My kitchen sink drains slowly even after I snaked it 20ft without hitting any consequential clogs. I'm thinking there could be a couple of issues:
The trap diameter (2.5") is too wide for the inlet diameter (1.5") and therefore the pressure from the draining sink is not great enough to force the water past the trap. In which case should I just put in a trap to match the inlet diameter?
The air ventilation in the system is insufficient. If so, maybe a vent on the outflow side of the trap would help?