Smaller pilot jet?

4 replies to this topic
  • rhinoracin

Posted December 01, 2014 - 01:03 PM


I decided to swap the stock 45 pilot jet in my 2009 YZ to a #42, because I saw it recommended somewhere and my bike would always start easier when using hot start (even with the engine cold). So now, after re-adjusting the fuel screw, starting the bike is how it should be (using choke and hot start only when necessary). But for the first time ever, there is sometimes a slight bog or hesitation (1/8-1/2 throttle openings) when riding.


I'm not really a jetting expert, but I don't think it has anything to do with the leak jet (bike has a R&D power pump AP cover set to #60 leak jet). I feel like the bike is running lean now (mid-throttle openings), and I should raise the needle a clip or 2, and probably turn the fuel screw out more. I just want to make sure that this really is what I should do before I do it...and how many clips should I raise the needle?


My current jetting is 42 pilot, 165 main, NFPP needle 4th clip. Full DRD exhaust, riding in SoCal deserts.

  • grayracer513

Posted December 01, 2014 - 05:02 PM


#60 is bigger than the original LJ (#55) by a considerable amount.  That makes the pump discharge weaker, not stronger (leaner, not richer).  The first thing I would try is returning it to #55 and go from there.

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  • rhinoracin

Posted December 15, 2014 - 12:14 PM


I adjusted the leak jet to #55 and replaced the spark plug (maintenance). The bog is gone, but now the bike usually runs a little rough/sputters at smaller throttle openings. The idle is lower than it used to  be (1700 rpm, used to be 1900-2100), and I havent touched the idle adjustment screw. The engine also dies every now and then when I abruptly open the throttle a small amount from closed. And every time I try adjusting the fuel screw, nothing happens at all except for when I turn it in all the way, the idle will go up a little. I dont know how it does that because the pilot jet is already smaller than stock. All these problems seem worse when I ride in cooler temps. So I'm assuming my bike is running too lean.

The only thing I can think of is putting the 45 pilot jet back in, and maybe raising the needle to 5th clip. Suggestions?

  • grayracer513

Posted December 15, 2014 - 12:33 PM


Leave the needle alone, and go back to the 45 pilot.

  • rhinoracin

Posted January 07, 2015 - 02:08 PM


Put the 45 pilot in, and the bike seems like it only wants to start with the hot start lever pulled in, and it still idles low (1500-1700 rpm) even after turning the fuel screw. Except the idle rises a bit when I turn the fuel screw all the way in. Same problems as before. When I chop the throttle while the bike is idling, it will often kill the engine instead of revving as it should...I'm guessing because the idle is too low. So I ended up turning the idle up a little and have the fuel screw around half a turn out. After doing that, it seems much better.

But I'm wondering if there might be a problem I'm missing, and if it's normal for the fuel screw to only be half a turn out. if I turn it out past that the idle drops a little, and even more after 1 turn out. And why does it usually only want to start with the hot start?

If it matters...I have been testing/riding my bike in warmer, dryer weather at low elevation (San Diego and desert). And I know the DRD exhaust on my bike needs a repack...dont know if that can actually affect jetting.

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