Why does my engine spit or backfire?

When you hear a backfire, you need to determine whether it is backfiring out the exhaust or back up through the carb. We usually refer to an exhaust backfire as a "backfire", and backfiring through the carb as "spitting" or "coughing".

"Backfiring" is usually caused by a spark plug "sparking" when it isn't its turn and the exhaust valve is open. If your air/fuel mixture is too rich and you have some unburned fuel in the exhaust system, cross firing from one spark plug wire to another can occur if they are touching each other which will send juice to a sparkplug that isn't supposed to be getting it. When this happens while the exhaust valve is open, it will ignite the rich / unburned mixture inside the exhaust manifold (and tail pipes) and result in a big bang. This is why they make spark plug wire separators - to help keep cross firing at a minimum. A great way to check for any cross firing is to park in a dark place at night and open the hood with the engine running. If there is any cross firing going on, you'll certainly see the blue spark when it happens. It'll usually cause a "snap" sound as well, but not always. A sticky or burned exhaust valve can also cause some backfiring out the exhaust. 

Having timing that is too late (retarded) can cause this same thing sometimes if your engine is running too rich. When the exhaust valve opens, the mixture isn't done burning so it ignites the unburned fuel in the exhaust manifold (or header collector) and causes a loud bang. If you haven't messed with your timing though, then this is probably not your problem. This is usually only an issue when starting up a new engine or when you changed-out a distributor or something and you don't know where the timing is exactly at until you run it and get a timing light on it. 

Backfiring can also be cause by a cracked distributor cap, or one that has carbon tracking inside which causes cross firing between the terminals inside, which in turn, sends spark to a spark plug that isn't ready for it yet. This can also happen if one of the plug wires isn't in the right spot within the firing order on the distributor cap, or down at the spark plugs themselves. For instance; if you accidentally plugged one of the sparkplug wires onto the wrong spark plug, it is going to cause spark to ignite that plug at the wrong time which could cause an exhaust backfire, or an intake backfire, depending on where either valve is at the time the spark occurs. If it sparks when the intake valve is open, you're going to get a pop back up through the carb. If it sparks when the exhaust valve is open, then it'll cause a bang out the exhaust. Basically, when that plug sparks out of turn, it lights the fuel in the cylinder - and that pressure has to go somewhere... so if the intake valve is open, it goes right back up through the intake manifold and out the carburetor with a "spit" and sometimes even a flame, or it goes out the exhaust and creates that loud BANG. 

Assuming all of your plugs and wires are in the correct locations and you haven't messed with the timing at all, then the most common cause of backfiring through the carb (spitting or coughing) is in the morning when a carb's air/fuel mixture is a bit too lean. This usually goes away once the engine warms-up. A cold engine needs more fuel to run, and a warmer engine runs more efficiently and needs less fuel, so if you are only spitting when the engine is cold, then most likely it's due to a lean condition. You could also check to make sure your choke is working. If it's stuck open when the engine is cold, then it isn't doing its job by richening-up the mixture and it could definitely cause spitting.

Spitting and coughing can also be commonly caused by the accelerator pump in the carb not squirting enough fuel before the main jets start working. If you spit the instant you blip the throttle, then it's probably the accelerator pump in the carb not working, or not squirting enough fuel, so the engine takes a gulp of air and goes lean, which causes the spit back-up through the carb. This could mean that it's plugged up with some dirt, or a piece of something that got through your fuel filter, or it could just be out of adjustment or maybe the accelerator pump itself has just gone bad and can't squirt any fuel anymore. That's not too uncommon if the carb is a bit old. 

So... when, and how, it backfires, or spits, will give you an indication for where to start looking so you can fix it.