Unless it’s a specific form of fuel starvation praps, like something intermittently blocking your fuel like a plastic bag getting sucked over the outlet or a hidden fuel filter that’s almost blocked. The dizzy you have, I think is an 009. This type of dissy was mainly used on static styled motors like pumps and generators etc etc. It was also taken onboard initially by some parts of the racing communities that required idle and flat out and for racing bangers it was spot on. In a van, it’s fairly similar in that mostly it’s idling then all power required to get that brick of a bus moving. But it does have advance inside the dizzy but it’s nowhere near as refined as an SVDA which I think is single vac, duel advance. There’s also several different jets in your carb for various stages of the revs and loads. Sounds like your idle jets are fine but might be worth cleaning out the rest and possibly look for spindle wear ? Did you have the same problem before you fitted the new carb ?
Ozziedog,,,,,,,,,,I’m grasping at straws here