How do I tell if pilot screw needs adj.?

7 replies to this topic
  • 4:20six

Posted January 16, 2002 - 12:34 PM


Still kinda new to the 426 seems like a lot of you guys are constantly adjusting the pilot screw. I re jetted for altitude(5000ft) by changing the main jet as stated in the manual to a 158 main, and 1 1/2 turns out on the screw (stock was 1 3/8 so I barely turned it at all). How can I tell if that's optimum? Will I just feel it in the power, or are there specific symptoms that the engine will have if not properly adjusted (e.g. bogging, stalling etc.). The bike seems to run fine and I haven't fouled the plug since the re-jet, so basically how do I know if I need to mess with thew screw? Thanks Guys, this site rocks.

  • skthom2320

Posted January 17, 2002 - 03:27 PM


Turning the fuel/pilot screw in will lean the mixture off idle (0-1/8 throttle). Too lean will result in slight (or not so slight, depending on the situation!) popping on deceleration and/or an idle that "hangs" too high. Turning the screw out will richen the mixture. Too rich will have a sluggish response when blipping the throttle off-idle.

Note that you may not experience both of these conditions (too rich or too lean) with a single pilot jet unless you are jetted perfectly now...

The response from blipping the throttle should be "snappy" with no bog, etc.

Since you already know your stock settings, just experiment (but don't go past 2.75 turns out).

Good luck,

Steve T

[ January 17, 2002: Message edited by: skthom2320 ]

[ January 17, 2002: Message edited by: skthom2320 ]

[ January 17, 2002: Message edited by: skthom2320 ]

  • Bill

Posted January 17, 2002 - 04:53 PM



Turn the fuel screw in until the motor burbles and then (while counting turns) turn the screw out until the motor burbles. Then turn the screw back, half way between. Does that make sense?

Then ride the bike. If it runs well (no popping on de-cel or bogging on accell) your ok. Then turn the screw in (while counting) and you should not more than 2.75 turns out. If you are, go up one on the pilot jet and start with the fuel screw at 1.25 turns out.


  • Boit

Posted January 17, 2002 - 08:39 PM


Good responses to 4:20six's question. In the carburetor jetting section of the manual, there are some very helpful drawings of how each jet, needle, clip position, and throttle valve affect engine operation as you open the throttle. At first glance, it can be very confusing, but once you get an understanding of what each circuit does, the drawings begin to make sense. As an example, look at the pilot jet drawing. It looks very similar to a valve. Notice that above 1/4 throttle opening, it's effect quickly becomes nearly constant. As long as the engine is running, fuel is ALWAYS flowing through the pilot circuit. Changing the pilot jet size has a resulting small affect above 1/4 throttle which can have a fine-tuning effect at 1/2 throttle and above. Conversely, you can see by the main jet drawing that this jet has absolutely no affect at throttle openings less than ~1/3. My personal experience has been that once I have the proper pilot jet selected, I make small tuning adjustments with the fuel screw according to relative humidity and ambient temperature. For snappy throttle response, I adjust the fuel screw to give a slight popping on decel. If it's a cold day, I usually will increase to main by one size to compensate for the denser air. The main jet is very easy to change. Just drain the fuel bowl, take off the bowl bottom plug with a 17mm wrench and use an 8mm socket to remove the main jet. The pilot is recessed further and requires a small flat blade screwdriver and a small mirror to remove.

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  • Ga426owner

Posted January 18, 2002 - 05:37 AM


Boit, good point as temp. changes definitely effect how my bikes run. On colder days I will adjust as on hotter days(to richen or lean). The symptoms that you will notice that tell you, you need to adjust are popping from the exhaust usually on decelleration. & Sluggesh throttle response. Also the kouba is the easiest tool to adjust air/fuel/pilot screw with........

  • Scott_F

Posted January 18, 2002 - 10:00 PM


Boit, FYI, the main jet is 6mm. A 1/4" also works.

Are you saying you can change the pilot without removing the bowl or turning the carb?!?

  • Boit

Posted January 18, 2002 - 08:37 PM


Scott. Thanks for catching my mistake. It IS a 6mm for the main jet.

With some patience, I can change the pilot jet without removing the bowl or turning the carb. The mirror is essential for getting the little screwdriver in place.

  • MN_Kevin

Posted January 19, 2002 - 01:28 AM


A Kouba T-handle will PERMANENTLY fit onto your pilot Screw (fuel screw), and will not require you to fumble w/ those stupid little screwdrivers!

I am making THE FINAL KOUBA ORDER OF ALL MANKIND right now. Jump in if you want one.

Please see "Final Kouba order" in the forum.

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