Both of my Rossi's (a .45 side exhaust and a .61 rear exhaust, I purchased new without mufflers and I think this was common for this engine. I used aftermarket Full wave pipes for both and both use pressure taps for the tank.

Can you post a photo to show what kind of muffer you may have.

When operating an engine that does not need a muffler pressured tank then the tank vent is with a short fuel line looped upward to face forward or angle cut to face forward.

John