Very High Idle.... then revs down....?

9 replies to this topic
  • lftdjeep426

Posted August 24, 2010 - 10:57 AM


Started last Wednesday, Went to ride and on the 4th lap I noticed that it (2007 YZ450f) wasn't decelerating all the way to idle at first. after two more laps of it getting progressively worse, I pulled off. I sat the bike on the stand and would rev the bike. It would rev clean, the throttle would snap back shut when let go but the rpm would only come down to maybe 2500 rpm..... hold for a second or two and then come all the way down to idle. Over the next few days I:
1. Changed the gas in case of bad gas
2. Took the carb apart to find nothing wrong
cleaned the carb, jets, accellerator pump, etc
3. Checked wiring and re-lubed with dielectric grease.
4. with running, sprayed the fore and aft carb boots with chemtool and
did not change the idle (no air leak).

Still does the same thing, not as bad and not consistent. Sometimes it will decelerate all the way to idle and then sometimes it still hangs up. I checked the slide and it is shutting properly without any hesitation. Also, when trying to adjust the idle screw, I can adjust almost 2 full turns without a change in idle speed and then all of a sudden it falls off and dies. I tried to make sure that the fuel screw was set correctly but without it idling properly I can't really adjust it. I set it at 2.25 turns out which is where I had it set prior to these problems. Nothing was different from the time before riding when it ran fine. Came home, washed off, started back up after washing to let it heat up and then put her in the garage. three days later is when all of this started happening.

Any ideas as to what could be occurring? :bonk:

  • grayracer513

Posted August 24, 2010 - 11:04 AM


Hanging idle is usually the result of a lean condition. Other than what you checked, a couple of common causes are the hot start plunger not closing or sealing, and tight intake valves.

  • lftdjeep426

Posted August 24, 2010 - 11:40 AM


I lubed the Hot-Start and made sure that it was working freely and not hanging up. The typical lean condition is what led me to believe that I might have had an air leak but the boots were still very playable and not cracks were apparent, I also lubed these slightly to ensure a good tight connection to the head/carb.

I didn't think about the intake valves. It did all of a sudden get hard to start but only after I took the carb apart repeatedly trying to find this problem. Just curious, how can the valves effect the idle? Not revving down? I'm not discounting it, just would like to know how they can effectively create this condition?

  • grayracer513

Posted August 24, 2010 - 01:52 PM


If the intakes become tight enough, they will start to lift much too early because they will come in contact with the quieting ramps of the cams too soon. They won't lift a lot, but it can be enough to cause exhaust pressure to leak into the intake tract, and that will dilute the intake charge and upset the strength of the intake vacuum signal to the carb, making it behave lean. It's not a theory, BTW. :bonk:

  • stroker101

Posted October 14, 2010 - 05:03 PM


just noticed the "hanging idle" on my 09 YZ450Fthis evening. i left the track early becuase i wasn't comfortable riding with it acting this way.

i will check the hot start tomorrow and a few other things. the bike sat for 2 wks before this evening. i put fresh gas in it tonight. i sure hope at 16hrs, i dont need any valve work

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  • mdkcrf250r

Posted October 15, 2010 - 11:19 AM


Not enough hot start lever freeplay?

  • stroker101

Posted October 15, 2010 - 05:52 PM


Not enough hot start lever freeplay?

well, there is slack in the hot start cable, but not sure if the plunger is actually seating 100%. more work to do. gotta wait for a day off from work.

like mentioned earlier in this forum...valve simming may be in order.
if i need shims, where would be the best place to get a kit from?

thx for the help :smirk:

  • crossedup1

Posted October 15, 2010 - 06:59 PM


hot cams shim kits are cheap and accesable at your local dealer through marshall, parts unlimited, tucker rocky or you can just order one off the net

  • grayracer513

Posted October 15, 2010 - 07:27 PM


If you need shims, the best thing to do is to just go buy the one or two you end up needing. If the Yamaha dealer doesn't have them, the Honda dealer is sure to.

Lean IDLE jetting can be the cause, but only the idle. Main jet and needle position won't do it.

Also check that your idle mixture screw is still in the carb, that the O-ring is not broken or missing, and that it's adjusted right.

  • locomike

Posted October 16, 2010 - 07:36 AM


My bike starts doing this when the weather cools off. If it's jetted for 90 degrees, and you operate it at 50 degrees, it WILL be lean, especially at idle. Usually the fuel screw can get it adjusted, but with the extremes in temperatures here in Idaho, I have to go up to a bigger pilot about this time of year, and usually drop the needle one clip position.

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