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sputter/low rpm problems, Yamaha Yz250 02

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Hi everyone!

 

Hope someone can help me out here. I have a 2002 yz250 that won`t cruise in low rpm..  (Converted to sm so I want to be able to use it on the road without chasing the bike in the low rpm`s) The bike rides fine else, but at low rpm it is "ready to take off" if you understand. It does not bog. I think this problem is called "sputtering"? Its not any false air, I`ve changed all gaskets as the gearbox wouldnt live anymore.

 

The mods to bike are:

 

UNI Pod Foam Air Filter (I`ve fitted this because I have installed a battery in the airbox since it has lights and everything)

Fmf Fatty

PC 304 Factory Sound

 

The specs of the carb:

 

Pilot Jet: 48

Main Jet: 175

Needle: N3CW

Needle position: 3rd

Mixture screw: 1.5 turns out

 

All help is really appreciated because Im sick of taking the carb off and on, but I dont get rid of the sputter. I`ve tried to change needle clip up (2nd from top) and adjusting the mixture screw while I was driving (from 0.5 turn to 4) but that ended up with a bit better low rpm cruise but got a minor bog in the middle.

Edited by _Niclas_

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A large part of your problem is the pod air filter. Stock air box and air boot 'straighten out' the air flow.

 

Air screw is for idle, set it for a perfect idle and leave it alone.

 

Just off of idle and below 1/4 throttle is mostly the stright diameter of the needle, you can onlt change that by changing needle. I'd try one two sizes richer, if the problem is worrse, try on a size leaner and do it on step at a time until the roughness is resolved.

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A large part of your problem is the pod air filter. Stock air box and air boot 'straighten out' the air flow.

 

Air screw is for idle, set it for a perfect idle and leave it alone.

 

Just off of idle and below 1/4 throttle is mostly the stright diameter of the needle, you can onlt change that by changing needle. I'd try one two sizes richer, if the problem is worrse, try on a size leaner and do it on step at a time until the roughness is resolved.

Do you think that if I modify the air filter placing a bit could help? See if i can find a rubberhose or an elbow so the air flow maybe gets a bit more even?

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My 01 had kind of the same thing going on until I put a JD kit in it. It hated a cracked throttle. With the new needles it runs awesome all over the throttle range and cruises a heckuva lot better, even with the jetting a hair off.

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Do you think that if I modify the air filter placing a bit could help? See if i can find a rubberhose or an elbow so the air flow maybe gets a bit more even?

No, an air boot starts out with a very large opening and then 'funnels down' to the carb throat. If a manuafacture could use just a length of hose, they would of, it would be a lot cheaper than custom molding a boot.

 

With a handful of needles, an assortment of air jets, you might git to work better.

 

Keep in mind too, a stock airfilter is protected from side drafts and it too, gets airflow in a controlle manner.

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No, an air boot starts out with a very large opening and then 'funnels down' to the carb throat. If a manuafacture could use just a length of hose, they would of, it would be a lot cheaper than custom molding a boot.

 

With a handful of needles, an assortment of air jets, you might git to work better.

 

Keep in mind too, a stock airfilter is protected from side drafts and it too, gets airflow in a controlle manner.

Thats true! I`ve ordered some few new jets and checking out the #8 slide to see if thats a possibility. Meanwhile I`m going to just make a little shield or/and add an elbow between the filter and carb to get a more stable air flow. It doesnt cost me anything so ill have to try:)

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