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cursed450F

weird running condition

7 posts in this topic

I have a 03 yz 450f with about 20 minutes of run time on a fresh complete top end.Almost everything up top was replaced:valves,valve springs,valve stem seals,cam chain,cam chain tensioner,complete piston kit O.E.M,machined and magnafluxed head,and all necessary gaskets.I'm having a problem with the way it runs now.When you rev it the motor revs up but kind of stays up at a high idle before settling down.I've checked both clamps for proper sealing to the motor and the airbox on the carburetor.The bike has been to two different shops,one an authorized Yamaha dealer,and the other a service shop close to my house with nobody seeming to know what's wrong.I'm ready to just sell the bike because I haven't ridden it in 3 months.Anybody with any ideas or suggestions would be greatly appreciated.

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Hanging idles are caused by a too lean condition, usually an air leak of some sort. Check the obvious and quick external things, like the carb isolator. Then, seat the fuel screw, counting as you go to determine where it was set, and back it out to 1 1/2 turns, and see if it runs better. Run it out 3 to turns. If it has improved before 2 turns, you're OK. If it takes 3 turns to get better, you may need a larger pilot jet. If it hasn't really helped by that time, check your intake clearances; they could have been incorrectly set, or the seats may have settled a larger than normal amount. Tight intakes are a very common cause of a hanging idle, but it would be unusual for it to be the problem in your case.

Another possibility is that something crusted over inside your carb as a result of sitting dry while the work was done.

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It is most likely a lean condition that causes the hanging idle. 'Ol gray pointed you in all right directions. Check them all.

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I had the same problem with my 2000 426 after I changed my cam to the auto decom cam. When I would give it gas and let off the throttle it would idle high for about 10-15 seconds and then drop back to normal. I was able to resolve it by changing to the easy adj fuel screw. Then I was able to make adjustments to it while the bike was running. Before changing to that easy adj fuel screw I was to the point of almost selling my bike.

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Check your hot start cable for freeplay you may have pulled it out a little that will do it too

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Check your hot start cable for freeplay you may have pulled it out a little that will do it too
While you're at it, check to see if the plastic nut that anchors the cable in the carb has broken.

Good call, Jack :)

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You already checked for air leaks so..

1) Clean the carb really well. While it's apart look for a cracked slide, etc..

2) Check the hot start operation, make sure it's not hanging open

3) Check your throttle cable slack, make sure it's not too tight. Take the plastic cover off of the carb and while it's running and idling high, see if you can spin the throttle control wheel back towards idle. If yes, then something is holding the throttle open (maybe throttle cable adjustment?).

hmm... that's all I'm good for this morning, good luck!

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