Help 08 YZ450F jetting

4 replies to this topic
  • ridered22

Posted February 04, 2008 - 09:52 AM


I put a White Bros full system on and off bottom is sluggish, hesitant even and on top it pops and backfires when of gas. I am thinking I need to go leaner on bottom and richer on top. Please let me know if anyone has an 08 dialed in. I am in KY with temps being about 60 outside.

  • William1

Posted February 04, 2008 - 10:41 AM


Pipe should make little differene on the bottom end. You can try a larger main but... should insted select the correct one.
Starting with the recommended main, remove the airbox door and go for a ride (bike fully warmed up). Is it better or worse?
If it is better, you need a smaller main.
Go down one size, replace the airbox door, ride. Remove the airbox door and test again. Better or worse? If better, go down a size again. Keep repeating this till the test with the airbox door is worse.

If it was worse with the airbox door removed, tape over 1/3 of your 3X3, test.
If it is worse now with the tape and was worse with the airbox door off, your main is just right. You are done!

If it seems better, you need to go up a size in main jet. Test it again (remove the tape). Replace the tape, test again. If with the tape on it is better, go up another size in main. Keep repeating this till having the tape on is worse than with it off.
To finish up and ensure you are set accurately, retest the bike with the tape off, ride it, then remove the airbox door. Best performance should be with the airbox untapped, airbox door on.

Remember, the main only operates at WOT. Ideally, you want to be in 3rd of 4th gear doing the tests, hitting max revs (just shy of the limiter) for at least 10 seconds to get an accurate representation of the jet status.

Be sure to keep notes of your settings. You may find you need to make a change, say if you're riding in the mountains or down at the beach. Returning home, it is a simple matter of referring to your notes to restore things.

Once teh main is good, you can try to fix the bottom end, and once that is good, deal with the popping, if possible. A open pipe always pops a lot more than a restrictive system. The popping has always been there, just he quieter system tends to mask it.

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  • grayracer513

Posted February 04, 2008 - 10:49 AM


Starting with the recommended main, remove the airbox door ...

There is no air box door on any YZ450.

ridered22, can you clarify this a little, please?:

... on top it pops and backfires when of gas.

  • William1

Posted February 04, 2008 - 11:33 AM


Gray, it is from my "TT Cheat Sheet" and also means any airbox opening you have, LOL.

  • grayracer513

Posted February 04, 2008 - 04:08 PM


From a practical standpoint, taping over the air box to the extent that the remaining opening becomes a bottle-neck for the intake air would be somewhat challenging, and the opening that is left must be small enough to cause a vacuum to form on the the atmospheric side of the carb. The fact is that with the seat on, the air path into the open YZ air box is on the order of half the size of the box opening in any case. It's frankly a waste of time, IMO. A more productive approach is to use the available symptoms to get first the main jet in the correct range, then the pilot, then analyze what the needle needs. By actually changing jets, you'll be making a metered, quantifiable change, instead of guessing at taped openings.

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