Adam944S2

I have a 72 911 with a pair of IDT webers. Car was misfiring a lot. Ran through electrics with no apparent problems, so figured maybe the carbs needed a rebuild. Stripped the carbs down and sure enough one had been sucking in water due to the pancake style air cleaners fitted. Lots of waxy emulsion inside. Got a kit, cleaned and rebuilt them. Also obtained a pair of carb bowls so I could fit the stock air cleaner which I got with the car. Also bought a filter king filter and regulator combined, as whoever did the conversion had utilised the existing injection filter. When I removed the old filter I noticed there were 3 pipes; one from the pump, one to the carbs and a recirculating pipe that goes back to the gas tank (according to the haynes schematic). As the filter king filter only has 1 input and 1 output I blocked off the recirculating pipe (is it OK to do that?). Cranked the car and it fired right up after about 3 turns. However as soon as I put the air filter on, the car stalls and won't restart and I can hear fuel bleeding through the venturis and there's fuel in the air cleaner bowls. When I remove the air filter it fires up after some cranking and plumes of white smoke come out of the exhaust (fuel!). I check the air filter, no blockages, car runs fine with it off. Put it back on minus the element same thing happens again. Take it off, fires up with plumes of white smoke again. I notice when I go to clip the stock filter box on one side is always difficult to clip down as it doesn't seem to line up, but with a bit of downward pressure it sits down and clips in. I'm wondering if that's putting presure on the carbs and inlet manifold, but don't see how that would make fuel flood back through the carb. I've checked float levels, fuel pressure (3 psi), etc and I don't have this trouble with the filter box off.

Anyone got any ideas about this and blocking the auxilary pipe from the tank?

Thanks in advance