Q: I have been a longtime subscriber and only recently I was finally able to purchase my dream car, a 1970 Corvette convertible. I drive my Corvette most every weekend and have enjoyed every minute with the top down cruising the countryside.
A few weeks ago my car started running rough so I installed spark plugs, ignition wires, distributor cap, and rotor button. This seemed to fix the problem, but I would like to finish the job. I am not sure how to set the points, timing, and carburetor. I have never owned a car with a carburetor, so could you take me through the steps of what I need to check and how to set up my car for the best performance and fuel economy.
Happy to be part of the Corvette family,
A: Congratulations on getting your dream car and welcome to the Corvette owners family. I will be more than happy to go through the steps on preforming a tune-up on your Corvette.
It is important to make the tune-up adjustments in order. First is to set the ignition points, then the ignition timing, and finally adjust the carburetor.
The points should be set first because changing the point gap will change timing, but changing timing will not affect the point gap (or dwell).
The carburetor settings should be set last because carburetor settings will not affect timing, but timing adjustments will affect the carburetor settings.
To perform a tune-up we must first understand the ignition cycle. The function of the ignition cycle is to fire the spark plugs at the correct time, just before the piston reaches Top Dead Center (TDC) on the compression stroke, so the air/fuel mixture can be ignited at the right time.
All Corvette engines are four-stroke engines. The four strokes consist of intake stroke, compression stroke, power stroke, and exhaust stroke. The timing of the ignition refers to the point between the compression stroke and the power stroke – when the spark plug fires, forcing the piston down and creating combustion. This combustion, if completed at the correct time, results in your maximum power and fuel economy.
The spark plug normally fires anywhere from 4 to 45 degrees before the piston reaches TDC depending on engine load. This is to allow the fuel/air mixture’s flame to completely traverse the combustion chamber.
Breaker points should be replaced approximately every 15,000 miles. If your Corvette is stored for long periods of time, the points may become pitted and need to be replaced after storage.
Points are a simple on/off switch. The points’ job is to provide properly timed pulses to trigger the ignition coil to fire.
There is a lobed cam on the distributor shaft that pushes on a small rubbing block on the movable side of the points as the distributor rotates. This causes the points to open and close.
As the points close, current from the ignition 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. A few degrees of crankshaft rotation later the points open and the current is interrupted, causing the magnetic field to collapse and fire the coil.
Setting The Points
The replacement procedure is simple. With the key in the Off position, simply remove the distributor cap and rotor button to expose the points. Then remove the points and condenser that are held in place using small screws.
When selecting a replacement part, we recommend using factory Delco parts. It is suggested not to use uni-points as they seem to have a higher failure rate than the separate points and condenser.
Lube your points where they touch the distributor cam area using distributor cam lubricant to help reduce wear. Install your new points. Turn the engine over by hand until one of the eight lobes on the distributor shaft has opened the points as far as possible, so the gap can be set.
The starting gap for the ignition points is approximately 0.016-inch and can be set with a feeler gauge. The gap is approximately the thickness of a matchbook cover and can be eyeballed for a starting point if a feeler gauge is not available.
The Delco-Remy window cap distributor was an engineering breakthrough. The unique breaker point design allows you to adjust breaker points through a distributor cap window. The window in the distributor cap provides access to adjust the dwell angle with the use of an Allen head adjuster screw. This can be done with the engine running, allowing fine-tuning of the breaker points.
From 1958-’74, most Corvette breaker points distributors use these window style distributor caps. However, there are some exceptions to this rule. Some 1958-’65 mechanically fuel injected Corvettes feature a unique, dual-point style window cap distributor and some use a transistorized distributor. The other exception was the dual-point, two four-barrel carbureted engine. The installation of two WCFB carburetors required a smaller diameter cap to accommodate the two carburetors.
Start the engine and adjust the points with a dwell meter to approximately 30 degrees. Using an Allen (hex) wrench inserted into the adjuster screw, turn the screw to achieve the desired dwell. Refer to your service manual for the dwell specification intended for your vehicle.
Alternative points setting procedure: this procedure is not the best method to set points, but will work if a dwell meter is not available. Gap the points with a feeler gauge or by eye, then install the cap. Start the engine. With an Allen (hex) wrench inserted into the adjuster screw, turn the screw clockwise until the engine begins to misfire. Back the screw out 180 degrees. This brings dwell close to specification in most cases. Accurate dwell angle can only be set using a dwell meter.
A fluctuating dwell angle reading can indicate that the distributor shaft bushings may have excessive wear causing the dwell variation.
To check for distributor shaft endplay, turn the engine over by hand until one of the eight lobes on the distributor shaft has opened the points as far as possible. With your hand, try to move the distributor shaft and note the amount of change in the point gap. Excess movement is a clear sign of bushing wear and an indication that the distributor may need to be rebuilt or replaced.
Well, Eddy, that should get you started and next month we will talk about the different ways to set timing and how to adjust your carburetor.