Im dealing with a very stubborn hanging idle issue on my yz.
2023 250x. Fresh complete rebuild bottom and top stock bore and oem piston. Vforce 4x reeds. Apex XC head. FMF gnarly/shorty. Stock carb. Power jet is plugged and TPS deleted. Hitliss exhaust valve setup. Verified no air leaks, holds 8.5 psi indefinitely. 91 premix with Klotz kl-302 oil. The idle hangs regardless of jetting, needle position, air screw adjustment, or idle speed screw adjustment. Advancing and retarding the timing also does not fix it. Gas is fresh. Fresh plug. Filter is clean. Zero binding in the throttle cable and it closes all the way with some slack no matter the bar position. Exhaust valves are functioning properly. The bike revs up perfectly fine, no bogs, sounds very healthy. When letting off the throttle the bikes dips down to what would be the correct idle speed, but after 3-8 seconds it dips down way too low and it dies. Im at a loss as what to do next to fix this. Restoring the tps/power jet is not an option. Any suggestions at all would be appreciated
Throttle cable may be to tight or kinking, should have a little play in cable at idle.
My KTM with yz250 carb was running like shit. Turns out the cap to the carb that holds the slide and throttle cable had come loose. Must've gotten distracted in the garage and didn't tighten it down fully. Check your hose clamps that hold the carb to the airbox and reed cage as well.
Might get you a propane torch from home depot and apply some gas to various places and see if it revs up.
"The bike revs up perfectly fine, no bogs, sounds very healthy. When letting off the throttle the bikes dips down to what would be the correct idle speed, but after 3-8 seconds it dips down way too low and it dies."
This condition you describe is not a hanging idle. This is the opposite of a hanging idle. A hanging idle is when the bike is hesitant to return down to the proper idle speed and, instead, the idle "hangs" higher than it should, and then it won't die. It will sit there idling at a high idle, which is usually an air leak or lean pilot circuit.
In your case, the stalling issue sounds like it might be from something clogged in the carb or a leaky jet block o-ring. I would remove the jet block and clean all the passages well including the choke assembly and starter jet. Replace the jet block o-ring and also re-check the float height. Make sure the reeds are fine also and don't have edge chips or gaps. Also, completely remove the air screw and clean that passage out and replace the little o-ring and make sure all the parts are in the proper order and the o-ring is not pinched in there.
Verify that the tip of the idle screw is actually contacting the slide and lifting it and that it is not worn down or messed up. Most of the PWK's have a plastic idle screw that is a piece of crap. The tip can get worn down and the threads can get messed up easily.
Check all of the above first, but It sure sounds like something Is holding the slide up. Are you using the original carb and slide and cable holder or something else ?
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Thank you for the suggestions! Will check out the air screw o ring and jet block o ring. Will verify choke knob is sealing good as well. I am using aftermarket aluminum idle screws and air screws. May try the stock ones again to see if it makes a difference. To answer digger's question I am using the original carb, stock needle, slide, and cable holder.
Where is the best place to get a jet block o ring? And are they the same on all the pwk carbs?
JD jetting
Thank you
What does the plug look like immediately after a hard ride?
I had pretty much the same issue with uneven jetting - Main and needle were lean and the pilot was rich. It takes a little bit for the pilot to 'catch up' and richen a lean condition after holding it wide open, and this sort of thing will happen. Not saying that is your problem - but its something to look into. Try all of the above first.
You can also get them from Jets R Us.
Carburetor jets, parts and tuning items - Jets R Us
They are not the same on all PWK. The carb on the YZ250 is a short body or AG carb and requires that specific o-ring. If you look through the JD Jetting site you will see the multiple versions. This is the one for the short body carb.
https://jdjetting.com/product/jdkh47-keihin-pwk-ag-carb-jet-block-o-ring/
Got it thank you. I did order that one. I took the stock one out, and honestly it didn't look too hot. It was packed with that goopy orangeish sealant they use from the factory. (Same on the bowl gasket). Looked, at least to me, like they used too much...some appeared to even be spilling out and even partially blocking one of the jet openings. The mating surfaces were also not very smooth with a lot of casting marks in and around the sealing surfaces. Cleaned it all up really well and smoothed out the sealing surfaces with a fine brass brush and some 2000 grit. Threw everything in the ultrasonic cleaner. All looks good now just gotta wait for the new gasket
My spark plugs always come out a nice golden brown, never white or black.
I've never seen any orange sealant on any stock carbs. Have you had this bike since new? That almost sounds like maybe a previous owner threw some silicone RTV on it.
Give your slide a good look. If there is any wear, marks, etc. Replace it with a new one. Possibly your needle as well. Consider both of these pieces wear out parts.
You will notice the JD gasket is stronger than the OEM. Lightly smear some grease on it to help hold it in place when assembling .
I bought it new. Its that stuff that dries kind of hard. I believe they may just use it to hold the gasket in place when assembling? Could be wrong on that. My Beta that I also bought new had it as well on the carb bowl gasket
I have not heard of replacing the slide as a wear part before. What is the reason for this? Does it actually wear out and let air through? I find it hard to believe the metal slide would wear out before the cast aluminum carb body, or even wear at all considering is constantly covered in oily fuel. Obviously I'm missing something here
Pit Row
Yes correct, the slide is designed to wear out before the carb itself. The slide is made of Zinc Alloy, which wears out before the alum carb body. It usually takes alot of hours to wear a slide out, but it also depends on the maintence on the bike. If someone is bad at replacing air filters, etc. Or rides in a area with alot of sand or fine dirt. Its can wear a slide/needle out quickly. Anytime I buy a used 2 stroke, I do a top end, and replace the slide/needle and cable. Might be over kill but I've had one stick open on me before, and I don't want it to happen again...
Below is from google, you can check it yourself. They say 60,000 miles... But I've seen them wear out much quicker then that. I'm in SoCal, no shortage of sand here.
Interesting! I must have been lucky so far to never run across a bike that needed it lol. On my bikes I usually replace or clean the filter as soon as it has any visible buildup on it. Which is usually after one ride if it's dusty at all. Anyway the slide in it looks like new, I cant even see any scratches in it and definitely not any that you can feel. Same with the needle, appears new
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