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# All YZ450 Jetting specs

## 10 posts in this topic

Can someone sticky a thread to demonstrate all the jetting setups for our bikes like the WR guys have?

Post your jetting using the template below (copy and paste to your post)

Model:

Edit Revision Date:

Main Jet:

Pilot Jet:

Starter Jet:

Leak Jet :

Fuel screw/Setting:

Needle/Position:

Air box/Filter:

Pipe/muffler:

Altitude:

Temp:

Humidity:

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Can someone sticky a thread to demonstrate all the jetting setups for our bikes like the WR guys have?

I'm having a helluva a time with my jetting and altitude.

How are you with Excel? I have a formula you can use to compensate other peoples' jetting for your altitude. Interested?

The spreadsheet is based on the following formula:

Correction Factor (CF) = :

1.0778-.00111*T + (2.38554*10^-10)*A^2 - (1.0777*10^-5)*A

T is temperature in degrees F; A is altitude in feet

If you plug in A=0 and T=70 you get CF=1. This is your correction factor at standard temperature and pressure (STP). If you put in 5000 ft and 70 F (Denver), you get CF=.95. So your bike that was jetted for a 175 at STP now requires a 0.95 * 175 = 166 (or the nearest size).

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The spreadsheet is based on the following formula:

Correction Factor (CF) = :

1.0778-.00111*T + (2.38554*10^-10)*A^2 - (1.0777*10^-5)*A

T is temperature in degrees F; A is altitude in feet

If you plug in A=0 and T=70 you get CF=1. This is your correction factor at standard temperature and pressure (STP). If you put in 5000 ft and 70 F (Denver), you get CF=.95. So your bike that was jetted for a 175 at STP now requires a 0.95 * 175 = 166 (or the nearest size).

Grey wouldn't humidity also be a factor for jetting specs?

just curious if this would add too much complexity

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Humidity is a factor, but I think less so than altitude or temp. Higher humidity makes the mix richer by displacing air. When I found this formula, all it included was altitude and temperature. If you could find a formula to correct jetting based on humidity, it could be dropped in very simply.

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Only make one change at a time.

Making multiple changes at the same time will screw you up.

IT WOULD REALLY HELP IF ALL OF YOU WOULD POST YOUR JETTING SET-UPS ALA THE TEMPLATE. This is one of the only forums this data doesn't exist.

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I have absolutely no idea why people are suggesting the WAY overly rich jetting on these things at elevation.

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02 WR426 All Stock cept the zip ty fuel screw at 2 1/4 turns out..

I ride dumont, glamis, and all that fun jazz around sea level

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Here's a bunch of 06 model jetting specs.

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FEEDBACK FROM A ROAD RACE TUNING SHOP: Saturday, May 6th

Been racing motocross every weekend since mid-March. Was told to go to 48/170 but just never got around to it. Now I got the jets and was about to install them when I see that at sea level now (at 70+ degrees) the 48/170 bikes are beginning to run badly on this website. So now what? Summer jetting specs?

So I stopped by a really fine road race tuning shop (Loudon, Pocono, etc) and figured if anyone understands 4-stroke race bike tuning its them. I told them my bike was running beyond perfect in all aspects of the power range. Only issue is popping/backfiring on deacceleration. Feedback was:

"Basically he told me that it sounds like my bike is jetted perfectly. He cautioned me that if I was a play rider and was annoyed by the popping, or I was not a serious racer who was more worried about valve maintenance 3 years down the road, then he would have a different opinion. However, he explained my goal is to win races and if that bike is running perfectly than he thinks the popping thing is a red herring. He said; look at any GP bike. They all pop and backfire on deacceleration. The reason for this is they have spot on jetting when they leave the corner. Also he said the popping was, of course, not in any way related to the main jet. He cautioned that if a non-competing rider walked into the shop and asked the same question he would be giving out different advice. He thinks the GP bike analogy is appropriate and he also pointed out that I should note that the factories all tend to release their race bikes now with jetting that is "on the lean side". Remember, he said, this is WELL WITHIN the performance range, its just that if you choose to jet a bike slightly richer or slighter leaner from the optimal performance point, the factories all go a tad leaner for emission issues EVEN THOUGH its a race bike. To his ear he said the sound of popping and backfiring, on deacclereating 4-stroke race bike, would only tell him that we are dealing with a finely tuned racing motorcycle that is giving all its got. He concluded, that taken in isolation, the popping thing is a red herring. Ignore it and any advice based on it. Go home and think about winning tomorrow."

So with that I hesitate to do the 48/170 jets. Waiting for the weather to warm still further and perhaps the 45/165 is the best compromise. But right now, I think my stock 42/160 is optimal summer heat jetting (which will be here soon!). Perhaps I will install the "all season" jetting in the fall. But for the coming few months, stock seems to be the hot ticket.

Sic

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Its mystifies me that nobody has offered much jetting info on the '08 machine considering the amount of complaints regarding the standard exhaust system.

My experience so far has been this.

Average air temp 32c. approx 1000ft above sea level. humidity varies between 60-90% because i live in the tropics and i am in Northern Australia.

We have the model with the shorty muffler.

Ok. Best results have been. 48 pilot jet. Stock needle on one position leanest from standard ie second from top or even in the leanest position depending on conditions. Stops the hesitation during mid rpm acceleration.

And the main jet has been left standard at 160 due to the needle position.

Please remember i live in humid/hot conditions for most times of the year.