This has happened three times, only while parked outside, twice around 60 degrees and once in 40 degree weather, after several days of parked.

Good battery, cranks, stumbles, continues to crank then sounds almost like a hydraulic lock, then continues to crank but no start. Next day it started normally.

Next time: same symptoms, banged on fuel pumps and it started but ran rich until it cleared out.

Last time: same symptoms, banged on pumps and it started, ran rich until it cleared out.

It would be simple to change the 4 year old pumps but I do hear the prime pump run when the key is turned on. Somehow I suspect a check valve or something causing the rich mixture or flooding. Could the pump relay cause this issue?