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XT500 running too lean: hanging idle, backfiring, sloppy jalopy

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Hello thumper experts. My XT has recently developed an issue of running extremely lean. I've been trying to track this down for a little over a week and have tried a number of things but now I'm getting stumped. Here's why I think its running lean:

Hanging idle
When the idle hangs up after accelerating after about 15min of riding. It will climb to about 3000-4000 rpm. Not cool at a traffic light.
Engine speed has been is EXTREMELY sensitive to the choke position. Moving it ever so slightly has a huge effect on engine speed. Seems suspect to me...

It used to do this a little on deceleration, bit but now much more frequently and occasionally at idle. Happens in the lower rpm range below 3000 rpms.

Spitting/Lurching at crawling speed/low rpm
Throttle response feels smooth around 3000 and up.

Spark plug is WHITE
Used to be a nice golden brown

What I've checked:

- Timing: Timing and points gap are good. Checked it with a light while running and advancer seems to be ok.
- Cam chain tensioner is good.
- Spark: It exists and seems to be ok. No arching at the points so condenser would appear to be ok. Plug gap is also good.

- Checked for vaccuum leak:
- Replaced the intake boot, no difference.
- Sprayed WD 40 all around the boot and throttle shaft while running, no change in rpm.

- Checked for clog/Vent issue
- Blew air through the tank cap, feels like it breathes ok.
- Blew air through the carb vent and overflow tubes, they are clear.
- Flushed the tank and petcock, replaced with new premium gas. Gas flows freely from petcock.
- Checked the float level, no adjustment needed.
- Cleaned the carb thoroughly with dip bucket and compressed air.
- Cleaned the damn carb again just to be sure.
- Replaced the float needle and seat. Rechecked float level. no difference.

More info about the bike:
- Carb looks to be stock '77 carb - not a pumper.
- Carb has 340 main, 30 pilot, 2.5 inlet.
- No air box, large K&N pod (high flow?)
- Old style Supertrapp with 14 discs (seems like A LOT, but had been running fine with this setup)
- Piston appears to be 1mm over (marking seen through plug)
- Not sure if the cam is stock or not.

The bike has ran great until now, and I've changed nothing on it. Before I buy a new carb I wanted to hear if there were any other ideas... Let's hear whatcha got! ...Please :)


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The only thing I can add to the list of what you’ve done, this happened to me once. Bike was a 1975 Honda XL350.

The WD-40 spray check on the carb boot didn’t help me this one time. The air was sucking in at the mating surface at boot to head. The spray down showed nothing, but I went ahead and replaced the rubber O-ring and applied a liberal amount of Hondabond on both surfaces. Problem solved!

Good luck!

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