Anybody a jetting expert?

10 replies to this topic
  • steverad1

Posted February 02, 2008 - 11:37 AM


Hello everyone,

I just rejetted my new 2007 WR450 using the JD kit and the Zipty fuel screw. What a pain in the rectal area to get to the carb. Anyhow. everything seemed to go OK. But I think I have a problem.

I used the settings that most folks recommended in the jetting database. Main jet 168 and pilot jet 48. Used the red needle from JD with the clip in the 4th position. I live in GA in a low elevation area. Have done the airbox mod and replaced the pea shooter with the Pro-Billet exhaust nozzle.

The bike cranked right up with the choke and I let it warm up with the choke engaged. Once warmed up I took it out of choke and it didn't want to idle so I increased the idle adjustment till it started idling. I started with the fuel screw 1/34 turns out. Thought that maybe I was running to lean so I started backing the screw out more and more but it doesn't seem to have any effect at all. I even ran the screw all the way in and it still seemed to run the same. If I idle it up and ride it runs like a scalded dog. But when I stop running and let it drop to idle it dies quite frequently.

Just seems like the fuel screw is not doing didly.

Any advise would be greatly appreciated.



  • clark4131

Posted February 02, 2008 - 11:44 AM


You may want to up it to a 50 pilot, but before you do check and see if your 48 isn't blocked. Your carb is acting like it's dirty/clogged somewhere. A good douching with carb cleaner can't hurt. BTW, did you make sure that the spring, washer and o-ring went back in with the fuel screw? Those little suckers have a nasty habit of disappearing...SC

  • William1

Posted February 02, 2008 - 01:29 PM


I posted a response to this same Q in the Jetting area. However, clark4131 makes a good point regarding a potentially clogged jet. Fortunately, you can remove the pilot without pulling the carb.

When you pull the 48 out, make sure it is clean before you swap anything.

  • steverad1

Posted February 02, 2008 - 03:52 PM


Thanks William. I took the whole bike apart to rejet the 1st time. Do you have to have any special tools to swap it out without taking the bike apart?


  • Gary Murray

Posted February 02, 2008 - 04:03 PM


Check to make sure you do not have a vacuum leak where the carb connects to the engine side. If the carb is twisted slightly from where it is supposed to line up it can leak air.

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  • William1

Posted February 02, 2008 - 05:41 PM


Well, yes, there is a special tool that makes pilot jet changing a less than one minute job but it is not crititcal to have it for a WR or YZ as there is a fair amount of space under the carb.

If you drain the carb and remove the 17mm bowl plug on the bottom, a small mirro will enable you to easily see the pilot jet. A stubby screw driver or other too such as I mentioned above can remove and install the pilot.

By the by, when took the carb apart, you did not remove the slide, did you?

  • tweav

Posted February 02, 2008 - 05:49 PM


I'm running a 42 pilot jet in my 07 450 . the dyno said it was way rich at Idle so I popped in the 42 . it runs much better now

  • clark4131

Posted February 02, 2008 - 06:05 PM


I'm running a 42 pilot jet in my 07 450 . the dyno said it was way rich at Idle so I popped in the 42 . it runs much better now

His bike only runs when choked, i.e. richer...SC

  • William1

Posted February 02, 2008 - 06:14 PM


I also depends if and which extended fuel screw is being used. The aluminum (ZipTy, MSR and various others) ones tend to require one or two sizes leaner pilot jet. The Kientech stainless steel and Kouba brass ones tend to be capable of using 'stock' jetting.

  • steverad1

Posted February 02, 2008 - 06:18 PM


No I didn't. I was also very careful to make sure no dirt or debris fell into the carb. While I had it apart. Based on what you guys are saying I guess I'll 1st pull the 48 pilot and blow it out real good to make sure it isn't clogged. If that doesn't do it I guess I'll try putting the 45 that was originally in it back in.

The fuel screw - does it control the amount of fuel that feeds the pilot jet?

  • William1

Posted February 03, 2008 - 06:01 AM


Actually, the amount of air that emulsifies with the fuel in the pilot jet. Notice the tiny holes on the side of the pilot jet? There are three components of the idle/pilot circuit. The slow air jet (located in the carb bell), the fuel/slow air screw and the pilot jet. It is a balancing act between the three. Basically, the slow air jet is a coarse adjustment, the pilot jet size is proportionately sized to the slow air jet and the fuel screw is the fine adjustment.

I know too much information.

To correctly select a pilot jet and set the fuel screw:
Fuel screw settings in the 'book' are recommended starting points. Every bike is different, as is the temp and altitude. Set the screw according to this method.
Gently turn the screw all the way in. Now back it out two turns. Start the bike and fully warm it up, go for a 10 minute ride. Set the idle to speed to 1,500~1,800 RPM as best you can (I know, without a tach this is tough, just set it to were it idles relatively smoothly). Once warmed, slow the idle to the lowest possible speed.
*** When turning the fuel screw, keep an accurate 'count' of the amount you are turning it and record it in case you have to reset it for some reason. Makes life easier when you can just set it from notes Vs. going through the procedure again.***
Turn the screw in until the idle becomes rough or the bike stalls.
if it stalled, open the screw about 1/4 more turn. Restart it and slowly screw it in till you can just perceive a change.
If the screw can be turned all the way in and the bike still idles perfectly and does not stall, then you need to go down a size in pilot jet.
Now very slowly, open the fuel screw till the idle is smooth. Blip the throttle, let the bike return to an idle, wait say ten seconds. Confirm it is the same smooth idle.
If the screw has to be opened more than 3 turns to get a smooth idle, you need to go up a size in pilot jet.
If you find it does not stall with the larger jet but has to be open more than three turns with the smaller pilot jet, put the larger one in and set the fuel screw at 1/2 turn.
If the idle speed increased, adjust the idle speed knob to return the bike to a real slow idle speed. You must then re-visit the fuel screw. Keep doing this till the fuel screw is opened just enough to provide a nice steady idle at the lowest possible RPM. Once this is done, increase the idle speed to the normal one for your bike, typically about 1,500 rpm, but go by the spec in your manual.


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