I am putting back together a Victa 160 cc motor. There are two wires coming from the block, one a thick one that is the spark plug lead and a second, thinner black wire. On later models I note that this wire goes into the carby next to the wire that is earthed, but on this carby there is no second hole for the wire to go into. Is it just an earth wire or does it have some other purpose. Any help would be appreciated.