idle problem/ carb

7 replies to this topic
  • bt08231

Posted December 31, 2007 - 08:40 AM


Ok before i send this thing to a yamaha dealer, see if anyone can help me figure this out. We picked up an '04 yz450f, the float on the carb was stuck open so it would run it would just pour gas out of an overflow tube. So we pulled apart the carb, cleaned everything and put it back together. It was running before hand and the only setting we messed with was the pilot jet. We also had to remove the throttle positioning sensor but we scribed where it was a put back exactly how it was. Heres the problem, the bike will not run without the choke on. It will not start unless you pull the hot start lever even when it is cold. It back fires alot and acts as if it is not getting enough air. The filter is brand new and clean. I think it may be the pilot jet but am not sure and need it fixed and running strong by this weekend. Any help is greatly appreciated. Thanks in advance

  • bt08231

Posted January 02, 2008 - 07:45 AM


any thoughts. Anyone with jetting experienece that may have input?

  • Ga426owner

Posted January 02, 2008 - 07:58 AM


I would have removed all the jets to clean it. Possible that some trash is causing this.

The TPS - I would double check those scribe marks - there is another coupler - this is what should be disconnected

I assume fresh gas is being used correct?

Also did the carb get installed correctly with the front and rear boots? No air leaks.

  • grayracer513

Posted January 02, 2008 - 08:09 AM


Check the TPS voltage according to the procedure in the manual. It may be off enough to cause trouble, although it doesn't seem that likely.

If that's not it, it sounds like there is still an obstruction in the pilot circuit. The choke/hot start combo you're having to run is evidence of that, as this is substituting for the idle circuit. Be sure to check the two air jets at the front of the carb.

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  • sirthumpalot

Posted January 02, 2008 - 05:13 PM


Make sure you put the slide back in the carb correctly. On the older models backwards looks correct, the 04' models might be the same?

  • hillclimbguy

Posted January 02, 2008 - 06:21 PM


Check the pilot jet. The hole in this jet is real small and plugs up if the bike sits long enough. Also check the fuel mixture screw at the bottom of the carb. Make sure its there. I've seen these things fall out from vibration or adjusted out too far.
If you have a shop manual. Adjust the tps or check it. Check your readings at the connector. ( Disconnected) Readings will be in OHMS. You will have a reading with closed throttle and throttle fully opened. Readings vary from bike to bike. Good luck

  • grayracer513

Posted January 02, 2008 - 07:57 PM


The coil resistance test only tells you if the TPS is OK. Adjusting it requires reading the TPS output voltage with the engine running. The procedure is on page 6-10 of the manual.

  • bt08231

Posted January 03, 2008 - 01:17 PM


thanks for all the replies, they will give us a good starting point to figure this thing out. Probably wont get to it until this weekend so i will follow up with either more questions or let u know what it was

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