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98 yz250 backfires, runs, fouls plugs and lean

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Hey guys, first time posting. I picked up a 98 yz250 that didn't run. It has an fmf fatty with matching silencer. Found out carb was clogged, cleaned it thoroughly and replaced the stock jets with new ones of the same size. New spark plug and fresh gas at 32:1 with castor 927.

Got the bike started within a couple kicks, it had a high erratic idle and slight lean bog. I assumed air leak so I sprayed carb cleaner around the base gasket, carb and behind the stator, no leak. Checked compression: over 210. Reeds were in excellent condition.

Jets were:

172 main

Stock needle middle clip

48 pilot

Air screw less than 1 turn out

Checked float leavel, a little low so I raised it to within spec. Air screw needs to be less than 1 turn out to not lean bog off the bottom and to have a semi normal idle. I even tried up to a 55 on the pilot with pretty much the same results

So currently it's back to stock jetting with air screw 1/2 turn out. Rips and is crisp at all rpm's while on the stand.

Occasionally when trying to start, it'll backfire out the exhaust for a few kicks before starting. Stator looks like it's in the stock position.

I took it out a few days ago to test on my street, the bike rips off the bottom then sputters like crazy in the mid range from 1/4 to 3/4 throttle and then rips on top end. I tried changing clip positions but nothing had any effect. Leaned the clip and it sounded like an 80 and didn't want to hit the powerband. Richened the clip and it was super blubbery. The sputter is not noticeable when you roll smoothly on the throttle, only when trying to accelerate from a cruising speed. It can be quite smokey in the midrange but I had zero exhaust spooge after running a full tank of gas. It starts to jerk when idling in first or second gear and It also fouls 1 plug per tank on average.

Spark is excellent

It's getting fuel

Compression tested good

I can't figure out the back fire and the rich condition in the mid range. This bike is really throwing me for a loop and I've tried everything I know and have been reading forums for hours. Any help would be greatly appreciated!

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Dude, we need more details.  Ha ha. Just kidding.

 

Maybe a leak on the right side crank seal, and that mid range smoke when motor is under load is burning transmission oil.

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Sorry for the long post. I'm just trying to avoid the "check your reeds, try 40:1, try a new plug" Bla Bla Bla

Would a leaking seal cause backfiring?

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Sounds like an air leak, or dirty carb,

cracked carb boot, oil seal ?

Even though the jets are new,

an ultrasoninc cleaning does

wonders for getting the gum out

of the tiny passages in the carb.

Cheap ones on craigslist, etc,

and some people even advertise

a cleaning service for a few dollars

Also, even though the reeds look

new, are they fully closing ? Hold

them up to light, any gap means

they are floating, and breathing back

through the carb.

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Also, if a bike sits for awile, the

fuel and 2T oil in the engine will

drain down to the bottom of the

crankcase, and the fuel evaporates,

leaving a puddle of oil. Even though

the bike seams lean, when the engine

is revved, the oil will get picked up,

and can foul plugs. Takes awile of

mid to high rpm riding to get it flushed

out. Jet the carb to spec, and find some

wide open area's

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I can try an ultrasound on the carb, can't hurt. I've jetted it to stock specs and I've also tried fmf's recommendation, they run fairly similar with the stock being a little more rich. The reeds look good and seat well, I did the water test and they actually did not leak any water out.

Can you do the oil seals without splitting the cases? I feel like that could most likely be the cause. If they have to be split, I would just rebuild the engine at the same time

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Yes, seals can be done without

even draining trans oil. Lay the bike

on each side, dig out the seals carefully,

and put new ones in. Might need to

grind / customize an old screwdriver

to remove them. They are cheap

enough. Good trick i found is brake

clean the seal bores, and use a few

drops of green Loctite 680 / bearing

retaining compound to install seals.

They slip in way easier, and when

it dries, they dont come out til you

want them out. Also, the right side

bolt that holds the gear on the crank

on Yamaha's sometimes back out,

and ruin the cover. Loctite the heck

out of it after changing the seal,

and torque to spec.

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When you checked for leaks did you use a leak down tester and pressurize to around 6-8 psi? If so, how long did you wait to see if it held pressure? If it drops any more than 0.5 psi over at least 15 to 20 minutes, you have a leak that needs fixed. Ideally, it won't drop at all.

If you didn't use this method, there is a good chance that you do have a leak somewhere. You can build your own leak down tester for very cheap.

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I only did a compression test, leak down test sounds like it's a much more useful tool. If the seals check out ok, is there anything else to look into?

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If you don't know the history of the crank seals, and plan to ride the bike for a while, then I suggest you install new ones. Don't bother leak testing anything. Just replace them, and test by riding the bike. That's what I'd do!

With the right tools doesn't take much time to get to either seal. Be careful when installing the right side cover if you've not done it before. Jiggle and lightly tap, don't force.

If not the seals (I suspect the right one because it happened to me), then the 3 general areas are carb, spark and top end health (rings/bore). Maybe your carb is worn and malfunctioning? Maybe the flywheel is loose? Maybe the spark coil is faulty or has bad connections? I'm guessing. You have to keep going through things. Testing else replacing.

Edited by numroe

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I replaced the seals and the bike runs the same.

Is it possible that this carb is just busted?

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Still hangs slightly when coming off throttle and has no mid range, still backfires when kicking over. I'll do a leak down as soon as I get a tester

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You are the owner of the Keihin PWM 38 carb. It's notoriously shit. Go on JD jetting's website and order a PWK air striker 38 mm. You'll be much happier.

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If its coughing and spluttering from 1/4to3/4 of a throttle its odd on your needle is too rich, first adjust it up 1 clip and just see how it goes. Also replace needle and check what it is , 2 strokes are very simple.

Regards Dazz

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I read that the pwm carb wasn't the greatest, but it did run correctly from the factory so that's what I'm trying to get back to. I did move the clip up already and it felt like it wasn't getting enough fuel to get on the pipe and the engine was very loud and high pitched sounding. The needle is stock

Edited by Drew_Tdi

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On 1/8/2016 at 12:58 AM, Drew_Tdi said:

I read that the pwm carb wasn't the greatest, but it did run correctly from the factory so that's what I'm trying to get back to. I did move the clip up already and it felt like it wasn't getting enough fuel to get on the pipe and the engine was very loud and high pitched sounding. The needle is stock

Did you ever get this dialed in?  

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