A STARTING POINT FOR AN ARTICLE ON 510 DISTRIBUTORS
- by golly!
The distributor in your 510 has an important and difficult job. It is responsible for making your engine's spark plugs fire at precisely the right time in all combinations of engine speed and throttle opening. Although the coil actually creates the spark, the distributor tells the coil when to spark and sends the spark to the proper spark plug.
At the top of the distributor are the cap and rotor. As you know, the coil wire and spark plug wires connect to the cap, and the rotor connects the coil wire to each spark plug wire in turn as it rotates.
When you take the cap off of the distributor, you see a plate known as the breaker plate, to which the points and possibly the condenser are connected. The shaft of the distributor has 4 lobes that open and close the points as it turns. This is what determines the spark timing - the moment the points open, the coil sparks. If you have an electronic distributor, there are no actual points, but the principle is the same.
Because the proper time for the spark varies according to engine speed and manifold vacuum, there needs to be some means for changing the spark timing (when the points open and close). A mechanical advance (sometimes called centrifugal advance) mechanism is provided to vary the spark timing according to engine speed, and vacuum advance is provided to adjust the spark timing according to throttle position/manifold vacuum. Both of these features of your distributor provide a way for the timing to advance (for the spark to occur sooner), but they work in different ways. The vacuum advance works by turning the points, and the mechanical advance works by turning the distributor shaft. The net result is the same - the points close sooner.
By the way, you always want the spark to occur as early as possible. Any later, and you have wasted some time that would have allowed your engine to run more efficiently and make more power. But that's another subject entirely.
Unless your 510's engine is nearly stock, you could probably benefit from modifying one or both of these advance systems. Because the vacuum advance only affects cruising and part-throttle operation, most people are satisfied to leave it alone. Note that, while many race engine builders remove the vacuum advance to make the distributor more reliable (because a solid breaker plate rather than a movable one can be used), this really hurts the gas mileage on a street car!
It is the mechanical advance that you would probably want to change to improve your 510. (Yes, even the best cars on the road can be improved upon!) As you accelerate at full throttle, your engine can take more ignition advance (without detonation, the limiting factor), so you want your distributor to advance your timing as much as possible, as soon as possible, without over- doing it. So you are adjusting two things with the mechanical advance: when, and how much. Let's look at exactly how the mechanical advance works so we will know what we are doing.
Before we do, it's time for Bryan's First Rule Of Working On 510 Distributors. Except for adjusting the timing and adjusting points, ALWAYS remove the distributor from the car to work on it. It is much much easier to work on the distributor when it is on a bench or in a vise. To remove it, just take off the cap, unplug the wire(s) and remove the single adjusting bolt. The distributor can only be re-installed in one direction so there is no need to find TDC or mark which way the rotor is pointing or anything like that.
The entire mechanical advance mechanism is beneath the breaker plate. To remove the breaker plate, which is held down by two screws, the vacuum advance mechanism must first be removed.
You'll see two arms with slots, each of which has a weight and a spring directly beneath it. (See photo) As the distributor turns faster and faster, centrifugal force pulls these weights outward against their springs. When the weights move outward, they turn the arms that are connected to the distributor shaft, advancing the timing. The amount that the timing can advance depends upon how long the slots in the arms are. One of the arms will have a number stamped on it, indicating how many degrees of timing that arm allows. The weights are pulled back in by their springs as the engine slows. So the stiffer these springs are, the more resistance the weights have to being pulled out by centrifugal force, and so the more slowly your mechanical advance will kick in. And the shorter the slots are, the less total mechanical advance you'll get.
To increase the total mechanical advance, lengthen both slots. To decrease it, shorten one or both arms. You can modify your own parts or swap around different ones that you find from various Datsuns with 4-cylinder L-series engines that use points. Note that the advance curve will not change, it will just go further. To change the advance curve, the weights or springs must be modified.
