Jump to content
Sign in to follow this  

WR426 jetting for elevation

Recommended Posts

I just purchased my first dirt bike a few weeks ago. It's an '02 Yamaha WR426f and the previous owner lived at sea level. The bike has a Jardine RT-4 exhaust and FMF Powerbomb header, plus the airbox lid has been removed. I live and ride at 5000-6000' of elevation and have realized that the bike has trouble idling (it often dies) and also difficulty starting when warm despite using the hot start button. In all other situations it runs fine. My more experienced friends all suspect it needs to be rejetted for the altitude so I removed the carb yesterday to see what it currently has in it.

The stock jetting specs according to the service manual are:

Main jet = 165

Pilot jet = 42

Starter jet = 65

The CURRENT setup is:

Main jet = 168

Pilot jet = 65

Starter jet = 38

I am wondering if anyone can tell me what the correct jetting should be given the exhaust mods and the altitude where I ride? If you need any other information, please let me know.


Share this post

Link to post
Share on other sites

Your 38 pilot should of been lean enough for altitude. But you do not mention what fuel screw you use. Some alloy ones have poorly machined tips and a bike that uses a 45 at sea level uses a 38 because of the poor quality fuel screw. So... You need to test instead of being given blanket 'this is what you must do'

. I suggest testing using new pilot jets as any accumulated varnish will skew your results.

Fuel Screw/Pilot Jet

Fuel screw settings in the 'book' are recommended starting points. Every bike is different, as is the temp and altitude. Set the screw according to this method. Do it with the bike fully heated up.

Gently turn the screw all the way in. Now back it out two turns. Start the bike and fully warm it up, go for a 10 minute ride. Set the idle to speed to 1,500~1,800 RPM as best you can (I know, without a tach this is tough, just set it to were it idles relatively smoothly). Once warmed, slow the idle to the lowest possible speed.

*** When turning the fuel screw, keep an accurate 'count' of the amount you are turning it and record it in case you have to reset it for some reason. Makes life easier when you can just set it from notes Vs. going through the procedure again.***

Turn the screw in until the idle becomes rough or the bike stalls.

if it stalled, open the screw about 1/4 more turn. Restart it and slowly screw it in till you can just perceive a change.

If the screw can be turned all the way in and the bike still idles perfectly and does not stall, then you need to go down a size in pilot jet.

Now very slowly, open the fuel screw till the idle is smooth. Blip the throttle, let the bike return to an idle, wait say ten seconds. Confirm it is the same smooth idle.

If the screw has to be opened more than 3 turns to get a smooth idle, you need to go up a size in pilot jet.

If you find it does not stall with the larger jet but has to be open more than three turns with the smaller pilot jet, put the larger one in and set the fuel screw at 1/2 turn.

If the idle speed increased, adjust the idle speed knob to return the bike to a real slow idle speed. You must then re-visit the fuel screw. Keep doing this till the fuel screw is opened just enough to provide a nice steady idle at the lowest possible RPM. Once this is done, increase the idle speed to the normal one for your bike, typically about 1,800 rpm, but go by the spec in your manual.

If you find the pilot must be smaller and you have a alloy fuel screw, I suggest considering changing it out for a brass or stainless lone and then retesting.

Share this post

Link to post
Share on other sites

Thanks guys for the feedback. I noticed that I accidentally mixed up the starter and pilot jet numbers but your feedback was helpful nonetheless. I'm going to swap in some new jets and test accordingly and will let you know how it goes. Until then, thanks again!

Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this