More Fuel Screw Fun

4 replies to this topic
  • writebrian

Posted December 09, 2013 - 08:29 PM


OK, check this one out... My bike is running perfectly and has been for the 4 years I have owned it (2006 YZ450F). Starts first or second kick every time, idles with perfect pitch, no bog anywhere, no decel pop and takes off like a scalded cat.


It also has an aftermarket adjustable fuel screw. Just for fun on my last ride I stopped after about 30 minutes and started playing with the screw. I screwed it all the way both directions with little to no affect on engine idle. At the completely closed position the bike idled just fine and didn't even sputter. Now granted I did not ride it in this mode but it ran fine sitting on the stand. 


I intend to not change a thing, however I would like to hear your thoughts on why it doesn't matter on my bike. Also, any way I can damage my baby based on the screw not having the right affect? 

  • mrjata

Posted December 09, 2013 - 11:03 PM


When I had this issue, I found that I had increased the idle screw setting and raised the slide to compensate for the idle issue and moved the slide/needle into the lower midrange and was feeding fuel off another circuit rather than the idle circuit.  Was just me tho.


I backed the idle screwed out till the slide bottomed out and found that I had to re-calibrate the low speed jet and needle.

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  • grayracer513

Posted December 10, 2013 - 09:35 AM


There are two fuel discharge ports in the FCR-MX carb, only one of which is metered by the screw.  If your pilot is too big, shutting off the fuel screw will have little effect if any because the remaining circuit will carry the idle. 


You said you have "no decel pop".  That in itself is an indication that the pilot is too large.  Properly trimmed, it should pop a little bit at least half the time.


Read: http://www.thumperta...ircuit-setting/

  • writebrian

Posted December 10, 2013 - 10:43 AM


Thanks guys. So is their a reason I should look at changing the pilot if the bike is running fine? Will I see any performance gains by doing so? Will I cause any damage by not doing so? Keep in mind it has been like this for 4 years.

  • grayracer513

Posted December 10, 2013 - 12:13 PM


If the pilot is right, it will run cleaner and smoother at lower RPM, and be less prone to stalling, particularly when you chop the throttle from a moderate load at lower RPM.


Far too many people fatten the pilot beyond reason in order to get rid of every trace of decel pop, and/or to eliminate a bog problem (which may be a matter of rider error).  The pilot should be set up to meter the idle circuit correctly.  If the bike bogs unreasonably, the fault lies either with the needle selection (NOT needle position) and/or the accelerator pump.

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