Welcome back for Part 2 of tune-up tips for your older Corvette. Last time we talked about tricks of the trade when setting points. This month we will talk about setting ignition timing.
The most important thing when performing a tune-up on an older vehicle is to complete the adjustments in order. The first step 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 the timing will not effect the point gap (or dwell).
The carburetor settings should be set last because carburetor settings will not affect the timing, but timing adjustments will effect the carburetor settings.
Signs of Incorrect Timing
Signs of incorrect ignition timing can include: hard, erratic cranking while starting; spark knock on acceleration; low power on acceleration; vehicle overheating and decreased fuel economy. Remember, ignition timing is always set after you have adjusted your points.
Elements That Contribute to Total Timing
In vehicles equipped with a points or first-generation HEI (High Energy Ignition) distributor there are three different elements that contribute to timing:
a) Initial timing or base timing—this is what you set with a timing light by moving the distributor when the vehicle is idling.
b) Mechanical advance—which consists of the initial timing plus the centrifugal advance. The centrifugal advance system in the distributor advances spark timing purely as a function of engine rpm.
c) Vacuum advance is intended to advance the ignition timing at idle since the engine needs more spark advance at idle in order to fire the leaner fuel mixture.
What is Total Timing
Total timing is the amount of advance added by the distributor combined with initial timing.
Most stock Corvette V-8 engines will produce peak torque and power with a total timing advance of 36 to 38 degrees at approximately 2,500 rpm. This can be checked with a timing light that is equipped with a degree knob, and is one of the best ways to set timing.
If your timing light does not have a degree knob, timing tape can be used on the harmonic balancer to achieve the same result.
The centrifugal advance system in the distributor advances spark timing purely as a function of engine rpm. The amount of advance added by the centrifugal advance system is accomplished by using weights and springs on the autocam mechanism located directly under the rotor button. The spring length and tension determine what rpm spark advance occurs.
The faster the distributor shaft turns the more the weights move outward due to centrifugal force. This allows the rotor button to move on the autocam and the spark timing to advance.
The speed at which the weights move outward is controlled by the tension of the springs on top of the autocam.
How fast the spark advances is controlled by these springs and weights. A lighter spring will let the timing advance faster at a lower rpm. A stronger spring will slow the timing advance through the rpm range.
There are many different calibrations of these centrifugal weights. It is important to make sure the correct distributor is installed in your car. While any distributor laying on the shelf may fit in your Corvette, the point in the rpm range where the weights begin to move and fully extended has to be calibrated specifically for that engine and transmission package.
The vacuum advance is intended to advance the ignition timing at idle since the engine needs more spark advance at idle in order to fire the leaner fuel mixture. The vacuum advance is also designed to respond to sudden changes in operating conditions by helping provide the correct spark advance based on engine demand.
This is accomplished on pre-emission vacuum advance vehicles by using engine manifold vacuum to rotate the distributor plate counterclockwise, which advances the opening position of the ignition points, therefore advancing the ignition timing when vacuum is applied. When the engine rpm increases, it’s possible that an over-advance ignition timing condition could occur causing engine detonation (pinging on acceleration) and a reduction in optimum power. The vacuum advance is able to regulate ignition timing by using engine vacuum to retard the timing. When the throttle is opened, manifold vacuum will essentially drop to zero causing the vacuum advance to retard timing by rotating the distributor plate clockwise. So as you can see, the vacuum advance should use manifold vacuum as its source on pre-emission Corvettes.
When testing your vacuum advance, use a vacuum pump to apply vacuum to the vacuum advance and make sure the plate rotates. The vacuum advance should hold vacuum. If not, the internal diaphragm may be ruptured. These are calibrated units and must match the factory rate of pull. So make sure to replace your vacuum advance with the correct unit based upon the distributor number. An adjustable style vacuum advance can be found for performance applications.
The airflow through a carburetor and the intake system creates various low-pressure regions. These low-pressure regions are sources for vacuum. To better understand the differences between manifold and ported vacuum let’s look at the position of the orifice. If the orifice in the venturi for the vacuum port is below the throttle plates of the carburetor, that port will have manifold vacuum. If the orifice in the venturi is above the throttle plates of the carburetor it is ported vacuum.
To test a port to determine if the vacuum is manifold or ported, connect a vacuum gauge to the port. At idle, if the vacuum gauge reads 15-20 inches of vacuum this is considered to be manifold vacuum. If no vacuum is shown at idle; but as you open the throttle you start to read vacuum; this is considered to be ported vacuum.
After years of controlling vacuum advance with manifold vacuum—along came emissions requirements that forced the reduction of hydrocarbons and nitrogen in the exhaust gases. One primitive method employed to reduce emissions was to switch the vacuum advance from manifold vacuum to ported vacuum.
When the spark timing was retarded at idle by moving the vacuum advance from manifold to ported vacuum, cylinder combustion begins late and this does two things, which are important for emissions:
a) This causes a reduction in combustion chamber temperatures, which in turn reduced the nitrogen produced.
b) This also causes an increase in exhaust gas temperature, which helps burn off hydrocarbons in the exhaust gas stream when fresh oxygen is introduced by the air pump.
As a result of moving the vacuum advance from manifold to ported vacuum, engines ran poorly with enormous amounts of heat energy being transferred through the exhaust port walls into the coolant, causing the typical engine to run 15 to 20 degrees warmer. Combustion efficiency was also affected causing a reduction in fuel economy. A slightly rough idle also commonly occurred when the vacuum advance was moved to ported vacuum.
Well, now we have understanding of how points work and how to set them as well as all the elements that contribute to total timing, we are well on our way of performing a complete tune-up.