XR600 Fuel Screw Adjustment

4 replies to this topic
  • pd10913

Posted April 28, 2007 - 04:56 PM


98' XR6- have just fitted an FMF Powercore 4, ground header welds, airbox snorkel removed, everything else stock. I'm thinking 155/68 jet setup but can someone tell me the best way to adjust the fuel screw and if the suggested jets should be ok with this setup, thanks

  • Jeff@TheQuadShop

Posted April 28, 2007 - 06:10 PM


Its actually an air screw and needs to be 2 5/8 turns out from fully seated. You are pretty much spot on with the jetting.

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  • HawkGT

Posted April 28, 2007 - 06:35 PM


It is a fuel screw, not an air screw. Nearly all four stroke carbs have fuel screws while nearly all two stroke carbs have air screws. If in doubt you can tell which one you're dealing with by it's location on the carb. If it is on the engine side of the carb it is a fuel screw. If it is on the air filter side of the carb it's an air screw. Said another way, if it's in front of the slide it's a fuel screw. If it's behind the slide it's an air screw. The distinction is important because they work opposite one another. In is lean for a fuel screw while in is rich for an air screw.

The short(ish) answer on finding the best setting is:

--Warm up the bike to full operationg temp.
--Turn up the idle slightly (a few hundred RPM). This is only to help distinguish small changes in idlel speed during the following steps.
--Turn the screw in until the idle starts to drop and/or miss a little. Note that position.
--Now turn the screw out until the idle starts to drop and/or miss a little. Note that position.
--The ideal setting is half way between the two noted postions.
--The goal is to find the setting that provides the smoothest and highest idle speed.
--Readjust the idle back down to the correct speed after the screw has been set.

--Typical settings are between 1 1/2 and 3 turns. If the best setting is more than 3 turns out the pilot jet is too small. If the best setting is less than 1 turn out the pilot jet is too large.

  • pd10913

Posted April 28, 2007 - 08:09 PM


Thanks very much for the advice, it does make sense and I reckon I can sort it out.

  • creeky

Posted April 29, 2007 - 04:14 AM


HawkGT's advice is on the money. I have had two stock XR600s, a '91 and a '95 and a '98 HRC 628. Best jetting for all three was 160 - 162 main, 68 pilot. Altitude here ranges from 500 - 900 ft.

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