hi all new at this so sorry for the mistakes.i have a 750 dbl.pumper 4779 holley on my 406 pontiac that i have bought new a year ago but only used for a month or two then it set the rest of the time till i got my engine back together. now it has a bad spluttering just off idle at about 2000 rpms while driving down the road at a steady speed.i rebuilt it but it still doing the same.changed the jets power valve from 6.5 4.5 still no change any ideal ?also if ican't get it fixed was thinking of buying a 750 mighty demon are they any good thanks.