This is my prefered method of finding the proper setting for the pilot screw (aka fuel screw). This method can also indicate if your pilot jet is too big or too small. Towards the bottom is an explination about the difference between fuel screws and air screws.
You'll need to be able to access the fuel screw while the engine is running. This can be quite difficult on many carbs. FCRs have aftermarket fuel screws that are longer which helps. On a stock 400EX carb (and many others) I use a 90 degree screwdriver. I bit the bullet and bought this one from MotionPro. It works well but I'm sure there are others available. If you're lucky, your fuel screw is easily reached with a normal screw driver.
http://www.azjetski.com/catalog/images/03MotionProPilotAdjustSM.gif
Here's a couple examples of aftermarket FCR fuel screws:
http://www.roostracing.co.uk/images/Pulse/fcrscrew.jpg
http://www.xrsonly.com/content/components/com_phpshop/shop_image/product/3c154d81101dad29998bc1e6e1247c7e.jpg
OK. On to the procedure!
1) Warm up the engine to full operating temp.
2) Turn up your idle a few hundred RPM using the throttle stop screw (basically you want a fast idle). This will make it easier to hear small changes in RPM. Watch for overheating--pointing a big shop fan at your engine will help it from getting too hot during the fast idling. The whole procedure shouldn't take too long though.
Each time you change the screw setting 1/4 or 1/2 turn or so, wait about 5 seconds to let the idle speed normalize. It usually take the carb and engine a moment to react to the change.
3) Turn the fuel screw IN until the idle starts to drop and miss. The engine should die if you bottom out the screw. Your pilot jet is too big if it doesn't die when the screw is bottomed out--it should die before it gets that far in.
4) Then begin turning the fuel screw OUT. The idle should peak and become smooth. Keep going and look for the idle to begin to drop/miss again.
5) The goal is to find the setting that provides the highest and smoothest idle. If it's unclear exactly were that point is then set to the midpoint between step #3 and step #4. For example, if the idle starts to drop at 1 turn out and starts to drop at 2 1/2 turns out then 1 3/4 of a turn out should be the correct setting.
If the peak/smoothest RPM is reached somewhere between 1-3 turns then your pilot jet is correct (the 1-3 turns applies to most carb types). If you end up less than 1 turn out then your pilot jet is too big and you need a smaller one. If you end up more than three turns out or the fuel screw seems to make little difference as you continue turning it out than you need to go up (bigger) on your pilot jet.
To re-emphisize: If the idle never drops when you're turning the fuel screw in, you need a smaller pilot jet. If the idle never drops when you're turning the fuel screw out, you need a bigger pilot jet.
Typical fuel screw settings are in the 1 1/2 to 2 1/2 range.
6) Once you've got the fuel screw set, re-adjust your throttle stop screw (idle screw) to an appropriate idle speed.
And that's it! Your pilot circuit is now VERY close to ideal. From here you can experiment with how small adjustments affect low-end (i.e. small throttle openings) response and make adjustments for weather. The hardest part is usually gaining access to the screw while the engine is running. Also, you may really have to listen carefully to detect the rpm changes in the idle. 100 or 200 rpm differences can be tough to detect when the change happens over several seconds.
And lastely, a little bit about the difference between fuel screws and air screws:
Two stoke carbs normally have air screws and four stroke carbs normally have fuel screws. You can tell if a carb has an air screw or a fuel screw by it's location on the carb. An air screw will be on the intake side of the carb while a fuel screw will be on the engine side of the carb.
They sort of work opposite one another. An air screw adjusts how much air is being delivered thru the pilot circuit: in is rich (less air) and out is lean (more air). A fuel screw adjust how much fuel (or air/fuel mixture) is being delivered from the pilot circuit. In is lean (less fuel) and out is rich (more fuel).
This image depicts a carb (a Dellorto in this case) with an air screw:
http://img230.imageshack.us/img230/3148/fig19gi0.jpg
1 is the air screw
5 is the pilot jet
blue area is air
orange area is fuel
green area is air/fuel mixture
This image depicts another Dellorto carb but this one has a fuel screw instead of an air screw:
http://img230.imageshack.us/img230/7382/fig18hd2.jpg
5 is the fuel screw
6 is the pilot jet
blue area is air
orange area is fuel
green area is air/fuel mixture
