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Whats going on with this 250

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My brother @ebbskis has a 2003 kx250 with a fresh rebuild (top & bottom, replated cylinder, corrected squish.. yada yada)  Went out on first ride a couple weeks ago and it was running great then blew a base gasket (don't buy athena gaskets).  the bike seemed to step down when coming down in rpm, not sure if you would call it a hanging idle or not, but when it comes down its goes dun dun DUN dun DUN. with the leaky base gasket it seemed like an air leak was the likely problem. replaced head and base gasket with OEM and it still had the same problem.  Checked the new base gasket, carb boot, LS crank seal for leaks by spraying starting fluid on them while running, no change in the engine RPM.  Leak down test also held solid at 6 psi for 10 min.  He just rebuilt the carb and started it again and still the same issue. Says it seems to get worse as the engine gets warm.  I believe the jetting to be as follows 50 pilot, JD blue needle #3 slot, 162 main.  I know most of the time a bike doesn't come back to idle smoothly it can be a little lean causing the hanging idle, but can a carb being jetted to rich (particularly on the main circuit) cause a similar sympton?   Looking for suggestions on what to check next?  I have the same bike so we are going to start swapping parts with the carb being first just to rule that out.

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You check the reed block?  Make sure it's tight for on and not sucking air there or crack in booty..

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Yea Reed block looked ok, there is a slight gap you can see in this picture, but to me it doesn't seem like enough to cause a problem.  The air boot is new so that can be ruled out.  Thanks for the tips.

IMG-20170513-WA0002.jpg

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What do you run for feul? Ratio?  Only does It when you let off throttle?  I raced one of them back then I don't remember my jetting being that lean but was usually 170's mains and 55 pilets on and my 250s any brand. Is it pinging do you mean?  

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Stock is 52-165 on pilot and main but we ride from 6-10k elevation we have to run leaner settings.  Fuel is vp 110 @ 32:1.  Is most affected coming down to idle would describe it as a stutter or stumble.

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Here's a video from today. 60 degrees, 5,000 feet asl, 162 / 52 / jd blue (middle). Just testing around the neighborhood- on the seat it seems a little burbly down low, but climbs through rpms very smooth. Sitting in the drive way is very choppy after rev, fairly rough step downs in my opinion.

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Just don't ride it down low low don't sound that bad threw phone tho

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Here's a video from today. 60 degrees, 5,000 feet asl, 162 / 52 / jd blue (middle). Just testing around the neighborhood- on the seat it seems a little burbly down low, but climbs through rpms very smooth. Sitting in the drive way is very choppy after rev, fairly rough step downs in my opinion.



I'm not sure what you think the problem is lol. Hard to hear in the video.

Get a little seat time riding it with that jetting in it. Sitting and idling isn't going to give you a good reading.

If it is choppy surging sound on decel, then your lean on pilot jet. I would try to be a little rich on jetting.

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On 5/14/2017 at 9:25 AM, ebbskis823 said:

IMG_20170513_172327197.jpg Plug after about 5 minutes, this was fresh it probably needs a little more run time.

plug checks are only good for the main

rest is by seat of pants riding 

if i ran those jets at you temp and sea level ( elevation would be worse) i would foul plugs and bike would run like crap

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