I should have added one more item to my post above -- which rainman eluded to: the exhaust pipe.
So let's change my previous post to: a combination of weight and gear ratio, PLUS the power characteristics of the exhaust pipes.
Be on the "wrong" side of all those, and keeping a clutch alive for any length of time is a challenge at best. Be on the good side (light kart, low gear ratio, and an exhaust pipe with a broader/softer powerband), and a light, possibly single-disc clutch will last a long time.
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Ted,
Something to possibly consider is that friction discs for a motorcycle may not have the characteristics that are ideal for a kart. The key thing is the difference between static and dynamic coefficient of friction. If that "spread" is too wide, the clutch may tend to "drop-in" a bit much (rpm drop at engagement) and possibly not "unhook" very well as revs drop. Both of those issues can be solved (or "tuned") by adding more spring and more weight, possibly changing material/hardness of floaters, etc., but it could still be something to think about.
It's all tunable with some creativity, no doubt.
Would love to see your design!
PM