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01 YZ426 Idle Issue

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So I have been looking at the user jetting database on here and using it as a reference guide, but with no luck. My bike has a very high idle but throughout the rest of the powerband seems to be fine, with a little decel pop and it does backfire a little when the bike is turned off.

When i bought the bike, it idled fine but had the normal bog off the bottom

I bought a JD jetting kit and put on a Baja Designs Dual Sport kit, with new WR flywheel and stator to become street legal, 03 450 Auto Decomp Piston, added a Powerbomb header and FMF Q4 exhaust and added a new fuel screw.

I live in San Diego, CA... almost sea level

After this, the idle is extremely high. Jetting with the JD kit:

Blue Needle 4th clip

Main: 170

Pilot: tried from 42-38

Rest stayed stock

Ive even gone back to stock jetting and needle, (with 162 main and 42 pilot) but to no avail.

With the JD jetting and stock jetting, i can turn the fuel screw all the way in and the idle is unaffected, it was only affected slightly by the JD jetting and the 38 pilot and the fuel screw being turned all the way in.

I cannot figure out if I am running lean or rich or if its something entirely different from the jetting.


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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.

Your description is that of a bike too rich on the idle circuit.

However.... you may well have a too worn needle jet emulsion tube. That or you may have the idle speed too high, having you part way on the needle.

Fuel screw adjutment has to be done with the bikem idling as slow as possible to ensure you are not on the needle at all. The method above must be followed to the letter.

Bog is caused by the AP.

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