YZ250 Lean pilot issue

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11/30/2018 5:06 PM

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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11/30/2018 5:34 PM
Edited Date/Time: 11/30/2018 5:36 PM

Check stator side crank seal, 1 psi in 6 minutes is not good either, double check reed block to. Sometimes it's the small things, 55 pilot is big, what needle set at?

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11/30/2018 5:39 PM

93 octane is usually detonation happy on a yz with stock timing

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11/30/2018 5:49 PM

Kristen_Valentino wrote:

Check stator side crank seal, 1 psi in 6 minutes is not good either, double check reed block to. Sometimes it's the small ...more

Sprayed soapy water all around engine and saw no bubbles. Put propane in stator area while running and it made no difference. The 55 is big, you're right. That's my problem. If I go any smaller it's heavy lean on the pilot. Needle clip is 3rd but that's not my problem. Only problem is hanging idle(lean) and popping on decel(lean). I run less than 93 octane on my YZ125 and it runs like a top

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11/30/2018 6:20 PM

Your pilot is fhe right size when your air screw is 1.5 turns out. Go bigger than a 55. Also if your float height is a little low it will be leaner.

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11/30/2018 6:21 PM

Yeah, 55 pilot and closed air screw is definitely masking something. Try to swap carbs with a known good bike to troubleshoot. There are quite a few quirks with the PWK carb on the YZ250 that are hard to fix, especially as they get older. Also, a leaky right side crank seal can can suck air and cause a lean condition.

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Powerband in every gear !

11/30/2018 6:32 PM

Try brake cleaner or a shot of ether, double check float hieght, pressure test should be a sign something is leaking, I would recommend trying a 48 pilot if everything else checks out, see if it bring your airscrew back into range, not every bike is the same, on my stock bore yz I ran a 165 main, suzuki needle and a 45 pilot, it now has a 300 cylinder I run a 168 main, 45 pilot and suzuki needle with a small notch filed in the slide to get proper idle

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11/30/2018 8:07 PM

Could you not get it warmed up fresh plug, then do a 1/8 plug chop?

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11/30/2018 9:56 PM
Edited Date/Time: 11/30/2018 9:57 PM

kb228 wrote:

Your pilot is fhe right size when your air screw is 1.5 turns out. Go bigger than a 55. Also if your float height is a little ...more

Checked float height 3 times. I have it set parallel to the sealing surface. I ordered a 58 and 60, will try those when they come in.

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11/30/2018 10:02 PM

FGR01 wrote:

Yeah, 55 pilot and closed air screw is definitely masking something. Try to swap carbs with a known good bike to troubleshoot. ...more

Unfortunately I don't have another carb I can get ahold of at the moment. While I was air leak testing I put soapy water inside the trans vent line and didn't see any bubbles. Based on research and videos, I would see bubbles if the transmission side crank seal was leaking. I also put propane to the trans vent line while running and saw no effect. I guess I will go up in pilot size, 60, but I feel like that's definitely masking something for sure. I don't think it's float height because it was actually a bit rich up top with the 178 main(smokey and won't fully rev out). On 3rd clip on needle and it feels good at 1/2 throttle, plenty of power.

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11/30/2018 10:04 PM

Kristen_Valentino wrote:

Try brake cleaner or a shot of ether, double check float hieght, pressure test should be a sign something is leaking, I would ...more

Currently I have a 55 and airscrew all the way in. If I turn it out even 1 turn (55 degree day) it starts to hang and idle starts climbing(like I'm running out of fuel). If I put in a 48, it's big time lean and surges and sounds terrible.

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11/30/2018 10:08 PM

Jrewing wrote:

Could you not get it warmed up fresh plug, then do a 1/8 plug chop?

The problem is really on the pilot circuit. It feels good in the midrange (3rd clip N3EW) and a bit rich up top(178 main). I went to a 172 main but haven't brought the bike out to test with the main yet. Mainly been tinkering with the pilot trying to get it to stop surging after revving at idle. When I turn choke on(bikes already warmed up), the surging goes away and it sounds good(slightly loaded up but thats to be expected with choke on).

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11/30/2018 10:16 PM

Thanks for all the help guys, keep the ideas coming! Carb settings are 55 Pilot, 0 turns out on airscrew, N3EW 3rd clip, 172 main, power jet stock, float height parallel to gasket surface. Temps around 60 degrees and I'm at sea level. Lean on pilot(worse when cold outside), good on needle(based on feel), and was rich on main with 178, haven't tested with 172 yet. Figured I'd post settings so everyone knows what I'm working with. With bike warmed up, if I turn choke on, bike no longer surges on decel.

