Understanding and Adjusting the Timing
Adjusting the timing is a must for the engine tune up of an Aircooled VW. From the stock distributor to an 009, to correctly adjust you need to understand the role of each of the distributor components.
The distributor allows the ignition power flow from the high voltage coil to reach the spark plugs in the correct ignition order : 1-4-3-2 in your Aircooled VW engine .
The role of timing :
To allow to an engine to run at optimum capacities, the spark in the cylinders do not usually fire when the piston is at the top of it's cycle (Top Dead Center: TDC). The spark usually fires just a little before, sometimes a little after TDC. The “little before” and “little after” are measured in crankshaft rotation degrees. Advanced means that the spark fires before the piston reaches TDC. Static advance is manually adjusted with the marks on the pulley and by turning the distributor. Dynamic advance is made by the distributor itself depending on how fast your engine is turning. Spark needs to be advanced with each increasing revolutions of the engine to optimize the efficiency. If you set your engine at 7.5 degrees static advance while the engine is off, then by the time you engine is running at 3500 RPM it should be around 27 degrees.
Depending on the type of distributor, this advance in timing is created by the vacuum chamber unit (which is measuring the pressure in the intake manifold) or by a centrifugal mechanism (measuring engine rpm) and sometimes both.
The static advance plus the dynamic advance is giving you the total engine timing advance.
The breaker points (9)
They are used as switches on the primary circuit from the coil to generate enough power to obtain the spark. For each crankshaft revolution those switches are opening and closing two times, At 4000 rpm's there is 8000 open/close , 130 per second.
The condenser (2)
It is used to reduce the electric arc between the points at the time the are opening (breaking power). There is no adjustments possible on a condenser but can be changed easily. With the condenser there is net and quick power breaking, the best way to have a powerful spark. When it is defective there is the formation of an electric arc between the points and then the breaking time is longer, so the spark is less powerful and the points deteriorate.
The distributor rotor (6)
The role of the rotor is to distribute the high tension from the coil to each of the spark plugs when it is rotating. Some models are equipped with a fuse, usually this parts is often replaced if too corroded.
The distributor cap (5)
Indistinguishable from the rotor, the cap is assuring the high tension connection (from coil to spark plugs) and it's studs are in contact with the rotor to distribute the high tension to the spark plugs, while the central stud is in contact with the rotor's center.
The vacuum chamber unit (3)
This part makes the position of the point base change the advance according to the engine rpm. If there is no capsule then there are some little weights under the points base which do the same function, like the 009.
Usually the parts that degrade and need to be maintained or changed are the points. You can see when they are danmaged, the points are marked and one has a hole and the other one has a bump. Usually the condenser is replaced at the same time.
The distributor cap can also be cracked (usually the black and transparent models) and create problems.
You can clean the inside of a distributor with an air compressor and you can also clean the contacts between the cap and the rotor.
To limit axle wear you can lubricate the cams with a special grease, lithium or copper grease. You need to be careful to not put too much on or be careful not put grease on the points contacts.
A ) Points adjustments
- Remove the distributor cap
- Turn the engine slowly by hand to the point where contact between the points are at their maximum
- Slightly loosen the adjust screw
- Adjust the space between the points at 0.4 mm with the proper spacer. The correct adjustment is when the spacer is moving but is touching very slightly both points.
- Tighten the adjustment screw
- Put the distributor cap back on.
At this point, be certain to have a same constant opening of the points. It is also important that the distributor axle does not have any radial play.
B) Static timing adjustment (always after adjusting the points)
You can generally find three marks on a VW engine crank pulley :
1) 10 degrees before TDC (farther right of the TDC mark)
2) 7.5 before TDC (also right of the TDC)
3) 5 degrees after TDC (left of TDC mark)
Usually there is just one mark right of TDC and this is 7.5 degrees.
- Turn the pulley until the timing mark is facing the top of the pulley at the split of the case AND the distributor rotor is facing the mark on the distributor body (cylinder #1).
- Untight the nut (10mm) at the bottom of the distributor.
- Plug a light, or a voltmeter, at the wire that goes from your coil to the distributor and ground your other wire on some metal in the engine (usually the carb works fine for this). If you do not have a light just look closely at the points when you will rotate the distributor to look for a spark.
- Turn the key in the ignition to the power on position but do not start.
- Slightly turn the distributor clockwise until you can see the points in contact. Then turn the distributor counterclockwise (very) slowly, until the light turn on, or the voltmeter bip or..when you can see a tiny spark on the points.
- Tighten the 10mm nut at the bottom of the distributor.
- Replace the rotor and the distributor cap.
C ) dynamics timing adjustment :
Even if the static timing adjustment is totally descent, the dynamic adjustment with the help of a stroboscopic light is much more precise.
- Plug the strobe light, the red on the + battery, the black on the ground and the sensor around the #1 spark wire.
- Remove the hose from the carburetor of the vacuum chamber unit, (with the 34PICT3 solex it is the one on the back of the capsule who is plug on the front of the carburator)
- Untighten the 10mm nut at the bottom of the distributor
- Start the engine and wait for it to run on idle
- Point the strobe light to the pulley and then turn slowly the distributor until the pulley mark is aligned with the case axle.
- Turn off the engine
- Tight the 10 mm nut
- Restart the engine and check once again with the strobe light in case you moved the distributor accidently when tighting the nut
- Plug back the vacuum hose
If you heard some clicking, just reduce the advance a few degrees.
VW Aircooled Engine Specificities
The big majority of those engines are made for a 7.5 degrees advance, one mark pulley : 7.5 before TDC, two marks pulley : 7.5 degrees BTDC and 10 degrees BTDC, three marks pulley : 0 degrees TDC, 7.5 BTDC and 10 BTDC
The pulley from the E and F prefix case number have two marks, the left one is at 7.5 BTDC. The AB engine's pulley have a mark at 5 BTDC or at 7.5 BTDC, as describe in the following table for the engine made before October 1971, after that the mark will be at 0 TDC
The F engines pulleys have one mark, or three... For the engines at 7.5BTDC use the unique one or the middle one. For the engines at 0 TDC use the left one.
The B engines pulleys have three marks, the left one is 0 TDC.
As you can see the pulleys can be really different from one engine to the other, different models, different years. Look closely when you are replacing your engine, used or even reconditioned.
TDC (Top Dead Center)
BTDC (Before Top Dead Center)
ATDC (After Top Dead Center)