If the pistons are bad it should be smoking a bright blue color out of the exhaust.
Are if a piston skirt is broken it should be knocking all the time.
If there is a whole burnt in the piston you will not have any compression on that cylinder. Most of the time pistons just don't go bad.
No spark.
Bad coil Bad rotor
oil building up on the pistons-the rings are bad
What would cause a 76 model 350 Chevy to run bad when idling. Already replace ignition module Distributor cap and rotor
loud rattling noise when idling.
bad rings or really bad valve seals.
gas gouge no work
bad head gasket
It is probably a broken valve spring.
The most obvious symtom would be overheating of the engine.
Your question isn't specific enough. there are many reasons why an engine could run bad.
No electrial spark or fire at the plugs.