As stated by others Strombergs are prone to running on. This is due to their ability to supply a combustible mixture at very low air flows compared to Weber's. I had an early SAAB 99 many years ago that had an electric solenoid device retract the idle throttle stop when the ignition was tuned off so that the butterfly's fully closed when the ignition was tuned off to stop run on. The source of ignition will be a hot spot in the engine. This hot spot may be due to a combination of a number of possible causes and each needs to be analysed to see if its contributing. If the engine has only done a few km since a full rebuild the rings may not have been seated properly which may explain the oily plugs.
Options are:
1. Too high a compression ratio. You quote 9.6 but how has this been determined. Has it actually been measured or have you at least measured the head thickness. You would only need to fit a thicker head gasket if the head has been shaved and you want to lower the compression. If its truely 9.6 then it does not need lowering and the engine would probably run Ok on 91 Octane if everything else was right !
2. To low octane fuel - unless you have over 11.5 compression ratio 98 Octane fuel will be OK if everything else is right. With your quoted compression ratio of 9.6 then should not need 100 Octane
3. To advanced ignition timing - Advanced timing can contribute to hot spots. You're guru may have set it to what he thinks is correct but does he know whats correct and does he understand all the errors around ignition timing in various Lotus manuals and which curve applies to your engine. For trouble shooting purposes the timing needs to be per the curve in the manual which for the non emissions Stromberg engine is actually correct
i.e. 9 crank shaft degrees at idle moving to no more than 33 degrees at 6500 rpm, the details of the actual curve is in the manual.
4. To lean a mixture - Assuming your guru has the need wide band O2 meter and knows how to use it and knows how to adjust Strombergs and you have the correct needles, diaphragms and springs etc to properly service them. He should have the mixture in an acceptable range but it is easy to get Strombergs running "well" but too lean. This will encourage hot spots from carbon deposits or sharp edges on the cylinder head or on the spark plugs. Strombergs are not common in Italy so maybe he has missed something. Even experts can get it wrong sometimes which is why you always need to ask for specific numbers not just assurances that its OK.
5. Sharp edges in the cylinder head - if the head was machined to fit new valves and valve seats and a sharp edge left from the machining operation that edge can overheat and glow. Normally this goes away with time a the edge gets burnt and rounds itself off. But if oil is present the oil will burn onto the spot and leave carbon that will continue to glow.
6. To hot plugs - Champion N9YC plugs should be OK. You could try a colder plug such as the NGK BP7ES or Champion N8YC but if that fixes it then it says something else is wrong such as too lean a mixture and or to advanced ignition
7. Carbon deposits - With the oil in the engine carbon can be quickly deposited which then is heated and glows red hot and provides an ignition source after the engine is switched off.
If everything else above checks out OK this would be my best guess. Does the running on occur only after the car is driven and not occur if the engine is started, warmed up at idle and then turned off ?
regards
Rohan