Sti_Pete

Have just imported a colin mcrae 555 and after some help.
At the mo i can't really drive it too far coz i don't want to register it until 1st may to avoid paying the sva.
Had a few probs at the start to get it running but now that it is, i've realised the exhaust is blowing and it is misfiring, i know from past experience that it maybe the coil packs, but am wondering if it could be misfiring because of the exhaust blowing could anyone tell me if they have had this problem if there exhaust was blowing?
Cheers
pete