Unusual KX250 Problem

[Cannon384]

My son has had a 2006 KX 250 for about two years now. The bike is very competitive in mx and supercross and until recently has been trouble free. I want to stress trouble free because the jetting has been spot on and it never has fouled a plug.The problem now is that after a cold start up the engine fouls a plug within three to four minutes. If you clean and or replace the plug the bike will run great until it totally cools down again. I assumed it was time to freshen it up, so I rebuilt the top and bottom end with all new crank seals,crank, main bearings, piston and etc. I checked the reeds,wiring connections, silencer, timing, flywheel air gap, float level, and cleaned the carb. The bike starts on second kick runs about three minutes and again fouls the plug. The plug appears to be "water fouled" for lack of better explaination because it is a clear coating on the plug when I remove it. I pressure tested the cooling system and it holds 8 psig for 15 minutes.
I have always run high test pump gas, BR8EIX plugs, Klotz 40 to 1. I have always ran a 160 main with a 50 pilot. I dropped the pilot to 48 still no change...When it runs it has no hesitations, flat spots, or blubbering it just rips from the bottom to wide open. What am I missing?????

 

[oldguy]

Float levels might be set good but possibly the float is gas logged or the float needle or seat are worn (or a speck of dirt keeping them from shutting completely). Any of them would allow gas to seap in and when you kick the bike over it would get liquid gas into the cylinder and foul the plug

 

[KTA]

Is the spark a nice bright blue? If the spark is weak the plug will foul easily. Weakening stator, coil, or plug cap could all be culprits of a weak spark.

 

[whenfoxforks-ruled]

Seconds on the float valve. As long as everything else is good. Vintage Bob