While many car restoration hobbyists and backyard mechanics have been tuning their cars for decades, very few actually do so in the correct manner. We'll explain how to go about this most important task, but first we need to mention one little thing:
There is one, AND ONLY ONE, correct procedure to follow when tuning up a car!
Now that we've got your attention, let's start getting the old flivver operating the way it's supposed to.
Don't start replacing or adjusting anything right away. Spend a few minutes checking out the machinery first. This is important, since improper operation of some systems will affect the engine's tuning. Don't allow any of the following to go unchecked or ignored.
The first thing to do in a tune-up is LEAVE THE DAMN CARBURETOR ALONE! It's the very last thing you will adjust so don't touch it. Doing so will cause you to have to "chase" the adjustments with the ignition system and possibly never get things right.
The first rule of tuning engines is to get the ignition settings correct. The ignition is the "bandleader" and everything else under the hood is the "orchestra," so take the time to get it right.
You'll want to start with a fresh set of points, condenser and plugs. Plug wires may or may not need replacement depending upon mileage, and we recommend replacing carbon-core wires (resistance wires) after about 15,000 miles on older cars. Solid-core wires tend to last much longer but should be replaced when their rubber end caps deteriorate — this keeps the wires from arcing over to the engine block.
Step 1 — Crank the engine to Top Dead Center, as marked on your balancer or pulley. Open the distributor cap and see if the rotor is pointing at the #1 cylinder (if unmarked, follow the plug wire to the distributor). If it's 180 degrees away from that position, crank the engine one revolution back to TDC. It should be there now. Mark a line on the side of the distributor housing and continue it onto the engine block. This will allow realignment if needed.
The reason you're doing this is to prevent misplacement of plug wires later and to facilitate removal/replacement of the distributor if that should be required. The engine is now mechanically set to operate in the proper firing order.
Step 2 — Pull out the old points, condenser and rotor. Check for smooth movement of the vacuum advance plate, preferably by sucking on the vacuum hose. Mount the new points on the plate along with the condenser (on some cars the condenser mounts on the coil). Loosen the distributor's hold-down screw just enough to allow movement, and rotate the distributor until the wiper arm on the points is at the top of one of the shaft's actuating cam lobes. Tighten the hold-down screw and adjust the points for proper mechanical gap, according to your engine manual. Use your feeler gauge properly and take your time, because you are mechanically setting the proper dwell angle*. Install the rotor and cap.
*Automotive coils generally have secondary-to-primary ratios of 200 to 1. Therefore, a 12-volt input to a coil's primary windings will result in a 24,000-volt output from the secondary winding. That's where the spark plugs get their electricity. Inductance isn't perpetual motion, nor is it "free energy." There are many "howevers" and other considerations to worry about. The biggest one is the coil's inability to hold the induced voltage once it's been built up. In a very short time the voltage will "bleed of," leading to weak spark. Also, the coil takes a finite amount of time to build the charge up. That's the dwell time, normally defined as the degrees of rotation of the camshaft during which the points are closed. Too little dwell and the coil doesn't have time to charge up fully. Too much dwell and the coil has bled off some charge, causing a weak spark. Hesitation, low power, misfiring, pinging and a number of other conditions are symptoms of incorrect dwell.
Step 3 — Gap the new plugs and install them, being careful not to over tighten the threads into the block. Place the plug wires onto them while noting the firing order. Make sure the wires are seated into the distributor cap and coil.
Step 4 — Get out the dwell meter and connect it. The red lead goes to the points side of the coil (negative terminal) and the black clamps onto any good ground point on the engine. Start the engine and measure the dwell angle. If it isn't in the middle of the allowable range (say, 24 degrees when the range is 20-26 degrees) stop the engine and move the point gap closer to raise the dwell or farther apart to lower it. Keep doing so until the dwell is correct. Dwell angle has always been set by properly adjusting the ignition point gap. Your car's points gap was derived by engineers to approximate the dwell angle, but individual point sets can vary considerably in their mechanical and electrical characteristics. The only way to properly set up ignition points is with a dwell meter.
Step 5 — Get out the timing light, the most overused engine maintenance tool. Why is it overused? Well, because the engine's internal components only wear miniscule amounts over time, which means timing chains or gears don't "jump" or get loose between tune-ups. The reason an engine's timing changes is because the dwell angle is changing as the wiper on the points wears down.
Why? Since dwell is measured by camshaft rotation and the camshaft runs at twice the speed of the crankshaft, for every two degrees of dwell an ignition is off from its proper setting, the engine's timing will be off one degree! If an engine needs to be re-timed when it's periodically checked, the points have worn down (thereby increasing dwell). The timing chain hasn't slipped, as so many believe. Conversely, if you set the points correctly at every tune-up you will find that the timing never changes!
Step 6 — Set the timing by connecting the light (inductive or direct) and then mark the correct timing position on the crank pulley or damper. Follow your engine manual and make an easy-to-read mark with White-Out (liquid paper) or chalk. Disconnect the vacuum line from the distributor and stick a pencil or nail into it to prevent a vacuum leak. Start the engine and get it running about 500-600 rpm to prevent any action from the centrifugal advance mechanism (if there is one) and then slowly rotate the distributor until the timing mark lines up with its pointer. Tighten the hold-down on the distributor. If the engine won't run slowly enough back off on the accelerator linkage at the carburetor. If the engine runs too slow increase the idle speed. Check the timing again and then put the light away.
Step 7 — Now (FINALLY!) you can adjust the carburetor. First, you want to adjust the idle mixture screw (or screws, if you have a 2 or 4-barrel carburetor). With the engine at correct operating temperature (this is important, so wait for it to get warmed up!) adjust one screw slowly clockwise until the engine starts to stumble or run rough, then back it up about one turn. Adjust the other screw the same way until the engine runs smoothly.
Now set the engine idle speed according to the manual or personal preference (we like slightly faster idle speeds, around 800-900, but do what you like). Re-adjust the idle mixture screws if the engine is running slightly rough.
Step 8 — Drive the car and pay attention to any rough-running, hesitation or misfire. It should run great, but if not you'll have to diagnose the problem. If it's not something like a loose plug wire (oops!) or something else obvious, it could be faulty carburetor systems, lack of compression, etc., but at least you will know that the ignition and idle systems are set up correctly.