There is a procedure that should be followed for all two stroke carburettors. Fundamental, as all engines have to go up from the lower range & back down again is controlled by pilot in conjunction with the mixture screw.
Control of the tick-over (which might appear to now be the nub of the problem
) is in general set by an idle adjust screw. If the screw acts upon the slide, then if either wear, it may mean that a fast enough tick-over may not be achieved. In any case, presuming a spring & washer to be fitted, often a substitute shorter spring can be fitted, thus allowing a nat's more of the screw to go in as the spring can often get coil bound.
An easy-peasy check is to temporarily remove the spring completely from the idle adjust screw & check if that makes all the difference.
As it's been suggested, my opinion is that there is a herd mentality concerning what is a safe retardation of ignition, but the same could be said about the choice of carburettor bigger than OEM. That's not meant to be a criticism, honest(!) but my belief is that those owner/rider/tuners that are involved in all aspects of engine tuning/optimising are likely to have a far greater understanding of causes & effects whilst being open minded to things that get done to other two stroke engines besides Lambretta. Personally, my current rule of thumb for a static ignition is 19 DBTDC within the top half of the rev range & a tick-over will be achieved with a J/Lui/Vespa PX/AFR lightweight flywheel. That said, were I to build an engine that revved substantially more, I would expect the setting to be different, but by then I'm almost certain to opt for one of the retarding set-ups to hand.
Unfortunately, anybody can call themselves a tuner, even though, principally, all they do is assemble the parts. IMHO a 'proper' tuner will be capable of preparing an engine to a specification that will deliver good power & reliability, even if some elements of the work must be subcontracted out, such as welding, boring, plating etc.