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06 yz250f hanging idle

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Ive done a bunch of searches on this forum to try and solve but haven't been able to track down the source of the problem I am having - I have the hanging idle issue and for the life of me have tried everything i know to figure it out. The bike ran perfectly all last year without changing anything, but lately the bike will start up but then as you blip the throttle, it will stay at high revs for a few seconds before settling down.

2006 YZ250F riden at sea level, 75F temps.

I didn't look at the jets but am almost sure the jetting is stock, 45 pilot

FS 1-3/4 turns out (set using the highest RPM method)

Idle set to 1850 rpm

So far I have:

Cleaned the entire carb and replaced the oring on the slide plate.

Cleaned all jets

Checked for air leaks at the boots with the carb spray trick - no change in idle speed.

Checked valves - one intake is .004 to loose, the rest are in spec.

Proper slack in hot start cable and throttle cable

Top end is recently done

The only thing that is weird on the bike is that the cold start plunger will not stay open. I have to hold it out with my hand - I did spray some carb cleaner in the area and no change in engine idle. From what I've read the idle is a lean condition, but if I turn the FS any further out, the bike bogs noticeably worse when blipping the throttle. Is my only option is to possibly go a size richer in the pilot and leaner in the leak jet??

Also, the bike has a zip-ty fuel screw, but the oring is on the outside of the carb...it doesn't fit thru the passage in the bowl and it was this way when I bought the bike, so in order from the tip of screw it is washer/spring/oring. Is this correct? I haven't looked to see if there is another oring in the carb body.

Thanks for any help.

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Replace, do not clean the pilot jet - Most likely your issue.

Choke knob needs to be replaced, the detent is worn out.

Correct order on the fuel screw parts is Screw/spring/washer/oring. If there is an additional oring mid shaft too, that is not requred but is there to provide friction to keep the screw from vibrating loose.

ZipTy screws typcially make the bike pilot circuit rich.

Lower the idle speed slightly, ensure the fuel screw is set right.

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.

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