what the *%%#$!

4 replies to this topic
  • blaze

Posted September 21, 2001 - 09:10 PM


my 01 426 has no power on the bottom. i just put the fmf power 4 Q exhaust on and couldnt even make it up a hill in 1st gear that normally 2nd and 3rd rip up it. mid range to top end is great though. i even went back to check the settings of the bk mod, timing and duration of the squirt were right on. my settings at 8000' are 52 mj, #4 needle position, up from #3 after installing pipe, 1.5 on fuel screw, 42 pilot jet. i played heaps with the fuel screw to no avail, and the mj seems fine, and i dont think the needle could effect it this much. please help!

  • sirthumpalot

Posted September 21, 2001 - 09:22 PM


Did this start immediately with the addition of the new muffler?

If not, then check your valves. Overly tight valves on a 4-stroke can hang open and the most noticable loss will be down low. If this is the case I would adjust it before further riding. A leaking valve can burn the seat and the valve.

If this started as soon as you put the new muffler on then it sounds like either you've got some tuning to do, or the new muffler isn't so good for bottom end.

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • blaze

Posted September 21, 2001 - 09:57 PM


yes this started with the new pipe. does anyone know if this pipe is restictive in air flow? i called fmf and the tech person said it was restictive but then went on to say the fmf settings at sea level were a 168mj and 2.75 turns on the fuel screw. when asked about his contadictory statement he then said the pipe was not restictive. so now im not sure.

  • canavero

Posted September 21, 2001 - 11:31 AM


I ride at an average ele of 6000ft. I too experienced a huge loss of pwr when I put the pwr core on. (pwr core 4 squared). I thought of throwing it away but i need a spark arrestor.

Bottom line - my settingd that finally worked. BK mod apporx. 1mm or roughly .5sec, 158mj up from 155 after the BK Mod, stock pilot, needle one clip lean or up one from stock, and fuel screw between 1.25 and 1.75.

It finally works, be patient!


  • Hick

Posted September 21, 2001 - 12:08 PM


Try a 40 pilot.

Related Content


Is it worth it? (Cam and High compression piston) by macgi77

Dirt Bike   Make / Model Specific   Honda   CRF 150/230 F/L
  • Hot  36 replies

Is the 2016 GYTR Yamaha YZ450F too fast? by Fryboy

Dirt Bike   Special Interest Forums   Pro Racing
  • * - - - -
  • Hot  241 replies

yz250x vs 450 4t comparisons (preferably desert riding) needed by JakeNorthrupYZ450F

Dirt Bike   Make / Model Specific   Yamaha   Yamaha 2-Stroke
  • 9 replies
WR Camshaft Swap Info - last post by jamesm113

WR Camshaft Swap Info

  • 0 replies

2016 YZ450 by CaptainKnobby

Dirt Bike   Dirt Bike Technical Forums   Suspension
  • Hot  59 replies

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.