Good points. The fuel pressure might be a possible cause. When I got this car the previous owner said the tank had been cleaned, but the car started running poorly after a while and I took apart the SU's and noticed they were full of sandy crud. I removed the fuel line near the pump to change the filter and a bunch of crud came out of it and the pump. I cleaned the SU's, installed a filter before the pump and even installed a brand new pump. It ran better, but still stumbled a bit when giving it a lot of gas. When I switched to the Weber, I installed another fuel filter after the pump. I suppose there could be some crud in the fuel line further back before the pump that is restricting the fuel flow, or maybe the filters are getting clogged even though they look OK (they are the clear plastic type).

If I buy a fuel pressure gauge to test it, what PSI reading would be normal?
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Tommy '64 122S