I have installed a toilet on the rough-in that was in my basement (built in 1973).
The toilet flushes just fine on the 1st attempt
(or after it has been left alone for awhile), if I wait for the tank to refill to the proper water line level and flush again - no good, just swirling water no flush.

Can the fact that this is in the basement and that new toilets use less water require me to
Re-build the vent? I was wondering if back in the days when toilets used a ton of water there was enough pressure to create the syphon with whatever size pipe was used for the vent BUT nowadays with 1.6 gpf toilets could they require more/better venting?

If I pour a bucket of water in the bowl I get a great flush every time.

Thanks
Dan