Good news--finally got my Newport road worthy, put a high-flow air filter on it (the common kind with the round, chromed intake) and a 64 jet and it goes almost 30. Runs great, idles all day.
Here's the thing: a funny pop/hiss sound comes from the air filter, in time with the engine. If I put my hand around the filter intake, I can feel a slight pressure change every time it does it, so I would guess it's the sound of air hissing past the intake port as the piston travels up and seals it off.
Anybody know if this is normal? Maybe it's just because of the high-flow filter, but the same filter on my other Puch never made this noise...
The bike has good compression by the way, just checked it yesterday.