Well, if yours is a cowled installation, you have two choices. One, fly it and see what happens. Sometimes it is not an issue. Two, go ahead and install the vent as a precaution. I usually just go ahead and install a remote vent myself.

What happens, is that, depending on the installation, higher pressure develops in the cowl in flight. This gives the carb a false sensing of the ambient pressure, and the engine goes rich. So if the engine runs great on the ground, but goes rich on you in flight, that is most likely the problem.

The line is generally run inside the fuselage, where hopefully there is no ram air pressure effect happening. The same thing could be accomplished by running a static pressure line outside the cowl. As long as the carb isn't sensing a false high pressure due to pressure build up in the cowl, you're good to go....

AV8TOR