Ka-pow!

The PV anti-backfire protection ball used to be on the bottom of the throttle body. You had to flip it upside down to see it. It was set into a pocket and the opening was folded over to keep it from falling out. Modern ones are a little different.
You may not be able to see a rupture in the PV.They make a tool to test them. When they develop a leak in the diaphragm, the engine can inhale extra fuel right thru it at idle. This makes the low speed circuit very rich, and turns the plugs all sooty and the engine can run like crap. And the power circuit who knows how bad it will run. But the clincher is that when you shut the engine off, fuel can continue to seep through the ruptured diaphragm and dump into the intake through the vacuum circuit. When it drips onto the hot intake manifold it will flash to vapor and rise up thru the throttle bores past the valves, and on up thru the air horn where you can see it as a wisp of smoke, to a cloud,depending on the severity of the rupture. If this continues long enough the fuel may puddle there, providing a nice rich boost to the start mixture or flood the engine, depending on the time to restart. If left long enough it can all evaporate, and it's a non-issue. 'Course it can also run into a cylinder past an open intake valve......... which can lead to hydrolock, or a bent rod in a worst case situation, or a eventually,a crankcase full of diluted oil at the very least.
When you take the metering block off, there should NOT be fuel in the vacuum cavity that remains on the main body. That would be the clincher.
But I don't think you have this problem,lol

Tight bolts do not guarantee a leakfree seal.