 100 RPM. Go on to step #7.
If #3 is greatest, you need less fuel. Install the next smallest number pilot jet. Go to step #3 and repeat.
If #4 is greatest, you need more fuel. Install the next largest number pilot jet. Go to step #3 and repeat.
Adjust the fuel screw in 1/4 turn increments around 1 1/2 turns out and find the maximum RPM fuel screw position. If the idle is now above 1900 RPM, turn it down to be in spec.
If you get a little deceleration backfiring on closed throttle, try adding another 1/4 turn out. Remember you will need to redo this if the altitude or temperature changes significantly.