1976 K5 blazer 350 5.7 4x4 (quadrajet carb) Today I did a tune up and now im getting a back fire coming out of my quadrajet carb. When I pulled the old plugs out they were very caked with black crud. I replaced those. I put new plug wires on and changed my air filter to a K&N filter. I also put a new rotor in. (HEI) When I started the truck up after the tune up the motor seemed to be running very hard/rough (very shakey) when I gave it gas in a pumping motion this is when I got a back fire coming out of my carb. The truck does idle. Should I check my timing? I do have a timing light but do I need to unplug anything before I time it? Any other ideas? I was also thinking maybe the new filter might have something to do with the back firing? what do youi think? Any ideas will be great.