This could be many things, and you’ll have to chase down the various possibilities one at a time, but here’s my recent tale of woe.
My engine had been out of the car for about a month while some transaxle work was being done. Nothing on the carbs, linkage, or exhaust was touched and it was purring when parked.
With everything back together, there was suddenly a persistent popping that the mixture screws wouldn’t fix. It was happening at one particular rpm (2500), and I could make it pop at will by holding it at that engine speed.
It turned out to be just dirt in one of the idle circuits that somehow magically appeared sitting on the bench, unused, for a month.
The weird thing is that as many times as I have cleaned those circuits over the years chasing down one gremlin or another, that had never before been the cause, but it seldom sits that long without being run.
So, if it’s been sitting high and dry for a while, maybe check the carbs (including the float bowls) for dirt, even if you’re sure it can’t be that.