ORIGINAL: opjose
Well obviously you cannot plug them up completely...
I've had the same problem as he has with the Pitts and the TH .46 engines.
I had the outlets crimped down to the point of being 1/16th "slivers", then I tried plugging one side or the other as well. No luck on several different TH .46 engines and serveral different Pitts mufflers. (I also ended up giving up with the same problem on an Evolution 1.0NX and Pitts inverted muffler...).
The muffler was not producing any pressure with this engine. It was as if the fuel tank was vented to the outside air.
As a result the engine will run, but it varies all over the place when the plane's attitude changed.
e.g. with the engine mounted sideways, making a right turn caused it to go rich, left turn lean, etc... due to the lack of pressure.
The pump does fix this though, as did switching to another muffler. The stock TH muffler did ok, but I had best consistency with a baffled muffler.
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We know that if the engine is running, there MUST be positive pressure in the muffler. The real problem is that the pressure tap nipple is in the wrong place. I would remove the tap nipple, plug the hole with the appropriate size and length bolt, then drill and tap another location on the muffler. It's pot luck, so pick a place that is convenient for you to handle when flying the model. The chances of finding a null pressure location in a positively charged muffler is very low, so just about any other location will statistically have to provide better results.
Semco made a Pitts muffler some years ago with the exact same problem. Relocating the pressure nipple was the cure then too.