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Kborr

1998 yz 250 fouling plugs

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My nephew's new to him 98 yz 250 in bogging out on take off in first and second then smooth after that. Starts hard most of the time and fouls plugs a bunch. 2 today and 2 the other day. Seems like it is running rich. Any help please

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Posted (edited)

Check and adjust the float level. Inspect the fuel inlet needle and seat.

 

If this motocross RACE bike is trail ridden or, it's engine not pushed very hard,

it will require leaner jetting and a premix oil more optimally suited for that usage.

Edited by mlatour

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I turned the screw on the side of the carb in a half turn and it seemed to run different. Then backed it out a quarter turn and it ran way better till it fouled a plug 10 minutes later. It is not being raced he is a young kid new to a race bike. Yamalube 2r mixture 40:1 what do ya think

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Posted (edited)

Sounds like a combination of many things,

a 22 year old MX race bike with likely many worn out components,

combined with a newbie rider that doesn't push the engine hard enough yet.

 

Running better as you backed out the air screw right a way suggest the pilot jet circuit is too rich.

 

 

Edited by mlatour

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Here's a how-to guide to jetting:

http://users.conwaycorp.net/virtus/page2/speed/jetting.htm

and reading plugs:

http://www.braigasen.com/howtoread.htm

 

Keep in mind fine tuning the jetting is the very last thing you do after you've confirmed the engine is mechanically sound

(good compression, reeds, float level, no carburetor blockage, air leaks etc.)

 

Perhaps not the easiest bike to get started on, both for riding and wrenching/tuning.

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We started him on a ttr 125 little trail bike for 6 months and he outgrew it already so we got him this. He has been pushing it hard in the open fields and a little MX track but fouling plugs still. It feels like it is getting 2 much gas  for the amount of air going in and needs to be leaned out a bit. 

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On 8/1/2020 at 8:23 PM, Kborr said:

We started him on a ttr 125 little trail bike for 6 months and he outgrew it already so we got him this. He has been pushing it hard in the open fields and a little MX track but fouling plugs still. It feels like it is getting 2 much gas  for the amount of air going in and needs to be leaned out a bit. 

That screw you turned out a 1/4 turn, that's an air screw.

Turn it out another 1/2, see what it does. That will lean it a bit.

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