Dependent upon the state of tune with any cylinder, there will be spit back from a piston ported engine, especially with any increase of inlet duration.
One method of reducing spit back is to fit a reed valve, which should increase torque @ the bottom end of the range.
Having fitted that type to a Rapido, the reed valve 'civilised' the engine, but it was not a massive transformation into a "poor man's" TS1, where the reed is better placed, closer to the bore.
However, if a decent sized elbow & a high flow filter is fitted (as I have on a 71mm x 61mm Rapido) the spit back no longer becomes an issue, though the carburation will need leaning off @ the bottom to mid range. (I am constantly amazed that there are cases of the standard bellows being utilised on tuned/bigger carb engines, because I could never get them to breathe sufficiently even on a 175!)
The downside can be that the Dell'Orto's PH range so beloved by many Lambretta owners, tend to flood as they have no built in overflow, unlike the superior PWK
Whilst other's on this Forum advocate drilling a hole in any elbow to prevent the occurrence of flooding, I prefer to run a small flexible tube down to the lower fan cowling pipe clip (if present) that has wire wool stuffed up the end to provide filtration.
Although I have applied the drain hole filtration technique to several engines now, I no longer think it necessary for superior carbs, like the PWK (
) because they have a built in overflow/anti-flood.
I hope I have answered your question, in a very roundabout way! I would advise you try a very good filter system first, because the cost of those RVM's are knocking on the door of a good, basic long stroke crankshaft, which I'd prefer as a means of providing a performance boost & obtaining better port timings........allied to a good filter, of course!