The carburetor has an acceleration enrichment function, Alistair, so if the throttle opens suddenly there should be a brief puff of petrol smoke. If you can remember the days when cars had carburetors, they did the same thing, for the same reason. If you advance the speed control smoothly rather than abruptly, it should not smoke, and there should not be any steady-state smoke regardless of the amount of load on the engine. If it is doing those things, all is correct and it will look as if there was no problem except for a clogged air filter element.
