Yep spot-on mate, the secondaries do have an idle circuit, stop's the fuel in the secondary float chamber from going stale if the motor isn't gunned for a while and as you say the secondary blades do need a feeler gauge at 10 thou, if too far open you get the problem I had, if too tight there's a chance the blade's will jam shut in the venturies.Mc Tool wrote: ↑Sat Aug 01, 2020 9:36 pm Ha , that's what made me think of it , I too have a 390 holley and it too has an idle circuit on the secondaries, I think from memory that the secondary throttle plate opening is set with a 10 thou feeler gauge down the bore between the plate and throttle bore wall .
Lovely sound when them secondaries open!
Wayne