Drop A Load (O-Matic)

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Save that SCV carb! Don’t toss it when you dump the Load-O-Matic!

Converting a Load-O-Matic carburetor to conventional ported vacuum

 

I already posted instructions for this modification on TFFN, but I’m including it here for anyone who stumbled across this site and missed my original post on TFFN.

Early Ford sixes didn’t use a conventional distributor with a centrifugal advance mechanism and a vacuum advance system using ported (above the throttle plate) vacuum. Instead, it used a system call Spark Control. This system used a vacuum advance only distributor and a carburetor with a specially calibrated vacuum port that was designed to work with the vacuum-only distributor. Together, the carb and distributor were calibrated to somewhat mimic a conventional centrifugal/vacuum advance system, but for lower cost. The system worked OK, but it was difficult (impossible?) to modify the advance curve by tuning.

When we scored a 1978 200 engine from Craigslist, it was already equipped with a Duraspark 2 distributor. An added bonus was the adjustable vacuum advance (adjusted by removing the vacuum hose and inserting an allen wrench). We knew we wanted to use this distributor. The problem was that our 1963 Autolite 1100 carb, with its Spark Control Valve (SCV) was incompatible with our distributor.

Conventional wisdom says you must obtain a carb with conventional ported vacuum if you want to run a conventional distributor. I decided that before I threw in the towel, I was going to see if I could modify the SCV carb to conventional ported vacuum.

First, allow me to give you a little background information. Ported vacuum isn’t rocket science. There’s a tiny orifice just above the throttle plate (or above one of the primary throttle plates in the case of a four barrel) that connects, through an internal passage, to a hose barb or fitting on the outside of the carb body. As the throttle plate is opened, it exposes the port to engine vacuum. Connected to a conventional vacuum advance diaphragm, this causes timing to advance as the throttle is slowly opened.

Upon examining our 1100, I was encouraged to see the two major requirements for ported vacuum already present and accounted for: the orifice above the throttle plate, and a vacuum fitting on the outside of the carb. Previous testing revealed that the vacuum fitting wasn’t true ported vacuum, so I began to investigate the cause.

I disassembled the carb and sprayed aerosol carb cleaner through the vacuum fitting and watched where it came out. It was immediately apparent that there was a labyrinth of passages in there, as carb cleaner sprayed out everywhere! Fortunately, it came out above the throttle plates, just as required. The rest of the passages, however, had to be identified and blocked.

All Paths

Here is a picture of the outside of the carb, with the internal ported vacuum ports identified with colored lines on the outside. The red passages must be blocked; the yellow ones are necessary.

The first place passage to block is shown here:

I drilled a shallow hole here with a #35 drill bit, tapped it to 6-32, and inserted a 6-32 brass set screw. Viola’!

Here’s the next passage to be blocked.

Again, drill a shallow hole with a #35 drill bit, tap it to 6-32, and insert a 6-32 brass set screw. 

The last passage was difficult to capture with the camera. It is shown here with a paper clip pushed through it. Drill a shallow #35 hole from the outside, tap to 6-32, and insert a 6-32 set screw into the hole.

That’s it! You now have an unobstructed path from above the throttle plate to the vacuum fitting on the outside of the carb… with no other passages. To maintain a stealth look, screw the SCV back into the carb, and no one will be wiser.

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#1 Larry DuBois 2017-04-30 20:22
Never knew this technology existed! Of course, I am new to Fords.

My predicament is the opposite of what is outlined above. I have a Load-O-Matic distributor but the carburetor apparently does not have the spark control valve. It has the mounting port but no threads.

Would drilling the ports you blocked turn this into a Load-O-Matic carb? It seems like there should be another hole where the control valve goes.

The truck is original but at this time I don't know if the carburetor or the distributor is factory. Maybe finding a standard distributor with centrifugal advance would be easier.
 

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