2003 YZ 450F idle problem HELP

7 replies to this topic
  • yamalama

Posted September 04, 2012 - 07:32 PM


I just bought a 2003 YZ450F the bike had the stock muffler on it. The previous owner drilled out the rivets and removed the Baffel and did his best to re rivet the end cap. well the bike was loud as al hell. so i wemt on line nd found a pro circuit 496 slip on pipe. wel i installed pipe and bike ran like crap. so i adjusted the air mixture screw under the carb or what i think is the aix mix screw. the bike began to run much better but the idle was off so i made adjustments. it idled good for bout 30 0r 40 sec then dies out. i thought i needed to open the air mix some more well the bike runs good but when i get off throttle it idles too fast for a few seconds then slows down to normal idle. i decided to burp the throttle again and same thing idles fast then ofter a few seconds idles down. if i adjust the idle screw down to wheir it idles after i get off the throttle it idles good for 30 or 40 sec then dies out I need help

  • grayracer513

Posted September 05, 2012 - 07:17 AM


That's a "hanging" idle, usually caused by a lean condition. Start by adjusting the fuel trim on the pilot circuit (not an air screw; meters fuel emulsion):


Use the information from this process to determine if the pilot jet is correct:


If these two steps don't straighten out the problem without changing the jet, check your valve clearance first. Tight intakes can cause these symptoms. Then, check that the hot start plunger closes completely, that you have no air leaks around the carb, and that the vacuum release plate on the slide is correctly positioned with the "square" end down, and there are no cracks or missing corners on it.

It's common for bikes that sit a while to partially clog the pilot jet. Because of their extreme small size, just a dry film of fuel varnish can cut the effective jet size in half:


  • yamalama

Posted September 07, 2012 - 01:21 PM


ok i tore the carb down and the idle jet and pilot jet plugged up. i cleanwd out the jets and carb and called pro circuit and the recomended settings for fuel screw is 1-3/4 out. once i re set and put carb back on she began idle real good. but when i took it out for test run. when i first get on the throttle it hesitates then revs up and runs good. and no mater what position i set the fuel screw it still hesitates. could that mean i still have trash in the carb.

  • grayracer513

Posted September 07, 2012 - 02:24 PM


It's good that you have the fuel screw set somewhere close, but "1 3/4 turns" is just a baseline starting point. Now you need to follow up by adjusting it correctly according to the RPM drop method that the link described. Once you have it set right, review the second link to validate your pilot jet size. Then, if it still hesitates, the next thing would be to check the accelerator pump operation and timing per the manual.

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  • yamalama

Posted September 07, 2012 - 03:18 PM


i did follow the instructions and no matter what i do it still bogs then takes off like a bat out of hell

  • grayracer513

Posted September 07, 2012 - 08:30 PM


Then it's time to look at the accelerator pump. Might also be useful to look at how hard you are busting the throttle open. You can get even the best tuned thumpers to stumble if you try hard enough, but if you are doing something less than snapping the throttle from closed to open in too high a gear, or something, then it shouldn't do that. Have a look at the pump diaphragm and the timing, check your slide plate, as mentioned, and look at the needle and the needle jet bore. I've seen needle jets wear into a long slot. Makes the bike unhappy.

  • yamalama

Posted September 07, 2012 - 10:54 PM


thanks in will and its not just snapping into the throttle when i ease into it its doing the same tinig funny.when i had the factory pipe that was gutted it ran fine just loud as all hell. i try to make it quieter and it turns to crap simply by putting a slip on pipe.... funny that bike would run just fine with the bored out exhaust now well thats just my luck

  • yamalama

Posted September 09, 2012 - 05:29 PM


Think i figured out the problem when i diled up he engine and began closing the fuel screw the motor never tried to kill when i closed the fuel screw fully engine kept running that tells me that the bike is way too rich and flooding out so im thinking change pilot jet

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