Here is a little clarification on how the "Load-O-Matic" and the infamous "Spark Control Valve" (or SCV) work. It's important you know how it works, especially if your planning a carb swap. Hot rodders know all to well, as they have been dealing with this issue since 1949. Basically, the same things happen when you try to install a bunch of Strombergs on a flattie V8 using the stock distributor!

The "Load-O-Matic" style distributor was introduced in 1949 and was continued on all V8's until the 1967 model year. This arrangement was considered to be far superior to the "vacuum brake" used on '32-'48 Ford V8's, which received a combination of vacuum and centrifugal advance to improve high speed performance. The "Load-O-Matic" was carried over to the Falcon Six due to it's "load sensing" design, which delivered good economy based on actual engine load.

While the "Load-O-Matic" distributors may be well suited for stock applications, they are a poor choice for the performance enthusiast. Unfortunately the "Load-O-Matic" distributors were designed for simplicity and economy, rather than performance. As such, the Load-O-Matic distributors have no provision for mechanical advance, relying solely on vacuum. As a result the distributors are very slow to provide spark advance when the engine is accelerated under load, making them a poor choice for performance applications. This is also why we don't like the Petronics Ignition for anything other than a stock application. While the Petronics improves spark quality, your still stuck with the poor operation of the "Load-O-Matic" distributor, and your carburetor choices are limited to one that is equipped with a Spark Control Valve. Therefore, if you want to improve throttle response and overall performance, ditch the "Load-O-Matic" and pick-up a late model or aftermarket distributor with mechanical (centrifugal) advance.

VACUUM ADVANCE
To understand how the "Load-O-Matic" distributor works, you first need to understand how Manifold Vacuum, Ported Vacuum, and Venturi Vacuum differ from one another.

"Manifold Vacuum" is taken directly from the intake manifold. The vacuum is created due to the down-stroke of the piston, which sucks air into the engine during the intake stroke. However vacuum is only created when there is a restriction at the other end, such as the carburetor. If the throttle plates are tightly closed and the pistons are moving quickly, a high vacuum source is created. If the throttle plates are opened, there-by reducing the restriction, less vacuum is created. If the pistons are moving slowly and the throttle plates are wide open, the pressure in the intake manifold will be very close to zero. As such, Manifold vacuum is used as a source to determine how hard an engine is working (or load).

Let's go over this again..... When an engine has a light load, manifold vacuum is high. At wide open throttle, or the highest load, manifold vacuum will approach zero, especially if the carburetor is the proper CFM.

"Ported Vacuum" is actually manifold vacuum, however it has the ability to be switched on and off. To accomplish this a vacuum port is drilled inside the carb body (not the venturi), with the port being positioned slightly above the throttle plate, when in a close position. At idle, when the throttle blade is closed (below the port), it act as an “off” switch by blocking the port from manifold vacuum. However as soon as the throttle plate is cracked opened, the port becomes fully exposed to manifold vacuum. As such, the throttle plate(s) act like an "on-off” switch, turning manifold vacuum “off” at idle, and back on as soon as the throttle plates are cracked open. This is called "Ported Vacuum".

"Venturi Vacuum" is taken from the narrowest point in the venturi chamber. As air enters the carburetor, it passes through the venturi chamber, accelerating at the narrowest point. When the air accelerates it creates a low pressure area, or vacuum, in the venturi chamber. When the engine is at idle it requires very little air, thus air flow through the venturi is relatively slow, there-by creating a weak vacuum signal. However as engine rpm's increase, the demand for air also increases. Therefore air flow through the venturi must accelerate to met the demand, there-by creating a stronger vacuum signal. As such, "Venturi Vacuum" can be used to measure the engines rpm (or speed).

NOTE: There has always been a huge debate over which is better, hooking the distributor to ported vacuum, or to manifold vacuum. Some say ported vacuum must be use, while others say you need to use manifold vacuum. We recommend using which-ever works best for your application. If the motor runs better on ported, great, but if it runs better on manifold vacuum, that's fine too. There is no right or wrong answer. Try both and see which one your motor likes best. In general terms, ported vacuum results in lower emissions, while manifold vacuum results in lower engine temps. Most engines idle a little higher on manifold vacuum, however this can be corrected by adjusting the idle speed on the carb. For more information, see our tech article on "Vacuum Advance".

