3 port / 4 port Fuel Petcock Valve.

[maciejp]

What is the difference between the 3 port valve fuel pump (one is IN and is OUT and one is vacuum). The 4 port is tagged as Vent. When I hooked up the petcock, It sprayed out gas. So does it go into the gas tank vent hose? What is the normal purpose for this 4th port?

[rks]

I would think the 4th port is used for a second cylinder/carburetor.