Stock Ford distributors are notorious for contributing to pre-detonation problems at high RPM because they tend to have infinite advance curves. Pre-detonation occurs when the timing is advanced so far that the spark plug fires in advance of the piston transition point and actually forces the piston back down against the rotation of the crank shaft. This will destroy your main and rod bearings and is a major cause of engine failure. This tech article shows how to limit your stock distributor's advance curve reliably and prevent pre-detonation.
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Mr. Gasket sells a spring replacement kit for Ford distributors. The replacement springs are at the top of the picture and the stock Ford springs are at the bottom. The replacement springs are much lighter tension than the originals and will give you full advance at about 2800 RPM. The problem with this solution is that by the time you dial in the distributor for say 35 degrees of advance at 3000 RPM, the engine won't idle because at 900 RPM the timing retards so much. |
| The way to attack this problem is to limit the amount of advance the distributor can generate. Lets get into a stock distributor and make the modification that will solve the problem. First, we need to get to the centrifugal advance mechanism. It's located under the top plate in the distributor housing. I use the vacuum advance pod diaphragm to hold the plate at full advance by removing the vacuum hose. Cap the tube with a plug to keep it dry and debris free. There is an E-clip that keeps the advance arm attached to the vacuum timing advance. (Yellow Arrow) Remove the clip and the two screws holding the vacuum pod in place and remove the pod. |  |
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Once the vacuum pod is out of the way, remove the two screws (Yellow Arrows) that hold the vacuum advance mechanism to the plate. |
| Now that the plate has been removed, the centrifugal advance mechanism is visible. The yellow arrow is pointing to the notch in the centrifugal advance plate. Note the stop in the middle of the notch. The springs have been removed so the mechanism will move freely from side to side. This notch determines the total amount of timing advance that the distributor can generate. The wider the notch, the more advance. |  |
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To remove the center shaft to make the modification a clip must be removed in the opening at the top of the shaft. The opening is directly under the rotor. When the rotor is removed use an awl or rigid piece of wire to locate the clip and remove it. The picture to the left shows the stock advance notch width at .625 inches (5/8 of an inch). We need to shorten the notch by about .125 inches. |
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I used a MIG welder to add material to the left side of the notch. The left side is the maximum advance stop. I used a grinder and a hand file to get the notch to .500 inches wide. This limits the travel of the advance mechanism to about 20 degrees. This means I can set the timing for 15 degrees advance at idle and I'll have about 35 degrees maximum advance at 3000 RPM. No matter how high the engine revs, the timing will never exceed 35 degrees. |
| The centrifugal advance center shaft has been re-installed. Note the narrow .500 inch notch at the upper right. The Mr. Gasket springs have been installed and the distributor is ready for re-assembly. I've used this technique for years. You can spend a lot of money on after market ignition hardware and get the same results. This whole project cost me $15 and a couple of hours on a Saturday. |  |
| For the rest of the assembly, simply reverse the disassembly order. Use a timing light and check your work. If you need more advance simply widen the notch. I've never had this method fail. Others have, so I've stuck with this one. There are varying levels of sophistication. This occupies the lowest. A complete replacement of the ignition system with an after market unit that can either be programmed or uses plug in timing and rev limiters, occupies the highest. The KISS principle seems to work the best for me. | |
That's it for this time and there's much more to come. See you in the next newsletter.
Joe Quaranto