SPARK CONTROL VALVE
Now that we have covered the differences in vacuum, we can talk about the "Load-O-Matic" distributor and how it works. In operation, the "Load-O-Matic" distributor and ignition system is very simple. The Autolite 1100 carburetor uses a mixture of manifold vacuum (load) and venturi vacuum (speed) to supply the proper spark advance. As such, the "Load-O-Matic" distributor has a vacuum circuit that supplies manifold vacuum to the distributor. This is accomplished by positioning a vacuum port in the carb body slightly below (not above) the throttle plate, when in a close position. As such the distributor delivers manifold vacuum advance to the ignition system, at idle and light throttle.

As rpm's go up, manifold vacuum drops, and the vacuum signal becomes to weak to provide adequate spark advance at speed. Since the "Load-O-Matic" distributors didn't have a provision for mechanical advance, like most other distributors, they had to find another way to compensate for high speed operation. This is where the "Spark Control Valve" comes into play. As the manifold vacuum drops, the "Spark Control Valve" closes, and the carburetor automatically switches over to venturi vacuum thru an internal passage in the carb. As the engine runs faster and inhales more air, venturi vacuum increases, sending a stronger vacuum signal to the distributor. Hence the "Load-O-Matic" distributor advances to meet the demand for high speed operation.

The same applies at "Wide Open Throttle" (load), when the throttle plates are fully opened. As manifold vacuum drops, the "Spark Control Valve" closes, and the carb automatically switches over to venturi vacuum to supply the proper vacuum signal. As such, the "Load-O-Matic" distributor advances to meet the demand under load. However, if the engine load is light the engine needs even more advance as rpm's increase, for better fuel economy. Therefore, when the manifold vacuum gets high enough, the spark control valve opens and control of distributor vacuum advance passes back to the stronger manifold vacuum signal, thus meeting the need for more advance at high speed cruise.

Since the "Spark Control Valve" is proportional, the vacuum signal is normally a mixture of manifold vacuum and venturi vacuum, rather than one or the other, therefore compensating for both load and speed. This vacuum arrangement is very simple and effective, allowing the "Load-O-Matic" distributor to provide adequate spark advance in most circumstances.

While this system may be adequate for stock motors, we highly recommend installing a Duraspark or DUI distributor for improved performance. The Duraspark and DUI, like most distributors, uses mechanical advance (centifugal weights in the distributor) to compensate for changes in rpm, which immediately swing out when rpm is increased. Whereas the "Load-O-Matic" uses Venturi Vacuum and a vacuum canister, which reacts slowly to the weak venturi vacuum signal. As such, the "Load-O-Matic" distributors are very slow to provide spark advance when the engine is accelerated under load. The result is soggy acceleration, which the "Load-O-Matic" distributors are notorious for. As we explained earlier, this is also why we don't like the Petronics Ignition system. While the Petronics increases spark quality, your still stuck with the poor operation of the "Load-O-Matic" distributor.

VACCUM CANISTERS

Load-O-Matic
As we explained above, the "Load-O-Matic" uses manifold vacuum to compensate for load, and venturi vacuum to compensate for speed. We should also note that venturi vacuum is a relatively weak vacuum signal, in comparison. The diaphragm in the vacuum canister is counter-balanced by two springs sitting at angles to each other, which are visible when the distributor cap is removed. The springs are attached to two eccentric post, which can be turned to adjust the advance curve. Therefore the "Load-O-Matic" vacuum canister replies on a combination of manifold vacuum and venturi vacuum to provide the correct amount of advance.

Conventional
A conventional distributor uses manifold or ported vacuum to compensate for load, and centrifugal weights to compensate for speed. The weights are counter-balanced with springs, which can be changed to adjust the advance curve. When an engine has a light load, the vacuum signal is high, whereas at the highest load or wide open throttle, the vacuum signal will approach almost zero. As such the vacuum canister on a conventional distributor, sensing manifold vacuum only, adds more advance when cruising and less to none when you have the "pedal to the metal". On the other hand, the centrifugal weights continually increase the spark advance as rpm's increase.

NOTE: If you are doing a carb swap, more than likely you'll need to swap out the distributor as well (see "MIXING IT UP").

NOTE: The spark control valve looks a lot like a Holley power valve. It works similarly to a power valve, but instead of passing fuel, it passes air.

Autolite 1100 carb with a "Spark Control Valve" (circled)

MIXING IT UP
The "Load-O-Matic" distributor and the "Autolite 1100" carburetor were designed to work together, so what happens when you replace one or the other.

