Carburetor Tuning — Ignition First

Story and Photographs Courtesy: Moore Good Ink

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.

But the distributor has vanished! Tuning contemporary hot rods involves electronics and computer software. Sensors abound. They sense Manifold Absolute Pressure, Mass Air Flow, crankshaft position and so on. They report to an ECU (engine control unit) that constantly ascertains all the variables and tells each spark plug when to fire. Where there was once a distributor, multiple coils now exist, often one on each spark plug.

Still, what a joy it is to understand the psychology of the hot rodder who lusts for a carburetor and a distributor. And, ironically, older vehicles can be simpler to tune. They require no fancy equipment or computer knowledge, often just a timing light, a screwdriver and a few wrenches.

background on points-and-coil ignition

Before sophisticated electronic management systems arrived, we used the points-and-coil ignition system that first appeared on the 1910 Cadillac. A distributor was employed to determine when each spark plug should fire. An engine-driven mechanical cam in the distributor rotating at camshaft speed operated a set of breaker points. The points switched electrical current to the coil which converted it to high voltage required to fire spark plugs.

The high voltage was delivered from the coil to the center of the distributor cap via a high-voltage wire. Inside the cap there are small metal tabs, one to serve each spark plug. A rotor mounted on the upper end of the mechanical cam and functioning within the distributor cap routes the high-voltage impulses to the correct spark plug. A condenser has the dual function of extending the life of the points by quenching the arc across the points and forming a resonant circuit with the coil that boosts peak voltage.

The rotor is connected through the high-voltage coil to the battery, and the small metal tabs in the distributor cap are connected via spark plug wires to the spark plugs. As the rotor spins, the current jumps across the tiny gap to each of the small metal tabs, completing the electrical circuit and sending short-duration, high-voltage currents to each spark plug on time.

Timing the ignition

Ignition timing is affected by a host of elements including fuel type, mixture strength, combustion chamber shape, compression ratio, temperature and humidity. Furthermore, the ignition is always timed to fire the spark plug before the piston reaches Top Dead Center (TDC) in the cylinder. Firing before TDC is necessary because of the time it takes for the flame front to ignite the air-fuel mixture in the cylinder.

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.

Source:

Demon Carburetion

Ignition Timing_Image01 (0785) Using a standard timing light (a non-dial-back type) connected to the number 1 cylinder and to both battery terminals and with the engine running on idle this engine is firing about 20 degrees before TDC. This ignition timing event is known as the static or initial timing and is a good starting point for high performance engines, particularly those with aggressive camshafts.

Ignition Timing_Image02 (0787) Firing at 10 degrees before TDC is a good initial ignition setting for a mildly modified engine.

Ignition Timing_Image03 (0809)Firing at 5 degrees before TDC is a typical initial ignition setting for a stock engine.

Ignition Timing_Image04 (0793) Firing at TDC or just after, as shown here, will probably cause the engine to stop. Engines, particularly high-performance engines are allergic to retarded ignition timing. Because of the time required to ignite the air-fuel mixture in the cylinder they need correctly advanced ignition timing to run properly.

How to check static and total ignition timing with a dial-back-style timing light

Ignition Timing_Image05 (0875) 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.

Discussion
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9 Responses to “Carburetor Tuning — Ignition First”
  1. Dave b.

    When doing the initial timing with the engine at idle should the vacuum advance hose be disconnected at the distributor and plugged?

    Reply
    • Joe M.

      The short answer here is yes, the vacuum advance hose should be disconnected and plugged. This is because an engine is producing vacuum at idle and this vacuum will advance the ignition above the base setting controlled by the distributor hold down. Therefore, disconnecting and plugging the vacuum advance allows you to accurately measure initial static timing.

      Reply
  2. Bert Cundle Sr.

    I’ve been Struggling to find someone to Fine tune my ’75 Pinto (3.2L / 2300c.c.) 10:1 Pistons ~ Isky 300 Dur. / .5″ Lift.{ (2) Webber (2) bbls.” I. D. F’s { I got it running… but ` It sure needs a tune-up to run right.} Thanks for your help!

    Reply
  3. Richard

    I need to purchase a timing light and never have used a dial type. I liked the info. thank you. Richard P.

    Reply
    • robert

      the first one I bought was a craftsman. it was fair priced decent (I think its called) “advance timing light”. I’m not too sure what your question is. but how its used is first having you timing indicator on the engine cleaned. it also helps to have TDC marked (with something like white-out) on the indicator on the engine (I’ve also heard it called the pointer) and also 0TDC marked the harmonic balancer. so with the timing light Is hooked up to the cylinder number 1 spark plug wire. point the timing light toward the marks and using the dial you line up both marks together. then you look at the number where the dial is pointed to, that is your timing .. so if your timing is supposed to be 0*TDC. when checking the marks will line up on the engine and the dial on the timing light will read 0..if your timing is supposed to be set at 10BTDC then you will turn the dial to line the marks on the engine again (to 0TDC) then check the position on the dial will be 10BTDC. when checking timing you will always line up the marks to 0TDC on the engine by turning the dial on the timing light and looking at the dial to read the timing

      Reply
  4. Jignesh Rana

    I want to change ignition timing & also i research on this topic for master level thesis but what is the procedure i don’t know how to change spark ignition timing

    Reply
    • Customer Service

      Hello Jignesh,

      On older cars (mostly 70s and earlier) simply loosen the distributor clamp and rotate the distributor to change the timing.
      This should be done with a timing light attached, to ensure it meets acceptable limits.

      On newer cars and/or engines, the timing can only be changed by altering the engines programing, and is often done with a laptop computer connected to the cars diagnostic port.

      Thanks,
      Mark CCRC Video Membership

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      Reply