Hello All. my stock 1987 gn has started to give me a little trouble lately. i brought it to a shop and had the rear main seal done because it was leaking oil and now i seem to have a much larger problem. all of a sudden the vehicle has a hard start sometimes it takes upwards of 8 or more key turns to start it. when it does start it has a very rough chug to it and puffs out black smoke. it is running very rich, then when i put it in drive it almost wont work at all. when i put my foot on the gas to move it just kind of dies out and makes a backfrireing sound. i had access to a scan tool and ran and i have no engine codes. my next step i put the car in limp home mode and it drove just fine. so far i have tryed replacing the cpu and the chip ( stock ) plugs and wires. i also checked the maf sensor and it dosnt seem to be that either. could it have something to do with the Cam sensor? or the timing? any input would be greatly appreciated as to where to go from here.