Distributor Swaps
The "Load-O-Matic" distributor uses the "Spark Control Valve" (a spring and diaphragm mechanism), to determine the proper amount of vacuum advance, which is commonly a mixture of manifold vacuum (sensing load) and venturi vacuum (sensing rpm). On the other hand, a conventional distributor uses mechanical weights (speed) and manifold vacuum (load), to provide the proper amount of ignition advance for any given situation.

While you can use a Duraspark or DUI distributor with a stock Autolite 1100 carb, both of these distributors utilize a mechanical advance mechanism that was designed to operate with ported or manifold vacuum. As such they will not operate properly using the vacuum port on the stock Autolite 1100 carburetor. To correct this you need to plug the port on the carb, so you don't have a vacuum leak, and run the vacuum line from the Duraspark or DUI distributor to a manifold vacuum source. This can be accomplished using an existing vacuum source on the intake manifold, or the bottom of the carb adaptor. Or you can create a new source by drilling a hole in the intake manifold and installing a vacuum barb. However, you'd need to remove the cylinder head before drilling, to make sure the manifold is free of any debris that could work down into the cylinders and severely damage the motor.

So...what happens if you do try to use the vacuum port on the Autolite 1100 carb? At idle, the "Spark Control Valve" is open, sending manifold vacuum to the distributor. As such, the engine will idle just fine. However the "Spark Control Valve" will not provide the proper vacuum signal under load and/or speed. All engines need more advance when cruising, and less to none at wide open throttle. However since you are now using the carb (venturi vacuum advance) and the distributor (mechanical advance) to compensate for speed, the ignition system will receive to much advance at cruise and wide open throttle. As a result, the engine may chug and jerk from the over-advanced condition at cruise.

Carburetor Swaps
If you're swapping carbs, there's a very good chance you'll need to swap the distributor as well. The "Load-O-Matic" distributor was designed to work with a "Spark Control Valve", which to my knowledge, was only found on the Autolite 1100 carbs. The only exception to this is the new Vaporizer carb, which we will discuss momentarily.

Most carbs, like the popular 32/36 Weber DGV, the Holley 350 or 500, or any of the Autolite 2BBL carbs, are designed to supply ported vacuum. Therefore the engine would idle just fine. However since neither the carb or the distributor have any means of sensing rpm's (or speed) the distributor cannot provide the proper amount of spark advance at cruse. With the spark advance severely retarded, the engine would lack power, run very poorly, and suffer from poor gas mileage.

As we explained earlier, installing a newer carb with a "Load-O-Matic" distributor should be avoided. However the one exception to this rule is the new Vaporizer, since it's offered with (or without) a "Spark Control Valve", depending on the model year. If you plan to keep the stock "Load-O-Matic" dizzy you would need to order the 63-67 Vaporizer, since it comes with a "Spark Control Valve" and designed to work with the "Load-O-Matic" distributor. However if you plan to swap the distributor you'd need to order the 68-69 Vaporizer, since it comes without the "Spark Control Valve", which is designed to work with the stock 68-69 distributor. Another option, altho extreme, would be to plug off venturi vacuum port so the carburetor supplies manifold vacuum only. For more information on the Vaporizer click here.

NOTE: Installing a Petronics Ignition in the "Load-O-Matic" distributor makes no difference in the outcome. If you keep the "Load-O-Matic" distributor, you must use the stock Autolite 1100, or the early (63-67) Vaporizer with a "Spark Control Valve".

NOTE: If you swap the carb to anything other than an Autolite 1100, or the early (63-67) Vaporizer, without a "Spark Control Valve", you must swap the distributor as well.

NOTE: If you install the popular 32/36 Weber DGV or an Autolite/Holley 2V carb, you must swap the distributor as well. You'll need a distributor that has provisions for mechanical advance, and designed to operate with ported or manifold vacuum.

NOTE: I should also note that we have noticed that some applications like ported vacuum, while others prefer manifold vacuum............... (currently being edited) So if your having problems, try using ported vacuum, then manifold vacuum, and see which works best for your application.

COMMON PROBLEMS
Typical problems with the "Load-O-Matic" distributor and/or the Autolite 1100 carb include leaky vacuum lines, failed or stuck "Spark Control Valve", clogged vacuum passages, worn out distributor advance plates, or worn out vacuum advance diaphragms. Any of these problems or failures can lead to a lack of full spark advance, causing poor performance, poor gas mileage, and potential overheating.

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