yep, top up every 3 months is very minor

black smoke is too much fuel, you will need to try to work out why, but the idle is controlled independently to the rest of the revs, so if idle is ok and rest blows black smoke it may just need the mixture screw tuned, but if the carby was never jetted to suit the a15 then it may need re-jetting

gear stick slop is almost definitely two little plastic bushes on the bottom of the stick, they brak up and disappear, putting new ones in is a 5 minute job