Many possibilities. Plugs, plug wires, coil, ignition module, fuel injector, head gasket. Essentially anything that interfers with combustion on 2 or more cylinders.
Faulty: spark plug, spark plug wire, ignition coil, ignition module, fuel injector are the most common. Coolant leaking into a cylinder is another. Internal engine problems such as burnt valve,...
it would be nice to know which engine we are talking about but generally no would be the answer ----most are not adjustable and those that are would most likely be destroyed by the rotating component
You did not say what you are working on. but you can check the plugs, wires, coil, and maybe the fuel injector low engine compresson, broken valve spring.vaccum leak... dist cap. Realey need more...