Hmmm. A few thoughts:
Sure sounds like it wants more fuel, and the air valve normally controls that function (along with the EHA, particularly on startup).

A big 'ol air leak (like maybe the backfire blew a hole in the lower rubber cover under the throttle body) would require a lot more fuel (via the depressed air valve) but it would cause the engine to rev higher as it got more air via the leak and more fuel due to your finger on the air valve.

When you push the valve down to make it run, how fast does it run?

Does the air valve look like it's properly seated - if the pop was indeed a backfire, it could have damaged or unseated it.

Can you keep it running and check the vacuum gauge, should be pegged left if there's not a big air leak.

Might unplug the idle air control to take it out of the picture: it should rest partially open but stable when unplugged, so it doesn't change the air flow as you look for sources.

The EHA plays a big role in starting enrichment, but if adjusted properly doesn't do a whole lot during warm running conditions, so it may not be the culprit, just guessing.

A very rough pressure system check can be done by turning on the key a couple times, then pressing down on the air valve. You should get a noticeable resistance if there's fuel pressure. If you press and release it a few times you can feel the resistance drop as the pressure subsides. Then run the pump again, and wait a few minutes before pressing the valve, to see if the pressure holds. A good one will hold 30 minutes or more.

Thanks for the reply, I will check those things you mention.
If i depress the throttle plate more than just a crack it stops.
Open it too far it wont start.
Its almost as if the fuel system is getting a false reading and wont provide air to the system to start, butnis happy if you manually intervene but only up to a point.
I'll recheck the bellows later again to ensure its fit and integrity.

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