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Bogging...YZ400F Jetting with 2008 YZ450F Carb


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Just installed a 2008 YZ450F carb on my 1999 YZ400F. The old carb's slide was worn out and it was cheaper to buy a new one and upgrade. Anyways, a little background on what is going on. Bike starts on first kick and idles great. It actually hasn't idled this great without a hanging idle in 5 years! However, when I quickly open the throttle from idle to about 1/3 Throttle and above, it bogs bad. If you blip the throttle from idle to 1/2 throttle, it will die if you do not back off the gas. If I open the throttle quickly from idle to above 1/2 throttle or wide open, well forget it as it will just die. If I slowly roll the throttle, it will run out great. I have no lean popping or backfiring. I have noticed very little black smoke when revving in neutral at times, indicating a rich condition. Is this an accelerator pump issue that needs to be timed? Do I need to adjust the leak jet size? The Carb currently has these jetting specs...

Main Jet 160

Pilot Jet 45

Starter Jet 72

Leak Jet 55

NFLR Needle 3rd position from the top

Any help or ideas are greatly appreciated:worthy:

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William1, thanks for the reply, I appreciate your help:thumbsup: The bike runs great other than the bogging that is going on upon opening the throttle. I thought I read that the 2008 YZ450F carb couldn't be modded with the O Ring, is this true? If I gently roll the throttle, it won't bog and will run great. I know that a little bog is normal if the throttle is chopped open from idle to wide open, but if I chop the throttle from idle to almost half throttle, it bogs bad and dies if I don't close the throttle. Thanks again for the help!­čĹŹ

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Do not nail the throttle from idle. You never do that when riding, you are always above 2,000 rpm.

Do the oring mod/stiffer spring. Yes, it still applies to the 08's and 09's

Tweak the AP timing screw. Keep track of how far you turn it. Tighten it in 1/2 turn increments, then test by riding, not juast reving in the garage.

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The problem I'm describing is when riding, not on the stand in the garage. There are instances when riding if I open the throttle slightly faster than a "roll," it will bog down. I remember how to time the accelerator pump on the original carb on this bike with some rigging, but I'm relatively new to the 2008 design which is more advanced. I will search on how to time AP. Any tips?

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