I have 1977 280e fuel injected gas engine. After many trials I have gotten it to the point that it will start and run...roughly. It back fires through the through the air cleaner when the throttle is stepped on. I have checked and the plug wiring is correct. If you slightly open the choke plate the engine evens out and runs smooth but still backfires when the throttle is cracked. I am thinking a possible vacuum problem exists although the power brakes seem to be great. Any thoughts would be great.