James Dean's Jetting Guide 2002

There is something new out on jetting!

See: www.JDJetting.com

[ August 29, 2002, 09:40 PM: Message edited by: James Dean ]

Adding a little history to this:

In the spring of last year I bought a '00WR400. The stock throttle response was dull and left me thinking something had to be done. I'm used to open class 2-strokes, having owned 2 CR500's and 2 KX500's before. I ordered a pile of carb parts from Sudco with different tapers and needle diameters along with pilots and mains that weren't already in the toolbox.

Quickly the DRS needle was tossed (like the YZ400 DVR) and the EMP needle stood out as a big improvement. Then with a YZ exhaust and YZ timing it was even better. This is the same as a EKP, 1 clip leaner. EMN (EKN) was a close choice too.

I posted the results on the WR forum and LOTS of riders wanted to get the EKP. Yamaha couldn't keep it in stock fast enough, nor could Sudco keep the EMP in stock. Did Yamaha notice???

To my surprise later in the year, the '01YZ426, YZ250F, and WR250F ALL came out using the same needle with nothing other than differences in clip position. (EJP, ELP) This is what most of you have in your bikes right now.

Now to take it to the next level... :) Some of you REALLY should try jetting your bikes with the FHP, FHN, or FHM carb needle. The spreadsheet helps to understand. This adds fuel to the mid-range and is an absolute BLAST to ride when traction is good. A little wild when it's slippery though. If your manual shows an OBFJP needle, this would be close too.

Good Luck!


JD: while I have the utmost respect concerning your jetting expertise, I have reservations concerning your jetting recommendations with regards to fuel selection. I noticed that there are no fuel specific sheets. Of course, that may be due to the fact that I did not buy into your spreadsheet. From my personal experience when using fuels such as VP C-12 and MR2 in the 426, all that it takes, are relatively small changes in main jets in relation to ambient temparature changes.


The spreadsheet is designed to show "relative change" from your current setting. What if temperature drops, altitude changes, or someone posts a new idea saying it works awesome? Your fuel choice may make his setting unacceptable. But how can you match the same principles he is using and make them work for you? Not many people know enough about the codes and interactions to make an informed decision.

If you have never run your bike with needles DVR, EJP, and FHN, then your skepticism is not surprising. These are night and day apart, but only in specific throttle ranges.

The spreadsheet is based on measured jet area. These are hard numbers that can't be disputed. The nozzle is 2.90mm, pilot #42 = .42mm, main #165 is 1.65mm. The needle dimensions at the nozzle go directly into the graph and change exactly as shown versus throttle position. I don't know how anyone could find that information useless. It gets graphed for your specific setup right before your eyes and updates as you go. Compare it to any of thousands of possible setups at any temperature and altitude.

Look in the back of the Yamaha manuals at the tuning section. You will be able to make graphs just like those.


[ December 23, 2001: Message edited by: James Dean ]

OK. That cleared it up for me. Thanks.

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