Weber ADFA/ADM tuning?

[131]

I've removed the IDFs from the 131 and fitted a Frankenstein dual throat Weber. I've taken the venturi body from a Ford ADM Weber and fitted it between the ADFA choke top and throttle body. After a bit of chopping and changing main jets I've got the primary and secondary side of it running pretty well, it's much nicer to drive and more tractable than with the IDFs. Unfortunately I can't get a stable idle, the 290 degree inlet cam probably isn't helping. Anyway, it'll run at a bit over 1000 rpm, it sucks fuel in through the accelerator pump nozzle and the auxiliary venturi, the rich mixture causes the idle to drop to about 500, where the extra fuel stops and the revs pick up again, to then repeat. At idle it hunts continuously between 500 and 1100 rpm. So what causes the fuel to be drawn out of the pump nozzle and auxiliary venturi, and can it be easily rectified?