There is a strainer in the fill neck. The overflow would act as vent for the filler neck. Other than the stainer being clogged or the tank being full. There should be nothing obstructing a fill operation. Gas gauge broke?

There is a thread entitled "slow filling with fuel...or is it just me".Dave's post #15 says there are 2 checks valves, 1 of which cuts the tank off in case of rollover. Give it a read , hope this helps.