You've got your classic car looking great, but taking it out for a run you notice that the engine is misfiring. Here's a list of the things that could be causing the problem.

At all speeds

• Fouled spark plug or broken porcelain.
• Faulty spark plug cables.
• Low battery voltage.
• Low generator voltage.
• Burned or pitted ignition points.
• Incorrect ignition point gap.
• Faulty condenser of coil.
• Weak spark or no spark in one or more cylinders.
• Faulty distributor cap or rotor.
• Primary circuit restricted or open intermittently.
• Primary circuit detoured by short intermittently.
• Secondary circuit restricted or open intermittently.
• Secondary circuit detoured by short intermittently.
• Blown cylinder head gasket between cylinders. This can be noted when missing occurs in two adjacent cylinders.
• Sticking valves.
• Hydraulic tappet holds valve open slightly.
• Broken valve spring.
• Leal at intake manifold gasket.
• Mixture too rich or too lean.
• High-tension wire shorted in metal manifold.

At high speed

• Hot spark plugs. Change to colder type but not that a hot plug may be due to loose installation or lack of a plug gasket (if gasket is called for).
• Ignition point gap is much too wide.
• Breaker arm binding or sticking.
• Breaker arm spring weak.
• Sticking engine valves
• Valve springs too weak to close valve promptly.
• Valve springs broken.
• Valve springs shimmy.
• Intermittent delivery of fuel to carburetor so that momentarily the mixture is too weak for combustion.
• Mild vapor lock.
• Weak spark.
• Exhaust manifold clogged with carbon.
• Exhaust manifold, muffler or tail pipe restricted.
• Improper ignition timing.
• Centrifugal advice not functioning properly.
• Manifold heater valve held closed.
• Dirty carburetor air cleaner.
• Choke valve not completely open.
• Carburetor throttle lever loose on shaft.
• Improper fuel pump operation.
• Preignition.
• Incorrect valve timing.

At low or idle speeds

• Faulty spark plugs.
• Spark plugs gaps too narrow.
• Dirty or corroded secondary circuit connections or faulty ignition cables.
• Cracked or faulty distributor cap. Radical contacts in cap burned or worn.
• Dirty air cleaner.
• Leaky valves.
• Ignition point gap too narrow.
• Faulty carburetion due to: (a) float level too high or too low; (b) float valve leaking; (c) incorrect or loose jets; (d) restricted or partially clogged idle air passage or jet; (e) air leaking occurring between upper and lower carburetor body; (f) air leak occurring around carburetor throttle shaft.
• Air leaks in intake manifold or carburetor resulting from: (a) loose manifold connections or leaks occurring in vacuum lines; (b) loose manifold nuts or cap screws; (c) broken or damaged intake manifold or carburetor gaskets; (d) cracked manifold; (e) warped or damaged manifold contacting surface.
• Slight leaks occurring at fuel pump check valves.
• Air leak occurring around intake valve stem because of excessive valve stem-to-guide clearance.

When car accelerates

If the engine misses when the car accelerates, but does not miss when idling, the reason is that the spark plugs stop firing because of increased compression pressure caused by:

• Weak spark plugs.
• Plug gaps too wide.
• Plug fouled or damp.
• Plug porcelain below par.