setting fule mix.

[garage dweller]

Hi Guys,
Trying to set the mix on a 1970 2.2 engine with double Webber's.
The thing seems to be permanently running rich-black dry sooty plugs-blueish smoke. Finding it difficult to get a clear sense of when the engine is leaning out by watching the idle drop as it doesn't do so clearly/obviously.
Mix screws are out 2.5 to 3.5 turns from fully in.
Even so I am still getting small phut noises from the cabs and blowing back through the exhaust (2.5-3K Rpm). Carbs balance out ok using an SK-1 Unisync. Car other wise runs well but does blow back/spit when coming off the throttle.
Anyone used the gunsons color tune thing on a 911 as I am think of trying one to at least get six cylinders running even mix at idle. Is it possible the engine is over jetted? What would you guys recommend for Jet size?
brs
Neil