Jim Smart
November 1, 2012

When Ford introduced the Mustang in April of 1964, the base powerplant was the Falcon's 170ci six with 101 horsepower and 156 lb-ft of torque. Later that year, in August, Ford updated the Mustang's engine line-up with the seven main bearing 200ci six as standard equipment. Both the 170 and 200ci sixes are well known and respected for rugged dependability. But engineering shortcomings have plagued them since the beginning. The greatest shortcoming is a lackluster induction system, which is positioned directly over a hot exhaust manifold. The Mustang six's ignition system was the old-school Ford/Holley Load-O-Matic distributor, which also offered disappointing performance.

Six Faults & Fixes

Autolite's 1100 series one-barrel carburetor entered service in 1963 to replace the Holley 1900 series glass bowl carburetor. It was a Mustang six mainstay until '69. There were three 1100 venturi sizes: 1.00-, 1.10-, and 1.20-inches depending upon model year and application. From '63-'67, the 1100 employed a spark control valve for use with the Load-O-Matic distributor, which had vacuum advance but no mechanical advance. The absence of a mechanical advance hindered performance.

Although the spark control 1100 carburetor and Load-O-Matic distributor may seem unconventional in function, they're quite simple. The spark control valve gets its vacuum signal from two places in the carburetor throat. One is at the throat of the venturi while the other is in the bore just above the throttle plate. The spark control valve is calibrated to function at specific vacuum pressures (working against spring pressure) depending upon throttle position. At steady throttle, the spark control valve is held open by vacuum against spring pressure. What the spark control valve does next depends on engine speed, throttle position, and load. During acceleration, the vacuum signal is low enough that the spark control valve's spring pressure closes the valve and vacuum to the advance unit is cut off. Yet some vacuum at the venturi throat keeps the advance from going full spark retard.

As engine rpm catches up with throttle position, the manifold's vacuum signal acts on the spark control valve diaphragm, which opens the valve to provide a strong vacuum signal to the vacuum advance. When engine speed increases with the throttle open, manifold vacuum decreases and the spark control valve closes, preventing venturi vacuum loss due to bleed back caused by manifold vacuum at the throttle plate. This gives full spark advance at high engine speeds. By the same token, the spark control valve also has to retard spark when throttle position and load are such that too much timing could cause engine damage due to spark knock. The spark control valve has the job of spark advance and retard depending upon load and throttle position. What makes the spark control valve different than a Holley/Autolite power valve is vent holes. You will see them in the spark control valve, but not a power valve. These holes allow atmospheric pressure to act on the diaphragm while manifold vacuum acts on the inside to modulate the valve.

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Dual Advance

Beginning with the '68 model year, Ford switched its inline sixes to a dual-advance/retard distributor with both a vacuum and mechanical advance like V-8s of the period. Instead of a spark control valve, the Autolite 1100 carburetor provided a throttled vacuum signal instead of a valve-controlled vacuum signal. As a result of tougher emission standards, the dual-advance feature was also advance/retard, with spark advance during acceleration and retard during deceleration to reduce exhaust emissions.

The reason we've shown you both types of fuel and ignition systems is to show you what happens when you don't have a proper match. We've seen our share of mismatched carburetors and ignition systems, which cause serious drivability problems. If you run an Autolite 1100 carburetor with a spark control valve and a dual advance distributor, you won't get sufficient vacuum to operate the vacuum advance. If you run an 1100 carburetor without a spark control valve with a Load-O-Matic distributor, you will wind up with too much spark advance. If you're having drivability issues with your Mustang six, the first thing you need to check is carburetor and ignition for compatibility. During a recent visit to The Restomod Shop in Stockton, California, we witnessed a pristine '66 Mustang hardtop with 200ci six that wouldn't idle properly. It didn't take long to figure out why. It had the Load-O-Matic distributor with an 1100 carburetor void of the spark control valve. At idle, the vacuum signal was strong, causing excessive spark advance and making it impossible for this engine to idle properly. It needed the 1100 carburetor with spark control valve.

Terry Simpson of The Restomod Shop donned his troubleshooting skills and went to work on this Mustang with the installation of the correct 1100 carburetor with spark control valve. The more Terry examined this engine, the more troubling issues he found. You need to do the same in your driveway as a means to honing your own technical skills. Does the vacuum advance operate? Check this by connecting and disconnecting the vacuum line with engine at a fast idle. If rpm drops when you disconnect, you have a working vacuum advance. One cylinder at a time, disconnect each ignition wire at the distributor cap. Does rpm drop or remain the same? If speed doesn't change, you have a weak or dead cylinder. Spray carburetor cleaner around potential vacuum leak areas like the carburetor base plate and spacer. Does the engine surge? If so, you've found a vacuum leak. Pull the PCV valve and check for vacuum. At idle, you should feel suction and the engine should surge.

Is there valvetrain noise, meaning a clicking rocker arm or dry valve spring? If you can hear valvetrain operation with hydraulic lifters, something is wrong, mandating a visual inspection. There should be plenty of oil flow throughout the rocker arm assembly. Return flow should also be good. While you are tuning, check things like coolant flow through the radiator at operating temperature with the thermostat open. Examine the engine for oil and coolant leaks. Replace fuel hoses at every tune-up and use high-pressure fuel injection hose throughout, which is resistant to today's harsh fuel additives.

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