Lippitdoo You really need to state exactly what engine make and model you have or post a picture. Otherwise we are just shooting blanks in the dark. Many modern engines will not function properly without muffler pressure or some type of pump. The fact is most modern engines have a much larger venturi that will not provide as much suction and therefore will require muffler pressure, this is to provide more power

However

Many older engines were designed to operate just fine only on venturi suction BUT they will have a much smaller venturi for any given displacement to provide sufficient suction to move the fuel.

Soooooo Whatcha got?

John