I've been running inverted enya 60's and Webra 61's in various engines for a long time. I've always used a medium plug with an idle bar. I've never had any problems as long as I had them adjusted right.
Foofinator, what engine/muffler combination do you use? I need to get a pitts style muffler for my Webra on a Sig Skybolt, but I need to be below 89db. Any chance you know what yours puts out?
John