Does it run good all the time ?
If not, the B310 version of the "Charcoal Canister Purge Valve" may be bad, or some part of this system.

Debris in the float bowl can cause this, or as said before, float heavy from gas in-leakage.
Dunno if the 310 has a brass float like the 1200, most likely if so.
http://datsun1200.com/modules/mediawiki/?title=Float+repair

Remove the Charcoal thingy, if it's heavy, then further study on the service manual needs doing to see proper repair.

The early ones had a foam filter on the bottom of the ones I saw, dunno about your particular vehicle though.

I suggest checking the visible fuel filter for a dark line at the bottom that moves, that's water. Or some version of garbage that's not gasoline anyway.

Post updates to let us know what you found.