i just put my 94 supercharged cobra back together, had a blown head gasket. changed both gaskets and had the heads shaved just enough to clean them up. know i have a problem with backfiring out the exhaust and smoking bad only after getting on it hard (6000+) and then while coasting back down is when it does this, i have done a compression test and it's ok (150 on all 8) , checked the pvc valve, it seem ok. could it be possible that cause the heads was shaved i my have a leak between the heads and intake??? i am getting some oil in the upper intake too.