By Sam Logan.
Photographs by Moore Good Ink:
Download text and hi-res images here.
The cardinal rule of carburetor tuning is Ignition First. Once the static ignition setting and the ignition advancing mechanism in the distributor is correct, the air-fuel mixture can be tuned for full power and fuel efficiency.
High-performance carburetors, intake manifolds, cylinder heads, camshafts, and other tuning components are all dependent upon correct ignition timing; if the spark is not delivered at the proper time to the combustion chamber, the quest for optimum power or economy is impaired.
The two elements of ignition timing: static or initial timing and progressive timing
When you combine static or initial timing with progressive timing the result is total timing. Static timing can vary from as little as 8 degrees before TDC to over 40 degrees depending upon the engine. Tuning the static or initial timing is achieved by simply twisting the distributor body in relation to the rotor. Consequently, either the points or an electronic pickup will be triggered earlier or later.
Progressive spark advance is conducted by either mechanical means or by vacuum or both. Its function is to increase ignition timing beyond that of the static setting. As engine speeds increase the spark is required to fire earlier because there is less time for the air-fuel mixture to burn.
Mechanical spark advance mechanisms consist of weights on springs that are hurled outwards under centrifugal force within the distributor. As engine speeds increase, the weights progressively rotate on a wider radius, advancing the rotor relative to the cap and consequently advancing the ignition timing. Vacuum advance, on the other hand, accelerates the ignition timing by responding to low pressure in the intake manifold. The task for the engine tuner, therefore, is to fire the spark at exactly the right time throughout rev range.
Tuning the vacuum advance mechanism is achieved by use of an adjustable vacuum advance or changing the location where it senses the vacuum in the intake manifold. Tuning the mechanical advance mechanism is accomplished by replacing the springs or weights or both.
The most common symptom of inadequate ignition timing
Often highly tuned engines, those with high performance camshafts, cylinder heads, and intake manifolds exhibit a lazy response or, worse, hesitate under acceleration or die at idle. The solution is to increase the static timing and decrease the progressive timing (mechanical or vacuum) thereby limiting excessive total timing at high engine speeds. Carburetor tuners regard this as their most abiding problem but one that is easily cured by distributor modifications.
How to check static and total ignition timing with a dial-back-style timing light
Using a dial-back-style timing light, you adjust the dial until the line on the crankshaft balancer aligns with the TDC mark on the tab. Thus, at idle the number on the dial would represent your initial timing. Total timing is determined similarly, except the engine speed is increased, usually to 2,500-3,000rpm at which speed the weights and springs will have moved to their maximum advance position.
The advantage of the dial-back-style timing light becomes clear when determining total timing. In this example total timing is recorded at 38 degrees before TDC. With the standard non-dial-back timing light, 38 degrees will be a far distance from the tab and requires some form of measuring.
Source:
Demon Carburetion
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