First and foremost, order a Keihin FCR OCEMN needle. This can be ordered through a carburetor supply house. While you are ordering that, get 168, 170, and 172 main jets (or 160, 162 and 165 for a 520). It was recommending to me to go down to a 165 main for the 400 EXC at 5,000ft + altitude. I'm going to order an OCFHN needle and give it a try, as there seem to be claims that it offers more snappy throttle response. Make sure all your carb vent hoses are well clear of the header pipes. If one melts closed it'll make your bike run poor.
General Jetting Rule of Thumb
- Higher altitude: less air, jet leaner
- Colder temperatures: more air, jet richer
Humidity and barometric pressure also affect jetting needs, but this isn't a two-stroke GP road-racer. I use a digital weather station and an air density algorithm to jet my RS250R (shown at right). If your bike runs better on a cold day, you could try leaning out a bit when it's hot. If it runs better when it's hot, try fattening it up in the winter. The colder temps (and more air) will make any bike feel stronger than even an appropriately-jetted bike in the heat. The jetting change only orients the fuel mixture to the air density, it cannot compensate for less air. So if you ride the coastal mountains, then take your bike to the Rockies, expect it to feel like a slug even if you jet it right, it'll just be less of a slug than if it were still jetted for sea level!
KTM Keihin FCR Carburetor Accelerator Pump
Next, the tricky part of the carburetor modification, the accelerator pump. Pull the carburetor out (it helps to unbolt the two lower sub-frame bolts and swing the sub-frame up), re-prime the bowl by attaching the fuel line and opening/closing the petcock, detach, then point the carburetor intake manifold end AWAY from you, the kids, the fireplace, etc and twist the bell-crank (what the cables turn). Turn and hold, and either you will see the accelerator pump squirt fuel halfway across the garage, or up your nose and down your shirt, depending on the orientation of the carb (speaking from experience). It should flow for a good 3+ seconds, which is TOO MUCH FUEL. Now, put the carb down on its choke knob side so the bell-crank is facing up. Note the metal linkage running from the bell-crank to the black plastic dogleg, actuating a plunger going into the bottom of the carb, that's the accelerator pump. Next, note where the black dogleg stops against a metal fork (red arrow).
That fork can be carefully pinched closed. This limits the amount of travel the accelerator pump will have, and will shorten the squirt time. We want our squirt time right around 1 second, but I think +/- 20% is close enough. I started by pinching this fork stop all the way closed and resulted in no squirt (be sure to re-prime your bowl after each squirt). I then took a small flat-blade screwdriver to the pinched gap and started opening it back up little by little, then timing the squirt until I got 1 second. This will yield noticeably better behavior when you snap the throttle open. The bike won't be so prone to bogging or be so soft when you want to pop the front end over something. No, this is not precision engineering, but it yields the same result people are paying $80 for the P-38 for. Before you put the carb back on, set the fuel screw (up under the bottom of the carb) at 1.5 to 1.75 turns out. If you have a #45 pilot order a #48. When you get the OCEMN, set the clip on the 4th or 5th notch from top. The fuel screw, needle clip #, and even main jets listed here are only a starting point. If you have an aftermarket pipe or ride in high altitudes you'll need to try different jets for yourself. Just a rule of thumb, your fuel screw has the greatest effect on right-off-idle, your needle clip and taper effect the mid-throttle, and the main's biggest effect is at WFO. Each change can dribble into adjacent ranges.
Important: When you pinch closed the fork, focus on bending in the fork side that's butting against the dogleg, this is the only side that matters. To clarify, bend the right fork side TO the left side rather than simply pinching them together, as that may not yield enough movement on the right side fork. What we're trying to change is WHERE this fork stops the dogleg. With the carb off, the bowl primed, and these changes made, you will be able to time a shorter squirt and adjust the fork accordingly.
New Information: I did a guy's accelerator pump in a campground and found a LOT of mud on top of the diaphragm. The little rubber boot doesn't seal the plunger worth a damn and I don't know what you can do about it, so until someone comes up with a solution I recommend you pull your pump plate and diaphragm off to clean it once a month.
KTM RFS Case Vent Hose Modification
Another cheap modification that made a noticeable difference was to disconnect the case vent hose from the carb, cap the carb, and install a standard automotive inline fuel filter and some more hose down past the swing-arm. This vents the blow-by from the piston to outside rather than back into the intake, the filter just keeps dirt from getting in. Notice in the photo my non-committal approach, I didn't even cut the 90-degree bend off the vent hose!
Update: I bought one of those cute little K&N case filters because every time you dump the bike with that paper fuel filter, it gets oil-logged and stops breathing.
Credits: KTM RFS Carb Jetting article by Jon Delameter. Edited and published by 4Strokes.com.