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Another 2 stroke jetting question

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Hi all I have recently purchased a 2012 yz250 which has a pipe/silencer fmf combo and have participated in a few races as usual. The jetting for our 90 degree weather at sea level with 65-70% humidity is causing the bike to run rich. I have lowered the main jet to 175 which has made a minor differences in the top end but I I need to change a few other jets. Does anyone have any recommendations, information or even some website/articles to point me in the right direction?

Cheers,

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Hi all I have recently purchased a 2012 yz250 which has a pipe/silencer fmf combo and have participated in a few races as usual. The jetting for our 90 degree weather at sea level with 65-70% humidity is causing the bike to run rich. I have lowered the main jet to 175 which has made a minor differences in the top end but I I need to change a few other jets. Does anyone have any recommendations, information or even some website/articles to point me in the right direction?

Cheers,

If the 175 main caused your topend to go flat, the 178 isn't your problem. Have you adjusted the air screw properly to see if the pilot is too rich or lean? I'm assuming one step smaller on the pilot and possibly 1/2-1 step leaner on the needle will solve your rich condition. One step at a time.

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What exactly is your bike doing?

 

If it's running rich, it should be sputtering. Where in the throttle position is it acting up? This will tell you which jets need to be changed, or needle changed or moved.

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There are about 100 threads on jetting if you search and check out the FAQ, but I'll bite.

It seems that no two bikes are jetted the same.

Stock jets at sea level have worked well for me.

There are a ton of variations and opinions, but a step leaner on the main and pilot jets is very common practice. 48/175.

It would be rare to have to lean out the needle at all at sea level. You can swap needles to get a half clip difference...and that is a happy medium if you like to tinker with jetting.

 

HOWEVER,

Have you adjusted your mixture screw? (Lots of gains to be had.)

Have you cleaned your air filter? (A dirty filter or an over-oiled filter can both make it run rich.)

What kind of fuel are you using? (Different race fuels run considerably differently. I've found C12, C16 and 100LL to make my bike run rich.)

 

There are some other common problems out there, as well.

A fouled spark plug.

A leaky crank seal.

Worn out reeds.

A partially plugged silencer.

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Hi all I have recently purchased a 2012 yz250 which has a pipe/silencer fmf combo and have participated in a few races as usual. The jetting for our 90 degree weather at sea level with 65-70% humidity is causing the bike to run rich. I have lowered the main jet to 175 which has made a minor differences in the top end but I I need to change a few other jets. Does anyone have any recommendations, information or even some website/articles to point me in the right direction?

Cheers,

hi Yzrider

 

i am also in oz (queensland ) i ride a kx250 and my current jetting at our temp is 42 pj , suzuki NECW  needle (2nd clip position) and a 158 main , a/s screw 13/4 out

slide #8

 

yz have a #7 slide so i would look at a 40-42 pj , suzuki NECJ (2nd clip position) and a 158-160 main a/s 1 3/4 out

it should be farely close

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To answer a few questions I haven't touched anything apart from the main jet which did make the top end satisfactory. The biggest problem is from no throttle to full throttle where it has poor acceleration and lots of bogging. In Broome Australia temperatures are pretty warm with lots of humidity which could possibly cause the bike to run richer? Also the engine is fresh with a clean air filter. Thanks for all the responses too

Edited by Yzrider 09

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To answer a few questions I haven't touched anything apart from the main jet which did make the top end satisfactory. The biggest problem is from no throttle to full throttle where it has poor acceleration and lots of bogging. In Broome Australia temperatures are pretty warm with lots of humidity which could possibly cause the bike to run richer? Also the engine is fresh with a clean air filter. Thanks for all the responses too

Absolutely would run richer with more humidity and heat.

Generally a size or two is all you need to compensate for temp and humidity.

Just about every bike is set up differently and under different conditions, so the jet sizes can vary. The right way is to spend the time to take it down in steps. And I dont wanna flame the above poster but you never wanna stick jet sizes in from a different model or with a different needle.

Suzuki NECJ needles are great but you run jets that are much leaner.

It can be time consuming, but worth it. And youre guaranteed to learn a lot.

Also you may come to appreciate the genius of this little notch in the sprocket cover if youre using a 6mm socket on an extension to change your main jet.

Screenshot_2016-03-24-06-46-30.png

Edited by SellYour4Stroke
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To answer a few questions I haven't touched anything apart from the main jet which did make the top end satisfactory. The biggest problem is from no throttle to full throttle where it has poor acceleration and lots of bogging. In Broome Australia temperatures are pretty warm with lots of humidity which could possibly cause the bike to run richer? Also the engine is fresh with a clean air filter. Thanks for all the responses too

I feel you on temp and humidity yzrider

last weekend it was 32 degrees (8 degrees cooler then it has been and 80% humidity

With my settings listed above I still think mine runs a bit rich

Pic of plug

image.jpeg

Edited by kxrob
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