This is particularly endemic to the new engine.

It's not a fuel issue usually. I've seen it on practically ALL of the Evolution engines I've owned.

The new engine builds up a "sludge" from the metal bits coating the interior walls.

You see this as a greyish sludge in the exhaust ( everyone complains about this in the Evolution threads ) that disppears once the engine has had several gallons run through it.

That "sludge" in turn builds up exactly as you said, in the pressure fitting.

You could re-tap the muffler to install a larger fitting and pressure tube...

You can also just run it for a while, cleaning the fitting out after each flight until it stops happening with such frequency ( I check mine every trip out to the field ).

As stated, you can try to adjust the muffler so that the fitting is on top, to lessen the build up.

You could use a different muffler...

Anyone else have any ideas?