I'm a bit confused on which plugs I should be running... My bus is a 1974 1800c twin solex with electronic ignition. I pulled out the spark plugs as it was running rough, they plug looked ok except for No.1 which was black (dry), the existing plugs were NGK BR7ES, I went to a couple of places but they didn't have these in stock so they gave me Bosch WR7CC instead. Put those is but it didn't solve my issues so I took the bus to the mechanic who put it down to a loose earth wire from the battery. However the kombi is now backfiring when cold and gets mostly good after 15 mins of running, I pulled out the No.1 plug again and it black (dry), the same as the previous plug. I just looked up Ratwell and it says I should be running WR5CC as an equivalent for the NGKs. I will be taking it back to the mechanic to have a look but I just want an understanding of which plugs I should actually be using? The Bus is based in Sydney.