If you do your own ignition tuneup, you'll inspect and relace spark plugs as necessary.
When you remove the wines from the plugs, it's important that you replace each wire on its own plug, so you don't change the firing order. If the wires are hooked up wrong, the engine will run rough (if at all), and you may cause engine damage. How can you prevent hooking the wires up wrong?
All engines have a firing order. An in-line six-cylinder engine, for example, has six cylinders, with a piston in each going up and down and producing power every other time it reaches to top of its stroke, when a spark ignites the highly combustible fuel-air mix inside the cylinder and drives the piston back down and turning the crankshaft.
With our in-line six, the cylinders are numbered front to rear. To even out the firing impulses and reduce engine vibration, the cylinders do not fire one after the other. The distributor sends out the impulses so, for example, No. 1 would fire first, then, 90 degrees of crankshaft rotation later, No. 5 would fire, 90 degrees of crankshaft rotation later No. 3 would fire, and so on.
Let's say you accidentally crossed wires between No. 1 and No. 2. Then when No. 1 piston gets to the top of its stroke, with a highly volatile mixure of gas and air compressed and ready to ignite, no spark would occur. Instead, there would be a spark in No. 2 cylinder, which would not be ready for ignition.
To keep from getting the wires mixed up, place a piece of masking tape on each one as you remove it and number the pieces.
If for any reason you need to know the firing order of your car's engine, and you don't have a shop manual for your particular car, you'll usually find the firing order embossed on the intake manifold somewhere. The intake manifold is what the carburetor sits on.
Those raised numbers may not be easily visible - they may be hidden under the air cleaner or some other part. But if you look thoroughly (you may need a flashlight) you'll find them. If, for example, you have a V-8, and the firing order is 1-3-7-2-6-5-4-8, somewhere on the intake manifold will be a string of numbers: 13726548.
Things can get a little more complicated if you're replacing all the spark plug wires. To prevent mixups here, mark each wire with masking tape as you remove it from the spark plug. Then, after all the wires are removed from the plugs, replace one spark plug wire at a time, numbering that wire with masking tape so that it matches the number of the wire you're replacing.
Keeping the proper firing order is really not difficult. But it's very important.