Tech Article 5
Troubleshooting your XK Engine – The Distributor
Last month we started looking at and understanding the operation of the spark ignition system. In this month’s article, we are going to look at the maestro of your XK ignition system, the distributor. The distributor directs the bursts of voltage from the coil to the correct spark plug. It also establishes the best time to send the voltage with respect to the position of the piston in the cylinder. And as with the rest of the ignition system, it does this very very quickly.
As we established in our last article, the ignition coil is able to generate short pulses of high voltage electricity. This electricity is sent down the ignition wires to the spark plug, where it generates a spark that ignites the air/fuel mixture. This needs to occur at the precisely correct instant in order for your XK engine to operate at peak performance. And the distributor makes it all happen.
Electrically, the distributor consists of two separate parts. There is the part that routes the burst of high voltage from the coil and
the part that signals the coil when to fire. First let’s look at the part that directs the voltage. On the top of the distributor is the cap. This cap has sockets around it’s perimeter for the attachment of 6 spark plug wires. In the center is the wire from the coil. Pull the cap off (it is held by two clips on the sides) and you will see the rotor. The rotor directs the voltage from the central coil wire to one of the spark plug wires. If you look inside the cap you just removed, you will see in the middle a spring loaded rod of black carbon like material. When assembled, this rod bears on the center of the rotor,
establishing the electrical connection between the cap and the rotor. If you study the rotor further, you will see that the rotor has a metal plate on its outer end. This plate comes very close to the metal contacts (usually copper) on the inside of the cap. In this case, the burst of voltage will jump the gap from the rotor to the contacts. This is the second electrical “connection” in the arrangement, although technically it is not a connection since the two pieces do not touch.
If you were to put your car in neutral and manually rotate the engine, you would see the rotor revolve in a counter clockwise direction. This is because the central shaft of the distributor reaches down into the interior of the engine, where it mates with a gear that feeds to the crankshaft. So when the crankshaft turns, the central shaft of the distributor turns. On most cars the gearing is such that the distributor revolves at exactly ½ the speed of the crankshaft. This is important because if you remember, on a 4 stroke engine the spark only needs to fire on every other stroke of the piston. So as
the rotor spins with the engine, it can be adjusted so that it is always pointing towards the corresponding wire on the distributor cap that needs to be charged with voltage in order to fire the spark plug for that cylinder at the exactly correct moment.
Wow, it may be time to go get a beer out of the fridge. Or maybe something stronger!
So the rotor is spinning and the spark is sparking and if all the gears and shafts hold up, the spark will occur in each cylinder when it needs to. But we’re not done yet.
You may ask, how does that stupid coil up at the end of the high tension wire know when to fire. I’m glad you asked because that is the function of the “points” in the distributor. In modern times this all happens electronically but in the good old days when life was simpler, real men let the points decide when to fire the spark plugs, rather then leaving it up to a computer chip encased in black secrecy! I have yet to be able to look at a computer chip and see if it was set correctly but the points are easy. On the shaft below where the rotor sits there are 6 equally shaped cams or lobes. One for each cylinder in the car. Bearing on the lobe is the “wiper” of the points. The wiper is connected to the points assembly. Simply put, as the wiper rides up on the high point of the cam lobe, the points open and when the wiper rides on the low point of the cam lobe they close. And also simply put, every time the points open, the coil fires a shot of high voltage. Now why it does this gets into a little bit of physics but suffice it to say, there is a small wire going from the points to the coil and electrically along this wire is sent the message to the coil “the points have just opened and you must release your pent up energy as a burst of voltage.” Believe me it happens and it happens 6 times for every 2 revolutions of the crankshaft and that is just right to fire the fuel/air mixture in each cylinder!
Now there is an easy way and a hard way to adjust the points. The points can be moved closer or farther apart by the action of a small eccentric screw. The easy way is to adjust them so that a matchbook cover will just slip between the points when the wiper is up on its cam. This probably worked a lot better when a lot of people smoked but the idea is to statically set the gap at the points to a finite value, about 1/64”. Then, once you get the car running, you connect a moderately expensive “dwell meter” to the system and see what you’ve got and adjust as necessary. When the dwell is set per the factory specs, the points will be spending the right amount of time open and the right amount of time closed for optimum engine performance. Suffice it say that if you ask your mechanic if he “checked the dwell setting on your points” he will recognize you as an astute student of ignition systems!
Now many folks like to change out the points with a simple electronic setup. The main two available are the Pertronix Ignitor and the Mallory Unilite. Both are perfectly acceptable substitutes for the points and will increase reliability, if not the “adventure factor”.
Hand in hand with the dwell setting is the “timing” of the distributor. As we said above, the distributor is driven off the crankshaft via gears. But in order to allow the spark to fire at the exactly correct instance, this connection is left to be adjustable by the mechanic. On most engines, including the XK, the distributor fits into a socket on the side of the engine. After the distributor shaft is inserted into this socket, it can still be rotated by a large amount. But there is only one truly correct position and that is the position that causes the distributor to be “timed” correctly in relationship to the engine. The “base” timing specifications are provided in your XK shop manual, including the Bentley manual. It will be quoted as a number like 10° BTDC. If you remember from our previous articles, BTDC stands for “before top dead center”. What this means is that the spark should fire 10° of crankshaft rotation before the piston is at its highest point or “top dead center”. This gives the fuel/air mixture a little time to react to the spark such that good burning and pressure is established when the piston travels past TDC and is headed down in its power stroke.
For an engine that has been torn apart, the timing can be set statically. But for fine tuning of the engine, it should be set with a timing light. As we discussed in a previous article, there are marks on your crankshaft pulley that correspond to various degrees before top dead center. Base timing is set by attaching the timing light per manufacturer’s directions, starting the engine, and rotating the distributor shaft in its socket until the timing light indicates that the correct value (say 10°) has been met. Once the distributor is set in the correct position, a small bolt is tightened at the base of the distributor, which clamps it into position.
Once the distributor timing has been set, it will stay at the correct position for many miles, since these are robust components that do not wear out. The points, on the other hand, are a wear item and will change their gap over time, affecting the performance of the engine. So the points need to be checked for wear and possibly re-gapped on a routine basis, say every 20,000 miles. Next month we will look at “ignition advance” and some possible problem areas in that department that may affect performance. Further excellent reading regarding ignition systems can be found at the website http://www.centuryperformance.com/timing.asp