Hi all,my sons prem keeps back firing mainly out of the left exhaust, we have changed carby,distributor,leads and it has fresh fuel and have just done a compression test and all are 150 even. we took this car off the road about 18 month ago and it was running fine but on gas,we have since removed the gas and put it back to petrol, it doesnt seem to have any vacuum leaks and the timing is good (have tried advancing and retarding it with no change) hope some one can help thanks