If you want less advance, you can increase the spring stiffness or you can make the weights lighter. I find it much easier to work with the springs. (As far as I know, only one size weight is available from Nissan, but I'd be happy to be proved wrong on this), You can make your springs stiffer by finding or buying different springs, or by bending the arms that the springs mount to. If you're bending the arms, you are limited by how much room you have (the arms will touch the distributor housing), but you can make the springs a good bit tighter this way. Note the term tighter was used because the springs themselves have not been stiffened, they are just pre-loaded more, so the advance curve is just moved, not actually changed.
If you want less advance, you can make the springs less stiff, or you can make the weights lighter. To make the springs less stiff, you can replace them, or you can bend the arms in, but only a little bit, or the spring will not be tight at idle, and it might even come off. You can also remove one of the two springs completely. Confused? Don't worry. So am I. But I never let that stop me, and neither should you!
First, you need to decide what change you want to make, and it may be one or more of the following.
If you want more power/performance:
- more total advance
- advance starting sooner
If you are getting detonation or spark pre-ignition:
- less total advance
- advance starting later
Answering these questions will get you on your way. But knowing just what to do, and how
much to do it, is the trick. Understanding the subtleties of how engines work and the
results of various combinations of changes can be a lifelong task. (And, as your editor
knows, maybe just as complex as writing computer programs!) So you are ultimately
subjected to good ol' trial-and-error, at least with the help of educated guesses. But
you'll learn a lot. Remember that everything is interdependent, and every change is a
compromise of some sort. You may find you can no longer get by on that 89-octane gas after
advancing your timing curve. If you have raised your compression ratio, you probably can't
use (and don't need) as much advance. And an engine that runs fine in a race car might not
work well at all under the load of a heavier street 510. Every car is different. Change
and drive, change and drive, change and drive, that is the story of a properly modified
car (as opposed to modi-fried). Good thing it's a fun process!
Sometimes you need to make sure the distributor is close enough that the engine should run. You have to make sure of two things: the rotor must be positioned properly so that the spark goes to the correct spark plug, and the points must also be properly aligned with the distributor shaft so that the spark occurs at the right time.
Start by turning the engine at Top Dead Center (TDC) of the compression stroke. The next paragraph explains how, so you may want to skip to the following paragraph.
You'll need to decide how to turn the engine. (The starter is not any help because you can't control when it stops.) Removing the spark plugs makes the engine much easier to turn. Often, you can then just reach down and turn the front pulley by hand. Manual transmission cars can be pushed in first or second gear. If you have to, use a socket on the front pulley bolt (I think it's a 1-1/16" socket), and be sure to only turn it clockwise (in the tighten direction). Use your timing marks to determine when the number one piston is at top dead center (0 degrees), but confirm that they are correct by touching the top of the piston through the spark plug hole using a wood dowel or a pencil as you turn the engine (obviously assistance is very helpful here), and by looking at the piston with a flashlight. Next you need be sure that you are at TDC of the compression stroke. It could be on the exhaust stroke, in which case you'll need to turn the crankshaft one full turn (which will turn the camshaft one-half turn). When on the compression stroke, both of the #1 valves will be closed. When the cam lobes are pointing up, their valves are closed. If you're familiar with what you're looking at, you can see them through the oil filler hole in the valve cover. (If not, it's not much trouble to remove the valve cover and make sure.)
Once at TDC of the compression stroke, your may need to install the oil pump/distributor drive in the front cover. Otherwise, you might just want to check its position, which would require removing the distributor. Stand at the side of the car leaning over the drivers side fender, and look down into the hole where the distributor goes. There is a drive gear that has a single tooth that extends into the distributor. Note that the tooth is offset from the center of the shaft, so the distributor can only go in in one direction. The shaft should be almost vertical, but slightly counterclockwise from vertical, at a clock position of 11:30 (where 12 is vertical). The drive tooth should be offset toward the front of the engine (see photo).
Now the distributor can be installed. The distributor's rotor should be pointing at the number one plug wire on the distributor cap. All that is left to do is to set the points. Turn the engine back to about 10 degrees before top center. Loosen the distributor adjusting screw, and turn the distributor until the points just touch the cam lobe on the distributor shaft - that is, until the points are about to open. That should put your initial timing at about 10ø - close enough to start the engine and adjust it with the timing light.