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YZ 250 turning off after 30 seconds

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I have a 99 yz250 that is freshly rebuilt and it will start with a bit of trouble, then run fine for about 10-15 seconds, then starts stalling but I can keep it running by reving it. I can get it stable enough to idle by itself by adjusting the idle and will run for about 20 seconds. Then it  starts to burp and shuts off as if I hit the kill switch, but I can sometimes save it by reving it again. It will eventually shut off no matter what I try to do... No idea what the issue is. Carb is clean, no sticky float, jetting is correct, brand new spark plug. Please help!

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How does it sound when it revs high? How are the reeds? Sound like at idle it may be getting too much fuel or not enough air? I'm not sure but I just got rid of my 98 yz 250 so I don't remember how the carb all works.

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Sounds normal when it's revved high. The reeds are ok and the jetting and airscrew is set to the setting for my elevation

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jetting is correct, brand new spark plug. Please help!

 

Did it every run correctly with the current jetting? Be honest.

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Did it every run correctly with the current jetting? Be honest.

I got the bike in pieces and put it back together. The guy who sold it used to ride it here.

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That doesn't sound like a solid conclusion of correct jetting. Could be OK. Just sounds vague to me.

 

What's your approx temp and altitude?

 

What size pilot jet and what needle clip pos?

 

When it's running badly, what's the exhaust smoke like?  None, minimal, lots, blue or white?

 

After it conks out and stops running, what does the spark plug electrode and insulator look like?

 

How fresh is the fuel, and is the pre-mix ratio known?

Edited by numroe

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the pilot jet would be the one for the idle, but I've had in a bike that the needle is not set correctly and let's to much fuel in the main and not enough air and it putters out. You can't take anything for granted when someone tells you something. People always lie it's their nature.

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I'm at about 3500 ft and between 10-30 degrees Celsius. Main jet is 168 and pilot is 48. Needle clip is the 2 position. The air screw is 1-3/4 out. White smoke when it's running and quite a bit. Spark plug looks clean and dry and I've got new gas with the same 50:1 opti-2 mix.

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Seems like the pilot is on the rich side, and add the low amount of oil, and you have an even richer setup.

You can start by dropping the pilot one number and maybe going to 40:1 on the oil

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I agree. For your altitude, try one size leaner on the pilot. Just a couple of $ to try it. It can make a big difference to idling quality. I found the air screw cannot completely compensate for the wrong pilot size.

 

Everything else you reported seems reasonable. Maybe after the rebuild your motor is more sensitive to correct jetting. That's not a bad thing.

 

Something else to check is your squish clearance. If too wide then your motor can be fussy with jetting and also hesitant to throttle opening.

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What is the squish clearance? Thanks I'll try the jet. I heard something about the crank seal on the left side being a factor, although it should be good.

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Search TT and the web re squish clearance. It's as fundamental in a 2 stroke as is jetting, porting, pipe and timing.

 

Being the minimal clearance between piston crown and head squish surface, at the points above each end of the wrist pin.

 

Better yet, search http://images.google.com for "2 stroke squish clearance"

 

Squish tight = deto and rough off idle.  Sometimes surging at idle throttle. 
Squish wide = hesitant and fussy jetting. 

I'd guess your pilot is too big, and squish too wide (like most stock YZ250s).

 

Lots of white smoke can mean a leaking right side crank seal. But your plug is still clean and you said your motor runs ok beside the idling, right?

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Ok I'll check the squish. Yea it was but now the bike won't even start even with a new spark plug. Could it sound like a magneto problem?

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Is the plug wet? Getting fuel at all? Did you ground the plug on a metal stud and kick it, showing a blue spark? Maybe worth popping the head/cylinder off (after checking the other things) and taking a peek...

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Yea checked all that, good blue spark, carb is clean an reeds are good. It doesn't make sense to me but I might have to pull the head off and check things out.

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