Check the bike starts - it should start about third or fourth time. If the bike does not start, the owner will go bright red and you offer him £50 for it. When (if) the bike is running, having made sure it runs smoothly without the choke, rev it gently a few times in succession to check the pick-up. The pick up should be almost instant - if it bogs down as you rev it, there will be either a carburettor problem or a dirty spark plug. It is always advisable to remove the spark plug if possible. The plug should be a golden brown colour, if it is wet and oily the bike is running too rich, if it is scorched, it has been running too lean.