Mk1 Cabby Digifant problem

Post

Hi all, got a problem since some time now with my 1991 Cabby.
Symptoms:
Car don't start well when cold, idle rough (like on 3 cylinders) for 10/15 seconds then adjust itself.
After a ride sometimes starts with a 1/4 of turn, sometimes takes forever…
When cold (never tested after warm) spits out black gunk from exhaust and smells terrible (see pictures)
Sometimes lose power when under load (very rare)
Idle, after a while starts to going down, almost like it wants to shut the engine off, then comes up again, this happens when warm too
Rattling noise under the car (could be the heat shield?)
Things checked so far:
Timing is perfect, igniton timing too (adjusted with a strobe at 2250, CTS sensor unplugged. 3 times revved past 3000 and plugged back.
No oil consumption, no strange oil colour
No russian salad in the valve cover, nor in the cooling fluid tank.
So far i already done:
New timing belt and full service, spark plugs, filters etc.
Already ordered (as a starting point):
New CTS sensor
New spark plugs (i suspect the Bosch Super 4 i mounted aren't good for this engine, so i bought a set of NGK BPR6ES)
New distributor cap
New rotor
New igniton coil
New igniton leads
What could it be? Someone said me piston rings or head gasket…but i never had to add any oil since almost 2 months of usage after serviced, and 1500km/900+ miles done