Rich...Plugs wet with fuel or oil?
motox4fun,
My guess is that your plug is fuel fouling as opposed to oil fouling. It may appear that it's oil fouling because the engine can't burn all the fuel/oil it's receiving, causing the spooge out of the silencer from the unburned oil. At 50:1 you have richened your jetting, that coupled with the already rich jetting from the factory may be contributing to your problem. Remember, less oil means more gas in a given quantity of fuel (i.e., rich).
Have you tried experimenting with other premixes? A friend had problems with Golden Spectro and switched to HP2 with good results. Each bike tends to like something a little different.
First I'd drop the main jet one size, then drain your tank (I'm not a big fan of avgas). Mix up some 93 octane at 32:1 (you may try HP2) and run it. It should be closer to what you're looking for.
If you want some information on avgas, read this.
http://www.ericgorr.com/techarticles/avgas_excerpt.htm
Once you've decided on your best fuel/premix blend, then complete the jetting process. Your blend affects the entire RPM range, not just a portion of the range like the main jet, pilot/slow jet, etc. As the temperature changes you can alter your premix ratios to fine tune your bike. Significant temperature changes still require jetting work.
If you've decided on using your current fuel/premix blend and do not want to change, the alternative is to lean out your carbs’ jetting across each circuit.
Here are great guides for carb tuning.
http://www.ericgorr.com/techarticles/carbtuning.html#m
http://www.ecmx.com/bike_tips/jetting.html
For what it's worth we use 93 octane with HP2 @ 40:1 for the bikes below. None have been highly modified other than pipes, reeds, etc. Very little spooge and they run crisp.
'01 KX250
'00 KX125
'99 CR250
'99 KX250
'90 RM250
Keep us posted and good luck!
Scott