340 not running good need help

Hi,
Perhaps if you could provide us a little more info on the issue. Certain idles/loads/speeds. Is it backfiring?

As you guessed this could be a number of things. My 318 did this a while back and it turned out to be a single bad plug.

Could be a number of things,

Check the obvious things first. Plugs , Wires, Distributor (check for moisture), check timing and vacuum. Then move to fuel delivery accel pump, carb and intake for leaks.