YZ250 surging when warm (over rev off throttle)

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6/19/2018 7:32 PM

Have been playing the “chase this issue down” game the last 2 rides i have been on. Getting tired of it. Bike has done this since after the break in period of full rebuild.

Video of what its doing: (not my video btw, but amazing example)



I started running a richer oil mixture (32:1-36:1) then what i ran before the rebuild (40:1) and didnt change anything. I am thinking it may be a lean pilot jet. Going up one might help?

Also had people tell me it could be an air leak somewhere. The 1st time it did it, i took ignition cover off to check for oil in there. Bone dry. Tighten down the ignition cover. Still dod it. Checked reed boot bolts/carb boots, still did it.

The 1st ride as well i did mess with the air fuel screw and all it really did was mask the problem. I have no way of checking the crank seals as i have very limited tools on me besides the basics as I am currently moving states.

Any ideas from others who have experienced this issue let me know what you did.
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6/19/2018 8:21 PM

Are the cases completely sealed with yamabond, or other case sealer? Ticking in that video is really weird though. Timing and all that is set properly?

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2017 RMZ450
2005 YZ250-sold :,(
1998 YZ250
2005 KX250F

80% of the time it works every time
IG @2HRacing
Thanks to : Factory Effex, N2Dirt, Acerbis, DT1, Fasthouse, Matix, FMF, ASV, 100% & Mika Metals

6/19/2018 8:52 PM

mattyhamz2 wrote:

Are the cases completely sealed with yamabond, or other case sealer? Ticking in that video is really weird though. Timing and all that is set properly?

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6/19/2018 11:31 PM
Edited Date/Time: 6/19/2018 11:33 PM

Definitely a lean condition... also the oil ratio while richer in oil to gas ratio is actually leaner in the air/fuel sense... I would definitely look for an air leak somewhere... you can do the old carb cleaner trick around all the mating seams and seal areas to see if you can pick up th idle to find the leak... also maybe the timings set a little (advanced... but I’d check for an air leak first... you can run the bike with the left side cover off and dust a little carb cleaner by the crank seal too to see if it’s leaking since it sounds like u don’t have the tools to pressure check it at the moment

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6/19/2018 11:33 PM

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6/22/2018 10:45 AM
Edited Date/Time: 6/22/2018 10:45 AM

Apparently nothing was wrong with my bike. I took it to the mechanic and he said if was crank seals or a gasket somewhere it woukd do it every time the bike got hot enough. He rode around for about an hour yesterday and it never did it once. I went and rang it out behind his shop and it never did it once either. I tried and tried... so i took it home. I an thinking it may be jetting or something along those lines now... i do ride up in the NC mountains everytime i ride unless its in SC. Maybe too lean of a pilot? I am going riding tomorrow and will update here i guess afterwards. Kinda stupid the bike wouldnt do ANYTHING but run normal while at the shop... lol

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6/22/2018 11:25 AM

Maybe the pilot had a piece of dirt in it that was restricting the flow.

Also, sometimes the vent tubes suck dust up them, and over time the slide area can get dirty enough that it makes the slide stick. Maybe that was the problem?

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6/22/2018 11:27 AM

CSAR FE wrote:

Maybe the pilot had a piece of dirt in it that was restricting the flow.

Also, sometimes the vent tubes suck dust up them, and over time the slide area can get dirty enough that it makes the slide stick. Maybe that was the problem?

No idea but he also said he messed with the air fuel screw. But i rang the dog s**t out of it and it never did it. When i change the oil and get my shock rebuilt next week i will pull carb off and clean it for peace of mind.

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