2007 YZ125 tuning issue

[Mufasa]

Hey all, have just today picked up an 07 YZ125 for my son.
We took it out for a bit of a ride this arvo.
After it was warmed up a pretty major issue arose.
Its a bit stubborn to start, but once its going, it sounds like the engine is "bogging down", for want of a better term.
It wont rev out cleanly or idle, and stalls when trying to get moving, it wont even turn the back wheel without stalling.
Before going out, I drained the fuel, put fresh fuel in, removed and washed the air filter in clean petrol, and put new
filter oil on it.
It sounds like its starving for air or fuel, what I might have done is either put too much oil on the air filter, or not let it dry enough before putting clean filter oil on,therefore
possibly starving the engine of air.
I plan on checking the air filter, and if needed, remove and clean the carb.
The bike ran fine when we started it in the back yard, albeit, it didnt really get warmed up, and didnt idle then either.
Any suggestions, tips etc would be greatly appreciated.
Cheers
Peter

Edit: Further to all this, Ive checked the plug, its black, with small carbon deposits.(not sure of the definition of a "fouled" plug)

[BERT]

Plug doesn't sound great,
Could also be exausht choked with carbon, it doesnt take much buildup to wreck the performance of a little 125

[Heretic]

a fouled plug would be wet. dark black carboned means rich and maybe someones fiddled with fueling, or its been idling or putting around (previous owner scared of it)

what i luv about two strokes is they really WANT to run (if you know what i mean, if it can it will) if it starts first or second kick then its a good sign

so i rekon always go back to basics - air / fuel / spark / compression.

air - probably ok, wash filter or remove any obstructions (filter temporarally)

fuel - fresh fuel at correct oil mixture, does it rev with choke on?

spark - take the plug out, check colour and gap, ground it neatly against the head and give it a good kick, (eeerrr the kickstarter, not the plug) what colours the spark? yellow - blue strong weak, have a spare plug or 2 ready if its fouling

compression - finger over hole, heaps?? ok then does it have some when you kick it.

when the bikes running hot what the exhaust gas look like?

hope that helps, should give you a direction before someone says crankcase compression or sumtin like that....

[bomoto]

Quick check just to isolate the air cleaner, pull the air cleaner off and see how it goes????

[seahorse]

.+1 bimoto, you may have excessed some oil and water into the carbs by doing the right thing, cleaning the filter. They only need a very light oil, then wring the excess out of the filter before install, clean the carb again, and do a bit of research on standard jetting for that model.

The previous may have it jetted way rich for for lots of high end WOT stuff, hence you cant get it going properly down low.
It may be geared that way as well.