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I bought a 1997 YZ250 a little while ago knowing it needed some work. Clutch was bad but it ran fine. It got into the powerband great and it idled fine. I replaced the clutch plates and basket and I rode it a few times and then it just bogged out and fouled a plug. I went through the carb multiple times, rejetted it, tuned it, and still no fire. Has great compression and spark, but it is getting too much fuel. I flipped the reeds over as a test and it fired right up. I put in new reeds and now it wont start again. Same issues. I am thinking reeds yet but was not too sure. Any suggestions or help would be greatly appreciated. Thanks, Kyle.

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So you've got a few potential issues.

As far as I can tell, your bike should have a Keihin PWM carburetor, correct? What are your current jet settings?

If you rejetted it and that still didn't fix the issue, you may have either a leaking main seal (from the transmission) or a leaking mid body gasket in your carburetor. How is your transmission oil level? Is it lowering at all as you run it? What about smoke? Once it's warmed up, is it still smoking a lot?

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You need new reeds and you need to test the seal on the block by removing the block/reed assembly and pouring in some gas.

It should drip very very slowly. If it drips more than that the block is NO GOOD and needs replacing.

If it ever ran well with the jetting you had originally, there is no reason to change it.

You need to replace the wear components first, before trying to fix problems with jetting: seals, air screw oring, emulsion tube, rings.

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If you still have a PWM on it, toss it in the garbage and buy a 38 PWK Air Striker for it, or at least borrow one off someone else's bike to see if it's the carb.

If you're determined to keep the PWM, I have two of them I'll sell you cheap lol

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I've done a few on these engines.  Honestly being a 97 it probably is worn out and needs crank,bearings and seals.  Unfortunately that year you can't get aftermarket kit. Can get oem rebuilt tho if trust washers are ok and not worn off. That engine is all down low and mid power Yamaha shorten the stroke that year. You get a 99 or newer you'll like better 

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On 2/2/2017 at 3:35 PM, k.bilitz said:

I bought a 1997 YZ250 a little while ago knowing it needed some work. Clutch was bad but it ran fine. It got into the powerband great and it idled fine. I replaced the clutch plates and basket and I rode it a few times and then it just bogged out and fouled a plug. I went through the carb multiple times, rejetted it, tuned it, and still no fire. Has great compression and spark, but it is getting too much fuel. I flipped the reeds over as a test and it fired right up. I put in new reeds and now it wont start again. Same issues. I am thinking reeds yet but was not too sure. Any suggestions or help would be greatly appreciated. Thanks, Kyle.

Best bet is to part the motor for what you can, and put a newer power plant in. Its going to be a pain in the A to get all the rinky dinky little parts to scrap the engine together. Their are a handful of engine builders that could do a rebuild for you, but honestly youll be into it for the cost of a used NEWER gen yz motor.

Edited by NEGbrap

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