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BrianM500EXC

Jets - Starter? Pilot Air?

5 posts in this topic

I know jetting is covered extensively by you fine people, but I need some more basic help. I can locate the main, pilot, and leak jets. What are the Starter and Pilot Air jets and where do I find them?

While you're helping, I just got a '04 WR450, and here's my plan:

Exhaust : Already has PMB tip

Grey wire: going to cut

JD Jetting on the way: Using Red #4

Leak jet: 40

Starter jet: 68

Pilot jet: 48

Pilot Air jet: 100

Main jet: 168

I ride mostly desert (60-95 degrees), elevation: 500-2000 ft. Bike has WR timing, Kouba fuel screw.

Please post your thoughts on the above - going the right direction?

Just got this bike, riding it Sunday for the first time - can't wait!

Thanks!

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Your starter jet is #21 on the diagram in your manual. It serves only the choke circuit, so if you're only riding between 60 and 90 degrees, the #68 should be fine.

Your pilot air jet is #22 and controls the vacuum signal at the pilot jet. As you increase the PAJ, you increase the vacuum at the PJ, which would result in an overly rich pilot. You would then need to lean the PJ back out to maybe a 42.

Since the WR's have an air cut valve (ACV) unlike the YZ's, there are two air passages to the PJ. One closes on decel via a vacuum signal, which temporarily richens the PJ to prevent popping (the air gets cut in half while the fuel remains the same). Only if you want to disable the ACV and keep one of the passages blocked would you need to change the PAJ and PJ to YZ specs, i.e. 100 PAJ and 42 PJ. If you don't plan on disabling the ACV, or if you are going to, but will leave both passages open, then leave the PAJ alone. You may also want to experiment with a 45 PJ at the higher temps.

The rest of your jetting looks good, but I found the #40 leak jet was too rich and use a #52 instead. Of course, your own trial and error will help determine what's best. Have fun frequently taking your carb off.

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The jets are pictured below.The starter jet and slow air jet are what you are asking about.

The pilot air jet is in the intake opening, while the starter jet is next to the main jet.

Thanks,

James

53264343.jpg

medium.jpg

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Your starter jet is #21 on the diagram in your manual. It serves only the choke circuit, so if you're only riding between 60 and 90 degrees, the #68 should be fine.

Your pilot air jet is #22 and controls the vacuum signal at the pilot jet. As you increase the PAJ, you increase the vacuum at the PJ, which would result in an overly rich pilot. You would then need to lean the PJ back out to maybe a 42.

Since the WR's have an air cut valve (ACV) unlike the YZ's, there are two air passages to the PJ. One closes on decel via a vacuum signal, which temporarily richens the PJ to prevent popping (the air gets cut in half while the fuel remains the same). Only if you want to disable the ACV and keep one of the passages blocked would you need to change the PAJ and PJ to YZ specs, i.e. 100 PAJ and 42 PJ. If you don't plan on disabling the ACV, or if you are going to, but will leave both passages open, then leave the PAJ alone. You may also want to experiment with a 45 PJ at the higher temps.

The rest of your jetting looks good, but I found the #40 leak jet was too rich and use a #52 instead. Of course, your own trial and error will help determine what's best. Have fun frequently taking your carb off.

Thanks, bg10459 and JD, really appreciate your help.

JD, I just ordered your jet kit for the '04 WR450. Planned mods while installing your jets:

Airbox: remove snorkel and open side holes

Pipe: Stock head pipe w/'04 YZF450 exhaust

Cam: YZF w/YZF timing

Jetting: Your kit.

With zero disrespect to bg10459 for his recommends, JD what are your recommends for desert (60-95 degrees), elevation: 500-2000 ft for all the jets:

Leak jet:

Starter jet:

Pilot jet:

Pilot Air jet:

Main jet:

Thanks again to you both.

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