I'll preface this by saying I'm no expert but have read up on two-stroke tuning and follow the rationale, also owned a few early RM's over the years. This may help you to understand mixture settings in two strokes.
Many believe that by running less oil in their premix it helps to stop fouling by creating a so called "leaner" mix. In fact it does the opposite. By decreasing the amount of oil in your premix, you're allowing more fuel molecules into the "fuel (oil)/air mix" that's being created by the carby. Think about it, by decreasing the amount of oil, the void created is taken up by more fuel. As most of us know, the more fuel in the air/fuel mix creates a RICHER fuel mix, while less fuel creates a LEANER fuel mix. In your case you've created a richer fuel/air mix simply by removing some of the oil thus more chance of plug fouling happening.
If you want to run 40:1 or 32:1 (less oil than manufacturers recommendation) then you will need to re-jet to a smaller main jet (top rev range) and probably lower needle position (mid rev range) so a 'light tan" is achieved in a high rev plug-chop test. If you're running standard jetting now, then go back to the manufacturers recommended premix ratio which I'm sure is 20:1 for those bikes and see how it goes, you may be surprised.
Hope this helps,
Cheers
