My 1977 280e runs roughly. New fuel pump, fuel filter, the usual tune up. It is a little hard to start when cold or warm. It backfires out of the intake when you step on the throttle. I have disconnected the intake vacuum line and plugged it that leads to the cabin. The EGR valve has been plugged off as well. Most places I have read always say the backfire issue is almost always vacuum related in this car.I've checked all of the rubber vacuum connectors and they seem to be good and snug fitting.THe car idles at around 550 RPMs. If I disconnect the vacuum line on the distributor (the one on the front not on the drivers side of the distributor) the RPMs increase but I still have the backfire problem. I am used to american made cars and I don't no where to look for a solution to this problem. Help!