To add a wee bit to Jim Sears response:

If the tank vent is not open, not only can a vacuum develop in the tank, but differences in ambient temperature can cause a pressure in the tank sufficient to cause carburetor flooding.

The lack of a functioning tank vent can cause some of the weirdest intermittent problems imaginable.

Jon.

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
If you truly believe "one size fits all," try walking a mile in your spouse's shoes!
The Carburetor Shop