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scraprecon Started a new thread YZ250 Lean pilot issue
3/23/2019 6:38pm Bike is a 2002 YZ250 2 stroke. It's acting lean on the pilot circuit. Hanging revs and popping on decel. Fresh top end, carb rebuilt. Float height set correctly. Reeds look good. New spark plug. Starts right up, runs great but sounds lean on the pilot. Stock carb pilot jet for the bike is 50. Went to a 55 and...
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scraprecon Added a reply to YZ250 Lean pilot issue
3/23/2019 6:37pm Unfortunately, I’m not sure. Have had the bike for a couple months, don’t know previous history of the bike.
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scraprecon Added a reply to YZ250 Lean pilot issue
3/22/2019 9:46pm UPDATE. Checked the timing with a dial indicator. It was heavily retarded. Brought it to 0.001 BTDC and it runs perfect. More low and midrange and 0 backfiring/surging/pipe bang at all. Less smoke and doesn’t load up at all. Only problem I ran into was I have the stator plate fully advanced(clockwise) and ran out of adjustment in the bolt...
scraprecon Added a reply to YZ250 Lean pilot issue
1/15/2019 3:42pm That's odd because my yz125 and brothers cr125 idle great and don't have any surging issues. I've even see people with YZ250s have their bikes idle perfectly with zero surging. This is definitely not normal but until I figure it out I'll just have it not idle. Really not a big deal but I don't like having to keep hitting...
scraprecon Added a reply to YZ250 Lean pilot issue
1/13/2019 6:36am Ok, update. For anyone following this post, I hate to see them left hanging. Replaced the jetblock oring. Unfortunately, the one I ordered was slightly different but I managed to modify it by cutting it. It might not have made a perfect seal, but the bike runs way better. No more bog or sputter down low, nice and crisp power...
scraprecon Added a reply to YZ250 Lean pilot issue
1/11/2019 3:47pm Yep
scraprecon Added a reply to YZ250 Lean pilot issue
1/10/2019 8:45pm Yep, replaced the air screw o-ring when I rebuilt the carb. Also made sure the old one was removed. Also removed choke assembly and cleaned underneath it, everything looks good to go there. She runs good up top if I run it hard but feels rich down low. Keeps trying to load up. And that's with a 48 pilot, 2...
BenG Liked a reply to forum topic YZ250 Lean pilot issue
1/10/2019 8:41pm
Have you had a look at this seal on the air screw. This was mine on my 98 125, I had to fish it out with a paper clip because it was stuck. It was rock hard and completely out of shape. I also had to replace the choke/idle assembly for it to idle just right. 
This forum topic has 161 replies.
scraprecon Added a reply to YZ250 Lean pilot issue
1/9/2019 12:13pm What would the symptoms of a bad jetblock o ring be? My bike seems to like to run rich at low throttle openings and the anti tamper torx screws look damaged like someone has been messing with them. Ive never dipped the carb or touched those screws so I know I didnt mess anything up.
scraprecon Added a reply to YZ250 Lean pilot issue
1/3/2019 7:13am Update Ended up being the jetting. Went back to 50 and turned the idle screw out(it was all the way in with the bigger pilot) and it surges way less. I'm guessing the idle screw too far in made the needle sit too far out of main when idling, so it was getting excess fuel. She runs great but has...
scraprecon Added a reply to YZ250 Lean pilot issue
12/9/2018 2:00pm Ok thanks for the info! My brother has a pwk36mm on his cr125(he changed out carb), could I try that on the 250? It's a 36mm and has no TPS or Powerjet. Also has the screw on cap, not 2 screws like the 250. I'm not sure if the 36 and 38 have the same inlet size, will check when...
scraprecon Added a reply to YZ250 Lean pilot issue
12/9/2018 12:42pm What actually happens in a carb that makes it run lean? And can only be cured with a new carb?
scraprecon Added a reply to YZ250 Lean pilot issue
12/9/2018 9:27am I wish. Went through it multiple times. That muttha is clean
scraprecon Added a reply to YZ250 Lean pilot issue
12/9/2018 9:25am I got a yz125 that runs like a champ, im staying blue for now lol
scraprecon Added a reply to YZ250 Lean pilot issue
12/8/2018 1:52pm Ok, airleak tested today after fixing tool and it only lost 1/8(maybe even less) psi after 6 minutes. After 16 minutes, it had only lost 1 psi. Sprayed soapy water around the whole engine. Underneath it too. No bubbles whatsoever.
scraprecon Added a reply to YZ250 Lean pilot issue
12/7/2018 4:35pm Can't wait to get to the bottom of this, I'm really curious what's causing this. I really think I still have an airleak, just have to locate. Both new crank seals so I know those aren't hit. Going to hook airleak tester up again next chance I get and make absolutely sure it loses 0 psi in 6 minutes.
scraprecon Added a reply to YZ250 Lean pilot issue
12/7/2018 4:12pm Also put propane around air screw, idle screw, and throttle cap while running. No difference. Also made sure to air leak test after replacing trans crank seal with everything dissassembled and saw no bubbles while spraying soapy water. Looked perfect!
scraprecon Added a reply to YZ250 Lean pilot issue
12/7/2018 4:09pm I rebuilt the entire carb and luckily the kit did come with a brand new airscrew, spring, washer and oring. I also double checked to make sure old oring wasn't stuck in bore, all was good. Good suggestion though!
scraprecon Added a reply to YZ250 Lean pilot issue
12/7/2018 4:07pm Right now I'm running 55 pilot with air screw all the way in. It actually sounds alright, very little surge, when it's around 60 degrees. The colder it gets, the worse it gets. The more I unscrew the airscrew, the worse the surge gets. 55 with AS all the way in sounds best. Factory pilot (50) with air screw 1...
scraprecon Added a reply to YZ250 Lean pilot issue
12/6/2018 7:02pm So why does it run fine at full throttle? You'd think if it's running lean at idle it would definitely run lean under throttle. I like to hear reasoning/theory behind doing something instead of just doing it.