Diagnosing and correcting a misfire that develops on the track can be far more frustrating than trying to figure out why a car will not start. There are, however, some similarities between trying to determine the cause of a miss and why a car will not start.
Troubleshooting misfires can be nerve wracking. Is it really ignition related or is it a fuel problem causing the misfire? It may be caused by a mechanical problem within the engine or even a cooling system problem. Other than mentioning the need to check for a blown or leaking head gasket and a bad or low-pressure radiator cap, we will not dwell on the engine internals. However, cooling system problems are responsible for over 20 percent of "ignition problems."
Fuel system problems are more often than not attributed to ignition problems. Before delving too deeply into the ignition, rule out any basic fuel system maladies: A fuel pressure gauge will identify a weak fuel pump, clogged fuel filter, or collapsed fuel line. A warped carburetor base, worn throttle shaft, or other vacuum leaks will show up as a lean condition. A clogged main jet may pass a visual inspection if it is blocked by a piece of sand or clear plastic. If in doubt, use a piece of thin wire to make sure that the jet is not clogged. A blown power valve should be obvious from an overly rich exhaust.
In the early days of the automobile, good wire insulation was poor at best. Some early engines even used bare metal for spark plug "wire." In those early times, simplicity was the norm and wires were routed as neatly as railroad tracks. Today, wiring has become more complex. Insulation quality has improved many times over, allowing wires to be bundled and be more tightly routed. Unfortunately, high voltage and high impulse carrying wires are bundled with signal wires that carry pulses that are affected by high voltage much the same as a compact car is buffed by a tractor-trailer on the highway. Critical signals are corrupt or lost. This can result in a misfire.
A short checklist will help in determining the source of the problem:
· Using solid-core spark plug wires, including the coil wire, can inductive an unwanted signal into the ignition amplifier, causing the amplifier to trigger at the wrong time.
· Poor connections on either end of coil and / or spark plug wires may cause a misfire.
· Bad crimps or connectors not seating on the plug or in the coil can both cause problems.
· Sometimes, a little moisture inside the plug wire boot turns to steam and blows the wire off. A thin film of oil will reduce corona and will help the moisture to escape.
· Spark plug wires must be separated. If possible, do not run them parallel to each other.
· In rare instances, having the coil mounted too close to the amplifier can cause problems.
· Do not run the coil secondary wire through the firewall with the distributor leads or the alternator lead as this can cause erratic operation due to "noise bleed over."
· A weak ignition can cause a misfire, reduced power, or poor performance. This may be the result of a low battery or a malfunctioning alternator.
· Poor power connections, or wires and connectors that are too small can reduce ignition output.
· A master switch not rated for the application can restrict current flow enough to cause poor operation.
· Weak or damaged ignition components may work satisfactorily with a well-charged battery and cool conditions, but when the battery loses its surface charge or the box heats up, misfiring will occur. Low voltage and higher temperatures both require the ignition to work harder.
· Parts store switches may work well for a 10-amp blower or a 100-watt light but the ignition switch on current racecars may need less than 100 milliamps. This small amount of current may not burn through a thin film of moisture or across an oxidized set of contacts. Quality switches are a better and more reliable choice. A Mil-Spec number on a switch gives you, or the supplier, a reference where you can check and compare the quantities and capabilities of a given switch. Make sure that the switch you have chosen or are using will work well in your application. A $ 5 parts store switch may work most of the time, but do you use supermarket motor oil in your race engine?
Misfire or Stumble
A voltmeter in the dash of the car helps you or the driver diagnose a problem. It can alert the driver to a malfunction before it develops into a misfire. If the alternator quits, proper power management may allow you to finish the race with all electrical power coming from the battery. To accomplish this, all non-essential electrical consumption needs to be curtailed as soon as possible after the alternator quits. If the tach starts jumping, or the car starts missing, it probably too late to start a meaningful conservation of power. How the engine acts, combined with when and where on the track the engine skips, misses, or flattens out are all factors that may be used to diagnoseose problems.
Some factors to consider when determining the cause of a load or RPM induced misfire:
What is the battery voltage when the problem occurs? Twelve volts or less? Or between 13.5 to 14.5 volts? A12-v misfire typically indicates a lack of reserve power. To remedy this type of problem, you can reduce the spark plug gap, retard the timing, and install an alternator or 16-volt battery.
A misfire at 14.5-volt is a little bit more complex. Does the tach fluctuate or get erratic? Does the problem occur early in the race or practice, or is it always after the midpoint in the race?
Always have a box and coil known to be good, along with an emergency or test harness and a direct coil-to-distributor wire available. If, after changing these components, the problem does not go away, look for a bad ground or a battery problem. Which way do the plates in the battery face? They should be aligned from left to right to prevent shorting in the turns due to centrifugal force. Where on the track does the problem occur? Diving into a turn? Off the turn or midway down the straight?
If a problem develops over time (10 or more laps), replacing components may "fix" the problem until everything heats up or the battery loses its surface charge. Tachometer operation may indicate the source of an ignition problem. An erratic tachometer reading or a zero reading may indicate a loose connection or low battery voltage.