Date:31 December 2009
Got a collector car with ignition points? Here’s how to replace the points and set the timing.
“It’s not a heap, dad. It’s a classic.” That’s harder to justify when your classic muscle car won’t start. Nothing like a high-compression engine combined with a battery that hasn’t seen a charge for a month and a half to make for slow cranking. Add in the indifferent, low-energy spark and incorrect ignition timing caused by worn-out points, and you’ve got an engine that won’t fire – oh, and wet spark plugs, too.
Modern engines use computer-controlled factory-preset self-adjusting ignition systems that never change their timing, have no moving parts and never need maintenance. Yay! A generation ago, every teenager, every mechanic and a lot of vehicle owners understood the theory and practice of changing points and setting the timing. Nonetheless, there are still plenty of older vehicles, outdoor power equipment, boats and tractors that need periodic adjustment or replacement.
The distributor on these older vehicles performs two related tasks. The first uses a simple on/off switch, the ignition points, to provide properly timed pulses of 12-volt electricity to the ignition coil. In the coil, essentially a transformer, it’s stepped up to 10 000 to 20 000 volts. Then, the high-voltage electricity from the coil returns to the distributor, where the rotor inside parcels it out to the correct spark plug to ignite the fuel/air mix.
There’s a lobed cam on the distributor shaft that pushes on a small rubbing block on the movable side of the points. As the cam and distributor rotate, the points open and close constantly. As they close, current from the ignition switch flows through the contacts into the coil’s primary windings and then off to ground. This current generates a magnetic field in the coil’s iron core. When the points open a few degrees of crankshaft rotation later, the current is interrupted, causing the magnetic field to collapse. This induces electrical current into the secondary windings of the coil, where the current is raised to 20 000 volts or more. The high voltage now travels over to the distributor, where the rotor metes the high-voltage pulses out to the correct spark plug.
All that current flowing across the points doesn’t like to stop suddenly, and can initiate a small arc, which eventually erodes the tungsten contacts. The condenser cushions that arc, making point life much longer. But not infinitely long. As the contacts and the plastic rubbing block, which contacts the point cam, wear, the ignition points’ clearance and timing constantly change. After thousands of kilometres, the timing has shifted enough to affect performance, and the ritual of changing the points and setting the timing becomes necessary. How often? Some vehicles need to have the timing adjusted as often as every 15 000 km to maintain peak performance.
High-revving engines will need premium points with a high-pressure spring to keep the points from bouncing at increased revs. Some points assemblies include the condenser; yet for others, it’s a separate part. Condensers are inexpensive enough that it makes no sense not to replace them with every set of points. They should last as long as a set of points, 30 000 km at least.
The function of the ignition system is to fire the spark plugs at the correct time, just before the piston hits top dead centre (TDC) on the compression stroke, to ignite the fuel/air mixture, thence producing high pressure in the cylinder to force the piston down and, subsequently, the wheels to move the car forward. The spark plug normally fires anywhere from 10 to 45 degrees before the piston reaches TDC, to allow the fuel/ air mixture’s flame front to traverse the combustion chamber. It takes a few milliseconds for the pressure in the cylinder to build, and waiting until TDC would make the pressure peak too late in the piston’s downward stroke to be most efficient. Under some engine-operating conditions, the advance might adjust the ignition timing to as much as 45 degrees before TDC. Signs of incorrect ignition timing include hard starting, spark knock, poor power, overheating and decreased fuel economy.
There are two main types of advance mechanism built into the distributor. The first is the centrifugal advance. A pair of bob-weights spins atop the distributor shaft, restrained by small springs. As the engine speeds up, centrifugal force pulls the weights outward, which in turn makes the top of the split distributor shaft advance. Missing springs or a gummedup linkage can give too much advance too soon, or none at all.
Similarly, a vacuum advance uses a rubber diaphragm to advance or retard the timing. Vacuum from the carburettor pulls on one side of the diaphragm, pulling the points around the distributor and making the plugs fi re earlier. A leaky vacuum line, a disintegrating rubber diaphragm or a sticky breaker plate can make the advance mechanism balky. Inoperative advance mechanisms can deliver too much or too little ignition advance. Too much advance can make the engine ping. Too little causes power loss and overheating.
Installing the new points and condenser is simple, and usually requires no more than a screwdriver. The proportion of time the distributor’s cam keeps the points closed and open is referred to as dwell angle. Adjust the dwell angle initially by using a feeler gauge. Typical four-cylinder engines, like early VWs, start around 0,014 inches or about 350 thousandths of a millimetre. Close is good enough, because the only really accurate way to set dwell is with a – wait for it – dwell meter. The dwell angle should be 30 to 35 degrees for V8s and 44 to 50 degrees for four-cylinder engines. Check the workshop manual for your car. Attach the dwell meter to the coil’s low-voltage leads and spin the engine with the starter motor to check and trim the dwell. Some cars have a small window in the distributor to let you set dwell with the car running, a real timesaver because you don’t have to crank the engine with the starter, adjust the points and check the dwell again.
Cleanup in bay 4
Before you button up the distributor, clean the point contacts of any oil left behind by your feeler gauges. Contamination will carbonise and become a resistance where there should be only metal-to-metal contact. I usually just use the corner of a business card to scrub any contamination off. Add a dab of point-cam lube to the rubbing block.
Adjusting dwell also changes the base ignition timing, so whenever the dwell is adjusted or the points replaced, the timing will need to be adjusted. Some engines call for vacuum lines to be pinched off or disconnected, so you’ll need to find the correct timing procedure for your engine in the shop manual. I’ll wait … got the timing specs? Find the timing marks on the harmonic balancer or on the flywheel. Use some contrasting paint or a felt pen to brighten the timing mark. Hook the timing light to the No 1 plug wire. Start the engine, and shine the timing light at the timing marks. Mind the whirling fan and the belts, reminds my old-timer mechanic pal Lefty. The strobing light will “stop” the spinning pulley when the No 1 plug fires. Loosen the clamp holding the distributor down and slightly rotate the body of the distributor to line up the timing marks. Revving the engine slightly should make the mechanical advance actuate – you’ll see the timing marks advance and retreat as the engine surges. Tighten the distributor clamp, reconnect any vacuum lines and drive.