I almost forgot about it, but some mufflers may not have much restrictions inside so that it develops some good muffler pressure. I remember some guys having problems with certain Pitts mufflers with glow engines before. This was on engines that had too large of a bore carburetor on them thus making the engines more sensitive to being able to draw fuel OK.  So it may be worth keeping that in mind if they run different mufflers or Pitts type mufflers on the Evolution 10cc engines.  The common fix was to restrict the muffler outlet or outlets some more, which increased the back pressure giving more muffler pressure.