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11/30/2018 11:55 PM

Try the bigger pilot and if it runs good in the full range, makes power, spark electrode and porcelain are right colour. Accept it and ride it

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12/1/2018 1:27 AM

mix fuel at 50:1. it will help lots

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12/1/2018 3:28 AM

Radfonz wrote:

mix fuel at 50:1. it will help lots

Isn’t that dependent on the viscosity of his oil he uses though

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12/1/2018 3:52 AM

Radfonz wrote:

mix fuel at 50:1. it will help lots

Jrewing wrote:

Isn’t that dependent on the viscosity of his oil he uses though

The yz250 calls for 32:1. Dont cover up bad jetting or air leaks with premix changes. Thats not the correct way to fix it.

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12/1/2018 4:31 AM

Throttle cable isn't hanging up at all is it? Reeds are good shape? How long have you had bike for, I would also check timing,

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12/1/2018 6:34 AM

Kristen_Valentino wrote:

Throttle cable isn't hanging up at all is it? Reeds are good shape? How long have you had bike for, I would also check timing, ...more

Throttle cable is good, has good free play. Reeds looked good. Got the bike a month ago. Had a bad transmission gear, broken power valve governor and bad ignition coil. Along with other things like a stuck rear suspension. She's come along way and is actually a beast besides this small issue.

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12/1/2018 6:35 AM

Radfonz wrote:

mix fuel at 50:1. it will help lots

I'm running lean. That won't do anything for me.

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12/1/2018 6:36 AM

Jrewing wrote:

Try the bigger pilot and if it runs good in the full range, makes power, spark electrode and porcelain are right colour. ...more

But deep down I'll always wonder why she needs a 60 pilot to run properly. Oh well lol

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12/1/2018 8:01 AM

I ran into a very similar situation over the summer with a 14' xc 300. I was up to a 65 pilot on a jetting setup that shouldn't have needed anything more than a 45 pilot to make work, and the air screw also needed to be just about shout also . I went throughout all of the basics as mentioned above and I even swapped carbs to one that I knew worked fine. The only thing that I didn't swap out were the vent lines, and guess what the problem was ? The vent lines were plugged. I simply didn't notice it and over looked it. How often do you here of the vent lines on a PWK getting plugged and needing to be replaced?

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12/1/2018 8:56 AM

digger wrote:

I ran into a very similar situation over the summer with a 14' xc 300. I was up to a 65 pilot on a jetting setup that ...more

Totally forgot that this bike has no vent lines, I haven't put them on yet. And I sprayed every passage including vent line holes with compressed air yesterday so I don't think that's the problem. But thanks for the idea for sure!

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12/1/2018 1:20 PM

what slide do you have? and is it cut? is the plating pealing off it

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12/1/2018 3:31 PM

pete24 wrote:

what slide do you have? and is it cut? is the plating pealing off it

Factory slide , number 7 I believe. How do I tell if it's stock, I never cut it so I doubt it but not sure. Plating is barely peeling, looking pretty good to me

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12/1/2018 3:57 PM

plating is barely peeling? how loose is it in the carb, put some lite grease on it and try it that will seal around the slide and see what happens and god dammit don't ride it till you wash the grease off, if its cut its easy to see it doesn't look stock put up a pik of bottom of slide

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12/1/2018 4:07 PM

pete24 wrote:

plating is barely peeling? how loose is it in the carb, put some lite grease on it and try it that will seal around the slide ...more

ok, next chance I get I'll check slide looseness and cutting and also snap a pick. Oddly enough, I have to have the idle screw all the way in for it to stay idling , and it still dies after a few seconds when it's warmed up.

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12/1/2018 7:55 PM

scraprecon wrote:

Totally forgot that this bike has no vent lines, I haven't put them on yet. And I sprayed every passage including vent line ...more

No vent lines might be your problem. There's a reason the vent lines loop up over the top the way they do. It affects the vacuum and pressure in the bowl. One of my vent lines got melted by my pipe one time and the bike didn't run quite right until I fixed it.

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Powerband in every gear !

12/1/2018 8:07 PM

Also, not sure if you're looking at the YZ250 service manual, but there is a specific way to measure float height. Note the part about not compressing the needle valve. If you bend the tab so that the float is level when it's compressing the needle valve the float level will be too low and could make the bike lean.

Make sure you take the petcock off the tank and clean the screen and blow it out too. And make sure the cap is venting well. Test run the bike with the cap off to check.

Photo

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Powerband in every gear !

12/2/2018 8:13 AM

On my yz125, I ran with and without vent lines and it never effected the way the bike ran but I'll try next time I mess with it. Yep, i set the float correctly, where it's touching but not compressing. It was parallel to the gasket surface at that point. Bike only seems lean at idle. Was actually a touch rich on the main. If it was float height it would lean everywhere, especially when demanding more fuel(WOT). Runs like a champ at wot

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