As a matter of fact is is only when idling, revving seemed to clear it so seems like a blockage that comes and goes.....
Last night changed needle valves, gaskets etc and checked float level @ 12mm. on both. One carb was slightly out - too big a gap. Noticed that the car started this morning with less time waiting for pump to prime and started first crank .... Ran well today although it slowed and stalled after idling ( warm engine) and ran rough at restart. It cleared after slight reving and then I reset the idle slightly faster and all seems Ok.