'03 YZ450F hanging idle, running rough.

2 replies to this topic
  • pace

Posted April 21, 2011 - 11:07 AM


Hello all,

I recently offered to 'fix' a buddy's bike, as he complained of stalling and difficulty starting. Since he is terrible with maintenance, I figured he probably just needed a valve adjustment or that his pilot jet was blocked.

I got the bike home, and this is what I found:

- Four valves were in the middle of spec. One exhaust valve was just past the tight end of spec.
- His pilot jet (#40) was partially blocked.
- He had a quarter-sized hole in his ignition cover (!). After talking with him about this, it seems like it could either have happened on his last ride or the one prior.
- Coolant was low (~half a quart).
- Oil was dirty. Oil filter was filthy.
- Air filter was extremely dirty.
- Dirt on the carb slide and under the carb top cover. I believe the latter is fairly normal based on my own FCR-equipped bikes.

Here's what I did:
- Shimmed the tight valve back to the middle of spec.
- Put new brass in the carb. Since the bike is jetted for altitude and he rides it down in Moab sometimes without re-jetting, I went up one size on the pilot and main, and lifted the needle. So it now has a #42 pilot, #160 main, and the needle is on the fourth clip. Set the fuel screw to 2 turns out.
- Fitted a new (used) timing cover.
- Changed oil + filter.
- Cleaned air filter.

Here's the problem:
The bike is idling very rough, with the RPMs jumping around. If I adjust the idle screw enough to prevent the bike from stalling, then after I blip the throttle the idle jumps up with the RPMs hanging. It kinda feels like it is super lean at idle, but oddly the situation seems to improve if I turn the fuel screw inward. I checked the clamps on the boot between the carb and head, and they are tight. Backing the idle adjustment outward helps with the hanging RPMs, but allows the bike to stall after running for a few seconds. Incidentally, when the bike first arrived with me, it would barely idle and would go 'buurrrrrr' and die when I cracked the throttle.

Secondly, after making the changes above I took the bike for a run up and down my dirt road, and at first it felt clean once above idle RPMs. However, after a few more runs it started hesitating/misfiring badly in the midrange. It also feels to me like the bike may be running hot, although I have nothing to accurately measure that.

My house where I was testing the bike is at 8,600' and I realize that the current jetting is going to be pretty rich, but I wouldn't expect that to cause the behavior I'm seeing.

Given the non-existent maintenance since he's owned it (~3 years), and the issues and symptoms I've described.... any thoughts?

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  • pace

Posted April 21, 2011 - 12:24 PM


..after reading some more, it sounds like a rich pilot might result in the bike exhibiting lean-like behavior.

I'm going to drop the pilot back down to a (new) #40. Also will check the accel pump diaphragm and drop in a new sparkplug.

Open to other ideas/recommendations.

  • pace

Posted April 21, 2011 - 03:12 PM


Think I have it figured. I dropped the pilot back down and fitted a new plug, and checked the diaphragm (it's fine). No improvement - the bike still acting like it's lean.

Sprayed some starter fluid at the carb-to-head rubber boot, and the motor immediately started racing. An induction leak certainly explains just about all the symptoms.

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