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I recently bought an 06 250X with the following motor work done to it:

08 R cam/head
Wossner 13.5:1 piston
"Special" carb tune by Eddie Cruz Racing in Long Beach CA
4"x4" cutout on top of airbox, top triangle cutout on left side
Twin-air air filter
Stock header, FMF Q4 pipe w/ spark arrestor

The jet/needle set up is as follows:
172 main
42 pilot
68 leak
NCYS needle, clip in 6th position down from top

The bike starts and runs fine, but the idle is high and I notice slight bog at lower RPM (nothing severe). I ride at 4000'-8000'.

Now to the problem. I recently pulled the plug to check it, and found it was wet and black, lots of carbon buildup on it. I also peeked down into the cylinder, and the face of the piston looks the same way, dark black flaky buildup that looks wet. It does not burn oil or smoke, and after some research it seems like this carb set up is very rich for my altitude. Will an overly rich mixture produce these types of buildups? What about an overly oiled air filter?

I am going to do a compression/leak down test as soon as I can pick up a gauge. If the leak down test indicates blow by or bad valve seals/guides, that will answer my question, but I'm wondering if mixture could do this too. Any help is appreciated.

Welcome to the site !!

Yea that sounds quite a bit rich , i would think somewhere in the 140 to 155 range would be more appropriate and the Needle Clip more towards the 3rd to 4th position

Its running high because its rich and it is most likely running Hot (temp wise/overheating) as well

Leaner it will run cooler and definitely not be sooty , the bog would also indicate rich , since a lean condition would bog , but also pop and die

A overly oiled filter will also change the air/fuel mixture unless you compensate for it thru leaner jetting , but it still makes it harder to tune , since the oil will dissipate , it becomes leaner , especially if riding in hotter temps where the filter oil thins out , so you actually have a combination of things that are each contributing to your symptoms

Yes welcome toofiddy,

Have to agree with Jim way rich, surprised you done have black spooge running out the exhaust like an over rich KTM 2 stroke.