Rejetted-now stalling off idle

I have a 2007 yz 450. I recently rejetted it, and it has way better response now, and pulls hard to redline, but every once in a while when i roll onto the throttle(even slowly)it just dies, and is hard to restart(4-5 kicks)used to be 1 kick. I rejetted it because the jets i got in the white bros. kit were ridicolous

I ride at about 1500-3000ft. Temps anywhere from 20f to 60f

Previous jetting was:




starter jet(stock)


White bros fuel screw 2 1/8out

Current jetting:




starter jet(stock)


White bros fuel screw 2 1/4out

One more question i have is i plan on doing the o-ring mod soon. Am i going to want to go back to a bigger leak jet?

has the slide ever been out of the carb?

if it was running with a 52 pilot jet the slid eplate is probably upside down.

Yea i've had it out, but i was running the 52pilot before i ever pulled the slide.

What size slow air jet is in it?

The oring mod will not require you to change the leak jet. After you do the mod, confirm the AP timing too.

slide plate goes square end down.

Slow airjet is stock,sorry im not real sure of the size. Slide plate is good.

First, do the pilot jet / fuel screw test. I really think your pilot is way big.

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.

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.

Many of the alloy fuel screws, due to the shape of the tip, actually require a much smaller pilot jet (for example, some who run a 45 pilot with a stock screw have had to go down to as small as a 38). Normal pilot size is a 45.

Thanks will, that helped me understand a few things. Im going to go try that now. I think i might end up back at a 45 pilot.Ill let you know how it goes.:busted:

Update: I ended up going back to the 45 pilot with the fuel screw 2 1/2 turns out. With the 48 the idle didnt even flinch with the fuel screw the whole way in. Now with the 45 the bike dies after a minute and a half or so of the fuel screw being in the whole way.

I decided to put a 165 main in it because i thought it still felt a tad fat on top, and it helped. I think a 168 would be even better, but i dont have one available so ill stick with the 165 for now. I did the o-ring mod, and confirmed my ap timing, and the bike is an absolute beast. I love it. Instant throttle response, and the bike seems less prone to stalling in tight corners.

I'm starting to get the hang of jetting, but there's still some things im unsure of, and im glad i can turn to you guys for answers.



Checking/Selecting the main jet:

Starting with the recommended main, remove the airbox door and go for a ride (bike fully warmed up). Is it better or worse?

If it is better, you need a smaller main.

Go down one size, replace the airbox door, test ride. Remove the airbox door and test again. Better or worse? If better, go down a size again. Keep repeating this till the test with the airbox door is worse.

If it was worse with the airbox door removed, tape over 1/3 of your airbox opening, test (airbox door on, of course).

If it is worse now with the tape and was worse with the airbox door off, your main is just right. You are done!

If it seems better, you need to go up a size in main jet. Then test it again (remove the tape). Replace the tape, test again. If with the tape on it is better, go up another size in main. Keep repeating this till having the tape on is worse than with it off.

To finish up and ensure you are set accurately, retest the bike with the tape off (airbox door on), ride it, then remove the airbox door (tape off, of course). Best performance should be with the airbox untapped, airbox door on.

Remember, the main only operates at WOT. Ideally, you want to be in 3rd or 4th gear doing the tests, hitting max revs (just shy of the limiter) for at least 10 seconds to get an accurate representation of the jet status.

if it take a minute and a half to dies with the fuel screw fully in the pilot jet is too big.

it should die immediately.

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