And where is the pilot screw?

4 replies to this topic
  • yzf_virgin

Posted July 02, 2001 - 05:42 PM


The factory service manual calls for the pilot screw to be 1 3/4 turns out. But their diagram doesn't point it out very clearly. Also, has everyone found that to be a good setting? Thanx!

  • Boit

Posted July 02, 2001 - 07:12 PM


The factory setting is an average setting. . . it's a starting point. You can take 5 bikes off the assembly line and each one will not run exactly the same. Racing bikes are tempermental by nature and the 426 is an excellent example. That being said, first you need to locate this screw adjuster. It's located in a recessed area of the carb between the carb and cylinder head but still a part of the carb. Get on the shifter side of the bike and look just aft(toward the rear wheel) of where the carb connects to the cylinder head manifold. There is a round looking tunnel with the pilot screw recessed up in there. It is held in place by spring resistance. It would help if you use one of those small automotive mirrors on a telescopic wand with a flashlight to first locate it. Pro Tec makes a dandy like screwdriver just for this specific screw jet. It IS a little hard to locate at first, but once you do, it becomes second nature after that. The rule of thumb is that if you have to adjust this fuel screw from the range of 1 turn out to 2 1/2 turns out, you need to change pilot jets. In other words....less than 1...or more than 2 1/2 turns out, a different pilot jet is in order. Once you find the proper pilot jet, it's perfectly normal to need small adjustments as the ambient temperature changes. What I have found to work best is to begin leaning it out slightly(clockwise) as the outside temp rises. Conversely for the opposite.
The most common mistake most guys make is that this screw is a fuel screw...not an air screw like on a 2-stroke. This is a 4-stroke pumper-carb.

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  • WR_Jason

Posted July 03, 2001 - 08:45 AM


Also, as I was first reading the book I got confused between the pilot air screw (mentioned above) and the pilot Jet. Not the same :) . The pilot air screw fines tunes the pilot jet. The pilot jet is inside the bowl. Funny, I never had to screw with the carb on my 2 smokes this much except if making major pipe or reed changes. Then it was mix up or down or clip up or down. The smoke was a good indication of what needed to be done. This thing is less intuitive. I had to make a table to keep track of changes!!!

  • sirhk

Posted July 03, 2001 - 08:50 AM


I've never messed with my carb but am going to. I've been keeping my eye on a lot of these jetting threads. I was told that mine is running lean because it's popping on deceleration and that I could raise the needle/lower the clip to fix this. Is there a idiots guide to jetting anywhere that someone can point me to?

"What's that?"
"It's a Yellow 99' YZ400!!"

  • Boit

Posted July 03, 2001 - 04:15 PM


Eric Gorr has an excellent site for jetting information. I don't have the URL but you can get to it from here His site has tons of other information as well.
Also, try this site

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