[Schulisco]

My experience: Grey/white smoke when engine is still cold is mainly water steam due to unburned hydrocarbons equals too rich mixture, condensed water in the exhaust and detoriated gas. As the gas is fresh I suppose a combo of both rich mixture and condensed water.
If engine is warm and smoke is black the mixture is still too rich. When smoke is blueish on warm engine then it's burned oil...

https://www.autozone.com/diy/exhaust/smoke-from-exhaust#:~:text=Dense%20white%20smoke%20indicates% 20a,of%20which%20are%20good%20signs.

As the 911G has no cooling water within the engine it's pretty clear...

The problem is with idling only in the garage you won't get the engine and the whole oil system and exhaust that warm to get reasonable results...and letting idling or even revving it half an hour or longer annoys the neighbours...and even it's not recommended for the engine (oil pressure and lubrication and even cooling is insufficient)

Check the mounted spark plugs too to see what color the have now. I suppose they look pretty dark too as tge car hasn't been really driven since...

The fuel pump must not run with ignition on on later CIS cars. I remember that the early CIS on '74 and probably '75 too didn't have the security fuel cut off switch at tge sensor plate housing. Check yours if it has one or not. Its a greenish 2pin plug on the "backside".