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Old 05-02-2011, 02:12 AM
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Unhappy 400ex carb problem

Hi, I have a 2002 400ex that I can't get to idle without bottoming out the throttle stop screw. The pilot screw has he effect. It appears to be rich on idle but I can't find any air restrictions. It has an after market exhaust and a K&N air filter. It does have the stock jets (main & pilot) and the needle jet is clipped on the third from the top. The plug is black and the fuel smell at high idle is strong. I can pull the vacuum line from the intake side of the Carb and it smooths out. Been through the Carb numerous times with cleaner, fine wires and compressed air all with no changes. I've check the valves and found no problems either.
Any suggestions will be greatly appreciated.

Thank you.
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Old 05-06-2011, 10:46 PM
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Quote: Originally Posted by 2leftover View Post
Hi, I have a 2002 400ex that I can't get to idle without bottoming out the throttle stop screw. The pilot screw has he effect. It appears to be rich on idle but I can't find any air restrictions. It has an after market exhaust and a K&N air filter. It does have the stock jets (main & pilot) and the needle jet is clipped on the third from the top. The plug is black and the fuel smell at high idle is strong. I can pull the vacuum line from the intake side of the Carb and it smooths out. Been through the Carb numerous times with cleaner, fine wires and compressed air all with no changes. I've check the valves and found no problems either.
Any suggestions will be greatly appreciated.

Thank you.
vacuum line smooths it out?? do you mean the crankcase vent line ?

ifso, it sounds like your air filter is dirty,

remove your filter and see if it idles, make sure inside air box is clean
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Old 05-06-2011, 11:00 PM
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No, its the vacuum line to the air valve. I'm testing without the air filter. Way past something that simple but thanks for the input.
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Old 05-07-2011, 10:02 PM
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The vacuum line goes to the air cut valve on the side of the carb. The air cut valve shuts off air to the idle circuit during deceleration (enrichening the mixture) to prevent backfiring in the exhaust. The valve is spring operated and relies on vacuum upon throttle closing to close. If the air cut valve holds vacuum, it is ok. If the air cut valve does not hold vacuum, then it is open and may cause a rich mixture at idle.

Also, I presume that although you removed the air filter you have the top on the air box? If the crankcase has filled with gas (may be caused by not closing the petcock when not in use) the crankcase vent line will supply fuel rich air to the air box causing poor idling and plug fouling. The problem may worsen as the oil heats and the fuel vaporizes out of the oil at an accelerated rate.

Don't forget that a tight exhaust valve can cause an appearent over rich carb problem.

Sometimes the carb just needs to be replaced.

Good luck
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