i am having trouble with getting my 78 F250 with a 400 to idol smooth. it idols rough and shakes the hole truck. it goes away at around 1100 rpm. I changed the plugs. Found a vacuum leak around the manifold. replaced valley pan with just regular intake gaskets. Couldn't find anymore leaks. Not sure were to go next. I could use some ideas and help. Thank you.
What did the plugs look like when you pulled them out? That will tell you a lot.
The best thing to do is take a compression test and if thats ok do a complete tune up, wires, cap, rotor and points if it has them