I have a fairly accurate monometer (reads muffler pressure to within .01psi) and my engine /muffler combos are producing between .3 and .6psi at WOT and .03 and .05 at idle. Is this too much muffler pressure differential for the uniflow system?
Kewl.... Not ever have I seen anyone with such a tool using it on a model a/c engine, and especially not seen anyone who had stuck it into their muffler pressure line with the engine running. Amazing.... and fun to talk to someone using that sucker (pun? intended). (Can I borrow it?
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I've read that muffler pressure is normally in the pounds range, not less than a pound. And it really looks like your WOT pressure is out of the norm for what the books said. Also, I'm fairly certain that most 2 cycle model engines produce sufficient muffler pressure at idle to easily keep the pressure line pressured. Are the engines giving you problems 4 cycle? I'd figure they'd be the only ones that'd be able to cause the problem from the engine side of it.
There is also the possibility that the pressure tap is located badly. Where do you have it? If it's in the OEM placement, then you really have a baffling problem, but the problem shouldn't be with the muffler or pressure tap location in the system.