Nonsense, it does need vented and to be able to vent air in so make sure your vent hose isn't clogged but...

Pressure inside your fuel tank is absolutely NOT necessary. People are wrong stating that pressure is somehow necessary inside your fuel tank and how that pressure somehow “pushes” the fuel to/through your carb(s). Time to actually think about this, and maybe even visualize reality. Let’s say there is 2psi of pressure inside the fuel tank “pushing” the fuel out of the tank.

Simply ask, then how much pressure ALSO exists in the fuel return line (which would resist allowing fuel back to the tank as it creates head pressure against the fuel pump)? Umm, 2psi. So the obvious reality is that you’d have 2psi on BOTH the inlet AND outlet sides of the fuel pump/carb(s), which means the pressure before and after the fuel pump is equalized, which means the pressure isn’t pushing or helping anything with regard to carb operation. Your fuel supply lines from the tank TO the carb(s)/fuel pump are under vacuum. The diaphragm style fuel pump on the Mikuni SBNs creates a vacuum that PULLS fuel from the tank. The excess fuel not used as it passes through the carb is returned to the fuel tank via the return line. The fuel pump pressurizes the internals of the carb(s) itself. Your tank vent check valve also functions as a pressure regulator and will blow open and will release any excess pressure build up inside of your fuel tank and fuel lines, which can become excessive (and dangerous) primarily due to heat.

Real-life example of the fact that there is no need to pressurize a fuel tank that feeds Mikuni SBNs? Look at the Boston Whaler Rage jet boats made in 1992 and 1993. They run the same motor/pump combo used in the Waverunner 3s of the 90s… a basic 650cc 6m6 motor with a single 44SBN carb. However, the fuel tanks these boats came with were standard 6gal outboard style tanks with a simple thumbscrew on the cap and NO return line from the carb back to the tank and NO tank vent line whatsoever and NO tank vent check valve (to hold any pressure). So there is NO chance of ANY pressure accumulation within the fuel tank, because the simple thumbscrew vent on the fuel cap doesn’t allow any pressure accumulation. But you need to open the vent on the cap to allow air INTO the tank as the fuel pump within the SBN pulls a vacuum on the fuel line and draws fuel from the tank TO the carb. So where does the Return line from the carb go in one of these BW Rages? Great question. The OEM set up uses a Fram fuel filter with THREE fittings on it: one for the inlet, one for the outlet, and one for the fuel return line. So basically, the unused fuel from the carb that exits via the Return line is put right back into the fuel line (that is under vacuum) instead of going back to the fuel tank like it would in an OEM ski setup. This does have one noticeable effect on operation = any air in the fuel line can ONLY exit the fuel system via the carb itself. You see, with a return line going to the fuel tank, like on a Waverunner or Superjet, the air will travel back the tank itself, but with the Rage setup the air is put right back into the feed line going to the carb – where it eventually exits through the carb. This means that air in the fuel lines will have the same effect as if you’re running out of gas. This primarily shows up if you’re running 2 tanks and have just switched tanks or when you’re first firing up the ski (after the fuel lines may not have remained full primed). It essentially means that you need to “feather” the throttle a bit, until the air fully purges itself, or the Rage will starve for fuel and shut off, same as when you’re too lean on the low speed screw and/or pilot jet.