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YZ250 Lean pilot issue

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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 the air screw all the way in and it's still a touch lean, very light decel pop. Runs good though, no bog. If I turn the air screw out 1 turn with the 55, it starts to rise idle, hang, like I'm running out of fuel. I'm at sea level and around 65 degrees. 32:1 ratio with 93 pump gas. Seems weird that I have to run a 55 while others are running 45-48 and I'm still lean with the air screw all the way in. With the bike warmed up and idling, if I turn choke on it sounds good(little loaded up), revs nicely, no hang or popping. Sounds like a textbook air leak but my air leak resulted in a loss of 1 PSI in 6 minutes, which I think isn't too bad. Anyone have any ideas? I could go richer on the pilot(58,60) but I feel like I'm just masking the problem. 

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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 the air screw all the way in and it's still a touch lean, very light decel pop. Runs good though, no bog. If I turn the air screw out 1 turn with the 55, it starts to rise idle, hang, like I'm running out of fuel. I'm at sea level and around 65 degrees. 32:1 ratio with 93 pump gas. Seems weird that I have to run a 55 while others are running 45-48 and I'm still lean with the air screw all the way in. With the bike warmed up and idling, if I turn choke on it sounds good(little loaded up), revs nicely, no hang or popping. Sounds like a textbook air leak but my air leak resulted in a loss of 1 PSI in 6 minutes, which I think isn't too bad. Anyone have any ideas? I could go richer on the pilot(58,60) but I feel like I'm just masking the problem. 

edit lol

 

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1psi drop within 6 minutes IMO is way too much, zero drop in at least 15 minutes is the rule of thumb, the longer the better.

Re-do your test and confirm where it's leaking out from, often the test equipment is to blame (homemade testers)

 

Ideally use a low pressure 0-10psi or so gauge, allowing to monitor smaller increments say 1/5th of a psi.

 

Edited by mlatour

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Couple things come to mind. How old are the crank seals? They can be very difficult to test and they aren't that hard to replace. Clean choke, pilot, carb again? Where are you losing that one psi? intake/read gaskets? Spray some soapy water during test?

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1 hour ago, mlatour said:

1psi drop within 6 minutes IMO is way too much, zero drop in at least 15 minutes is the rule of thumb, the longer the better.

Re-do your test and confirm where it's leaking out from, often the test equipment is to blame (homemade testers)

 

Ideally use a low pressure 0-10psi or so gauge, allowing to monitor smaller increments say 1/5th of a psi.

 

I was using a low pressure gauge. My tool had a very tiny leak so I will fix and retest

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1 hour ago, Kinger317 said:

Couple things come to mind. How old are the crank seals? They can be very difficult to test and they aren't that hard to replace. Clean choke, pilot, carb again? Where are you losing that one psi? intake/read gaskets? Spray some soapy water during test?

Not sure how old the crank seals are, haven't changed them since I bought the bike.  I've had the carb apart several times, it's definitely clean. Sprayed soapy water all over, very little bubbling at the tool fitting. No bubbles around reed, head, base, or cases. Will fix tool and retest. Also, I'm going to remove stator cover and put propane(unlit) near flywheel and listen for idle change. I believe the trans side crank seal would cause a rich condition from ingesting trans fluid. Thanks for the help so far!

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Not sure how old the crank seals are, haven't changed them since I bought the bike.  I've had the carb apart several times, it's definitely clean. Sprayed soapy water all over, very little bubbling at the tool fitting. No bubbles around reed, head, base, or cases. Will fix tool and retest. Also, I'm going to remove stator cover and put propane(unlit) near flywheel and listen for idle change. I believe the trans side crank seal would cause a rich condition from ingesting trans fluid. Thanks for the help so far!

Power valve shaft seal and breather tube can give a false reading also. Don't rule out "wet" side crank seal will usually suck air through the tranny vent. Occasionally gulps oil when splashed but is not submerged in oil. If you are riding the bike much you will notice a drop in tranny oil volume and experience weird lean conditions.

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3 hours ago, Kinger317 said:


Power valve shaft seal and breather tube can give a false reading also. Don't rule out "wet" side crank seal will usually suck air through the tranny vent. Occasionally gulps oil when splashed but is not submerged in oil. If you are riding the bike much you will notice a drop in tranny oil volume and experience weird lean conditions.

Pulled apart the carb and completely cleaned everything again. All passages, under choke, air screw, idle screw, everything. Cleaned all jets and double checked float height. Ran bike with ignition cover removed and put propane all around crank seal area, no change at all. Put propane around air filter and it shut right off, so I know the propane was working. Put propane inside trans vent hose, no change at all. Put propane around the whole entire engine, including carb top cap and choke. Even got under bike with propane and checked the cases, no change. It's not looking like an air leak. What else could it be? Bike sounds great at idle with choke on, I don't get it.

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How long have you owned this bike? Has a lot of ethanol fuel been run through it? The bike has a history of running well and this just started happening? where you getting it to "hang revs and pop on decel" when using the propane? Are all five vent lines clear?
Crank seals are difficult to test affectively. It's easier to change them then it is to test them. A lot of people have been frustrated by bad ones, keep that in mind if you start running out of ideas.

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47 minutes ago, Kinger317 said:

How long have you owned this bike? Has a lot of ethanol fuel been run through it? The bike has a history of running well and this just started happening? where you getting it to "hang revs and pop on decel" when using the propane? Are all five vent lines clear?
Crank seals are difficult to test affectively. It's easier to change them then it is to test them. A lot of people have been frustrated by bad ones, keep that in mind if you start running out of ideas.

Recently got the bike a month ago. Needed a ton of work. This has been happening since day one. It pops on decel and hangs when its running(Idling). I was using propane to find the air leak. All vent lines are clear because it did this even with them disconnected. I put propane around the flywheel and it had no effect, no point in throwing parts at it. Same with trans side seal, no change in idle when I put propane into the vent hose.

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29 minutes ago, HevyRotashunz said:
32 minutes ago, scraprecon said:
I'd rather not resort to throwing parts at it.

SOP bro. Good for you though if that's not something you ever have to do.

Every now and then(cdi), but I try to avoid it if I can

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I had issues awhile back. Felt like i was chasing my tail trying to fix it.

Ended up being the O ring on the air screw was gone. Deteriorated over time as i never pulled it all the way out before. Worth a check if you haven't pulled it out yet.

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8 hours ago, asian195 said:

I had issues awhile back. Felt like i was chasing my tail trying to fix it.

Ended up being the O ring on the air screw was gone. Deteriorated over time as i never pulled it all the way out before. Worth a check if you haven't pulled it out yet.

Great idea! Unfortunately, when I rebuilt the carb I replaced that whole airscrew with a new one including spring,washer, and o-ring, so I know that's not it. Thanks for the tip though!

Also got a vid of it running so you guys can see what you think.

 

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