Have my carb off now, looking for cause of only running on choke

6 replies to this topic
  • Nitrousman

Posted April 02, 2010 - 04:16 PM


I have the carb apart and from what I see all the jetting is stock for a 2005 YZ 450F. I pulled the jets out and cleaned them, making sure they would pass fluid and I could see through them. Fuel screw is about 1.6 turns out. The bike has a full Yoshimura exhaust but other than that it's stock. What else should I check on it before I put it back on the bike? Accelerator pump looks good. Thanks for any help.

  • timoyz1

Posted April 02, 2010 - 04:20 PM


Was there an oring on that air screw? Or even is the hot start cable still threaded in good. Sometimes the plastic nut strips, and slips out.

  • Nitrousman

Posted April 02, 2010 - 05:07 PM


Was there an oring on that air screw? Or even is the hot start cable still threaded in good. Sometimes the plastic nut strips, and slips out.

There's an O-ring, then a washer then the spring and then the jet. The hot start looked good when I removed it from the carb. I don't think the previous owner ever touched the carb on this bike. The screws and allen bolts don't look like they've ever been touched.

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  • sro2004

Posted April 02, 2010 - 05:12 PM


I fought the same battle on my bro's bike. A new pilot (42 to replace the stock 40) fixed it right up. 03 yz250f idles like brand new...

  • Nitrousman

Posted April 02, 2010 - 06:02 PM


When I was blowing it out with my compressor there was one port that points into the air box the one on the right bottom when looking into the carb venturi that had a lot of dirt coming out of it when sprayed cleaner into it and then hit it with the air. Any idea what that passage is for?

  • grayracer513

Posted April 02, 2010 - 06:14 PM


There are two such ports. The larger of the two is the air passage that leads to to the main air jet and runs back to the needle jet/emulsion tube. The smaller one in which you can see a jet at the end is the pilot air jet and air passage to the pilot jet. If this was clogged, the pilot circuit would not have worked.

Trace all of the passages in the carb and identify their functions, being sure they are open as you go.

  • Nitrousman

Posted April 02, 2010 - 06:21 PM


It was the larger of the 2 ports that may have been clogged. At least it had a bunch of dirt coming out of it when I hit with air so I just kept flushing it with cleaner and air until it was clean. All the other circuits appear to be clear now. Would this larger port being plugged have caused the bike to only run with the choke on? Thanks.

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