Perfect Jetting

4 replies to this topic
  • FlashGordon

Posted August 19, 2005 - 05:33 PM


Hey guy’s,

I was reading a thread from a couple days ago where people were listing what their jetting specs were. I was suprised to see a bunch of 450's with 158 mains, and 38 pilot jets. I have a 99 YZ400F, and it's way richer than that.

Main 180
Pilot 48
Needle 2nd slot from top
Stock pipe with PMB spark arrestor
Live at sea level, ride at 3000-6500 ft.

My bike runs great. Plug is not rich at all. One of the best starting machines I have ever had. I was tweeking the jetting when I first got it to help make it start easier, and to get rid of that annoying de-acceleration pop at low rpms. Going to a bigger pilot eliminated that minor backfire, and helped starting. My only complaint is the off idle bog when you quickly snap the throttle. I did the throttle pump mod, and shortened the squirt duration from 5 secs to .5 secs. Helped a bit. Did a homemade power now; helped a bit more. I can snap the throttle, and it has a bog, but won't kill the engine as it once did. Adding an 8oz flywheel weight made it better for riding, but also slowed down the quick rev. I wish it had no bog, as my buddies CRF450, but I got tired of tweeking.

My question is, how could there be such a huge disparity on jetting between this 400 and the 426/450, and still run so well, minus the bog off idle? I keep my pilot fuel screw at 2.5 turns out. If I screw it in just a turn, it will start popping on de-acceleration just sitting there at idle, so I need to keep it where it is, or go up one pilot jet size. Float is set perfect per the manual. At idle to 1/4 throttle, it's just the pilot, and the needle right? If I went to a different needle, could I drop down the main jet size?

  • StreetbikePimp

Posted October 02, 2005 - 03:27 PM


To answer your question about why 450 has smaller jets...

A 450 has bigger displacement which leads to more vacuum to suck fuel in. A 450 draws more fuel through a main jet then a 400 will. Hence smaller jets are needed, otherwise it would be too rich.

Can't help you with your bog problem as I have one of my own that I'm trying to solve on my 450.

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  • 642MX

Posted October 02, 2005 - 04:14 PM


I solved my bog problem with a Boyesen Quick Shot AP cover. It makes a night and day difference in throttle response. Maybe you guys need to try one?

  • StreetbikePimp

Posted October 02, 2005 - 06:15 PM


Thanks for the tip, but I'm too cheap and stubborn to spend a $100 on something that I can fix with correct jetting and proper set-up.

  • Matt96xr6

Posted October 02, 2005 - 08:33 PM


From playing on many FCR carbs, once you get the AP squirt dialed in (with either the BK mod for the 00-02 bikes or different sized leak jet for the 03+) and adjust the fuel screw a bit, if you still have problems it is in the needle.

Try a different needle or just get the JD Jet kit with the special triple tapered needles.

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