In case of Jetronic injection equipped 33s the system is very sensitive to vacuum leaks
between the vane air-flow meter and the intake ports.
The following things may need to be checked:
- Condition of high tension parts of the ignition system including rotor arm,
distributor cap [centre carbon brush length and the wear four segments],
spark plug leads [in a dark place you may see small sparks through the insulation
[when the engine is running], and the spark plugs.
All four spark plug electrodes should be light brown(ish).
Differences between the spark plug electrode colors may result of uneven compression.
- Basic ignition advance timing at idle RPM, with engine at operation temperature.
Checking with a timing light/strobe gun the advance should be stable, no jumps
allowed. (jumps in the timing advance may suggests failed ignition advance control unit)
- Air filter, fuel filter, correct operation of fuel pressure regulator.
- Throttle position switch and it's correct angular alignment.
- After many years of service the air-flow meter could be faulty as well. It may sends
incorrect values to the injection controller unit, mostly in the low RPM range.
I hope this may helps.