351 tunnel ram problem?

hi from new zealand i could use some help friend of mine has 351c not to sure of internals has twin 450 holleys on cain tunnel ram port matched to heads and extractors points dizzy convertered to electronic with an (ignitor) engine has quite bad stumble at idle but will rev up but standing still driving under load will only accept little throttle has a big spit with any more im thinking a vacum test and power valve/ shooters/jets/floats carbs found to be set up with secondries empty is this normal?at idle sounds like its running on six have checked and is firing on all eight!plugs fouling badly anyone with any advise would be great thanks