First, I'm no mechanic.......with that said I need some help. Driving my 4 speed yesterday. Got to 35-40 mph and started shaking and shuddering real bad. Back fired a few times throughout the process of getting it home. Engine dies several times on the route home. Thought I was out of gas at first, but same thing continued after adding fuel. Oil ok. Couldn't get it over 20 mph without knocking and shaking. Thoughts?