Degree of Difficultly: Intermediate
Keeping your BMW "in tune" can mean a big difference in its performance, fuel mileage, and engine life. In this article, we'll discuss condensers and points, distributors, and timing. In a subsequent column, we'll cover advanced topics such as installing a CD ignition system.
The subject of spark plug changing was covered in an earlier Bimmer Beginner column (Roundel Vol. VIl, No. 1), so please refer to that article for advice on changing your plugs. However, when you change the plugs it's usually also time to check and/or change the points, the rotor, the condenser, and the distributor cap; and the timing must be reset any time the points are readjusted or gapped.
Tools you'll need include the usual tool kit items:
- Dwell tachometer
- Timing light
- Leaf gauge (either metric or SAE)
- Old spark plug wire will help in connecting up the timing light.
Let's start with points. Remove the distributor cap and unscrew the old point set if you've determined from a visual inspection that the old points are too pitted for continued use (symptoms are substandard acceleration and missing). After cleaning the surfaces with a rag, insert the new set of points and fasten the screw loosely. Apply a small quantity of distributor cam grease to the cam and put a couple of drops of oil in the felt pad in the middle of the distributor shaft.
Put the car in fourth gear, release the brake, and push in either direction until the points are open as far as they will go/the lobe of the distributor cam is centered on the rubbing block of the point set. Now insert your leaf gauge, loosen the adjusting screw, and set the points to the gap shown in the owners' manual. Since the rubbing block will wear down in time, set the points to the minimum of the range shown N if it says 35 to 40 mm, set it to 35 mm.
If you don't have a dwell tach, you can still get a fairly accurate setting with the above procedure. If you do have one, connect it now and check the dwell angle. Attach the positive (red) lead to the + terminal of the coil, and the negative (black) lead to a good ground on the engine. Put trans in neutral and set handbrake. Set the dwell tach indicator to "dwell angle."
Now, with the distributor cap and the rotor removed, turn the starter, either with the key or with a remote starter switch, note the reading on your dwell tach. If you're aiming for 36 and the dwell tach reads 30, your point gap is too wide, and vice-versa. Adjust until you have it just right.
Before you replace the distributor cap, check the cap itself; if it has more than 20,000 miles on it, it may be badly "carbon tracked"check the metal contact points inside the cap. A cracked or broken cap will also put your car out of commission.
You may also wish to replace the condenser, although not routinely; I change mine only about every 25,000 miles or even a little more. The old tune-up procedure of "change plugs, points, and condenser" isn't always necessary.
Now replace the distributor cap (or change it if necessary, being very careful to get the plug wires in the right places). Since you have adjusted the points, it is necessary to reset the timing to the car's specs. Here's how it's done on a '76 530i; check your owner's or shop manual for the correct settings for your car (also see spec plate under hood).
- Disconnect vacuum hose(s) on distributor and plug.
- Remove #1 plug wire either from plug or distributor and attach correct lead of timing light here (see directions with timing light).
- Connect electrical leads of timing light per directions (to battery or AC outlet).
- Set engine to idle at 1700 rpm at working temperature.
- Turn off engine and loosen distributor (10-mm wrenches).
- Start engine again, aim timing light at flywheel, determine if silver ball lines up with mark on flywheel. Turn distributor each way until ball lines up properly.
- Shut off engine and re-tighten distributor (10-mm wrenches).
- Recheck dwell angle and timing; in tightening the distributor you might have jarred it.
- Remove dwell tach, timing light, and plug from vacuum hoses; reinstall vacuum hoses on distributor.
- Start engine and set to idle at 900-1000 rpm.
This sounds very complicated, and it is the first time you do it. But the whole procedure, including changing the plugs, can be done in less than an hour once you get the hang of it. Quite a bargain when you consider the cost of a tune-up at a service shop these days.