Pilot screw adjustment

5 replies to this topic
  • av8shunmeckaneck

Posted November 01, 2004 - 08:17 PM


assuming I have the correct jetting already, what is the procedure for adjusting the pilot screw?

  • Indy_WR450

Posted November 02, 2004 - 04:15 AM


This is directly from Thumperfaq:

:cry: :cry: :cry: :cry: :cry:
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.

:lol: :lol: :lol: :lol:
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.

  • av8shunmeckaneck

Posted November 02, 2004 - 06:01 PM


Thanks a million again Indy.

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  • redrider77

Posted November 03, 2004 - 05:06 PM


but here is the 64 million dollar question.... How far in is "all the way in"? I've read all the horror stories about tightening up the fuel screw until it stopped, and then having to replace the entire carb because the tip broke off inside. How much resistance is "all the way in"? Thanks.

  • Indy_WR450

Posted November 03, 2004 - 09:03 PM


Get a Zip Ty screw and you put it in very lightly with your finger tips only. You have to be gentle but the screw should never break in the carb this way. :cry:

  • ovrrdrive

Posted November 04, 2004 - 01:55 AM


Here's a great page on the fuel screw I bookmarked...


And agreed on the zipty screw. I love mine. A gentle seat is all that's needed to count the turns back out. I'll never understand why people torque that thing down just to count how far it's out.

Btw, word has it that if you do break off the tip and can't get it out, if you send that part of the carb tp zipty they will remove it for free for you. I think the last guy that did that even got a new screw for free too...


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