Badly missing
When I picked the car up and started to drive off it started to missfire badly like 1 or more leads were off. It would randomly come good and then get bad again. Its blowing out heaps of black unburnt fuel smoke which it never used to do.
The first thing i thought was they have stuffed my o2's since they moved them slightly (about 6 inches) so I took the car to my tuner who turned off the o2's in the tune which made no difference. If anything it made it even worse. I have changed my spark plugs also to no avail. The motor is a crate motor (l98/l76) with only 3,000kms on it so the leads are all near new.
So the only two things that were done on the day the problem started was new tank of fuel and exhaust work. They didn't take the headers off they just changed some flanges, put 2 new cats in and re-made some pipe for better ground clearance.
Any ideas what the hell is causing this? It gets so bad that the car feels like its gonna cut out sometimes. Could they have shorted something out when welding the exhaust?

