There's no question that a distributor cap and rotor must work together precisely to properly and efficiently distribute the spark. There are two areas where cap and rotor fit are critical. The first is rotor to distributor cap clearance. It must be close enough to allow the ignition spark to cross the gap easily.
In the second case that rotor blade must line up exactly with the distributor cap terminal precisely when the ignition spark occurs.
If the distributor cap and rotor blade clearance is to great, the spark can easily jump to the next cap post in the firing order.
The same thing can occur if the cap to rotor alignment is off, even only by a small margin. Not only will jumping spark result in a poor running engine, it can also destroy the engine, that you spent thousands of dollars to build.
To make it simple, the effect can the same as too much initial spark. It's no secret that it's physically impossible to adjust a conventional rotor. But that is not the case with the distributor cap. If you look closely at the cap, you'll notice that there's a margin of free play on the cap mounts.
Most caps can be twisted slightly clockwise or counter-clockwise even when the mount clips are lined up (but not completely fastened). In order to check, and properly phase the system, use the following process. 1. Make a test cap, by cutting a large hole in the side of an old cap for a viewing window. The window should be positioned high enough so that you can see the body of the rotor. It doesn't matter where you cut that hole in relation to the firing order, simply cut the hole in a spot where you can view that rotor as the engine in running.
2. Mark the distributor cap with a series of index marks, the correspond with the exact center of each post in the window, and be sure that the marks are straight. To make the marks visible on the cap, use a felt tip marker that will contrast that color of the cap you have.
3. Mark the rotor with an index mark (a light cut with a small file works very well). The index mark must co-inside with the center of the rotor blade tip exactly, in order to make the tip easy to view, fill the index marl with white typing correction fluid.
4. Install a timing light on one of the distributor cap posts in your window (it doesn't matter what post, as long as it is physically inside the window). Set the engine speed so that the rotor appears to have a steady spark. Use the timing light to view the rotor versus the distributor cap post position , as expected the timing light will slow down the rotor speed so the you can see the location of the rotor tip in relation to the distributor cap terminal. |
5. If your car does not have vacuum advance, phase the tips so the they line up precisely with the distributor cap terminals.
To accomplish this with a conventional cap, you will you will have to note the position of the test cap (mark it with a felt tip pen).
Replace the test cap with the conventional cap, but be certain that you have placed it in the same spot as the test cap (be sure to use the same brand of cap as the test cap once you have determined the location, and marked the distributor body, you will have to check the rotor phase again).
As indicated earlier, there's a slight amount of play available when installing a cap. In most cases this play should prove sufficient to phase, or index the rotor/cap position.
If the car has vacuum advance, you must take in to consideration, that the advance will change that phasing of the rotor according to manifold vacuum.
A distributor with a clock-wise rotation (with the vacuum advance disconnected and plugged) should have the rotor just to the left of the target distributor cap post.
When the vacuum advance is working, the rotor will appear just to the right of the target post. Similarly a distributor with a counter-clockwise , will be exactly the opposite of the above.
There's no question that a distributor cap and rotor must work together precisely to properly and efficiently distribute the spark. There are two areas where cap and rotor fit are critical.
The first is rotor to distributor cap clearance. It must be close enough to allow the ignition spark to cross the gap easily. In the second case that rotor blade must line up exactly with the distributor cap terminal precisely when the ignition spark occurs.
If the distributor cap and rotor blade clearance is to great, the spark can easily jump to the next cap post in the firing order. The same thing can occur if the cap to rotor alignment is off, even only by a small margin.
Not only will jumping spark result in a poor running engine, it can also destroy the engine, that you spent thousands of dollars to build.
To make it simple, the effect can the same as too much initial spark. It's no secret that it's physically impossible to move or adjust a conventional rotor. But that is not the case with the distributor cap.
If you look closely at the cap, you'll notice that there's a margin of free play on the cap mounts. Most caps can be twisted slightly clockwise or counter-clockwise even when the mount clips are lined up (but not completely fastened). In order to check, and properly phase the system, use the following process.
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Distributor Indexing