Thanks to the Mercedes-Benz Club of South Australia for this article which first appeared in issue 388 of their magazine. As always this article is for information only.
If you own any car that was made before the mid-90s or one that has a carburetor, the chances are it will be fitted with a conventional type distributor and vacuum advance unit. The vacuum advance unit is easy to spot - it is a large round unit that looks like a spinning top that protrudes from the side of your distributor and will have a vacuum line running from the inlet manifold. Here’s one we prepared earlier:
Personally, the topic of vacuum advance units failing has been fresh in my mind recently - both in conversation and in practice. Very little thought is given to these units - when they fail you can still drive your car but you will never get it to run properly - a non-functioning vacuum advance unit causes sluggish throttle response, especially off idle (like starting moving from a stop) and when accelerating from a steady (part-throttle) speed.
So what does the vacuum advance unit do? The vacuum advance system consists of a vacuum diaphragm mounted on the distributor body. The diaphragm is spring-loaded in the zero-advance position, and has a rod that connects to a hole in the breaker plate (see arrow in pic below), which is the movable plate the points (or ignition sensor) are mounted on.
Vacuum advance works by rotating the base plate to change the position of the points (or magnetic pickup) relative to the lobes (or stator) on the distributor shaft. By rotating the base plate in the direction opposite of distributor shaft rotation (counter-clockwise), timing is advanced because the trigger signal is generated earlier. A typical vacuum advance unit, when fully deployed, will add about 15 (crankshaft) degrees of spark advance over and above what the distributor’s centrifugal advance system is providing at the moment, which depends on engine rpm. They are two independent systems, but they work together to provide the correct ignition timing.
How is the system controlled? As you drive your car and move the accelerator pedal, the amount of fuel and air mixture entering the engine and how rich or lean that mixture is varies dramatically. Lean mixtures burn slower and rich mixtures burn faster. Engine load conditions (idle, steady cruise, acceleration) result in how lean or rich the air/fuel mixture is (the carburetor handles this), and the best indicator of engine load is intake manifold vacuum. At idle and steady cruise, engine load is low, and intake manifold vacuum is high due to the nearly closed carburetor throttle plates.
Under acceleration, the throttle plates open wider, and intake manifold vacuum drops. It is essentially zero at wide-open throttle. As a result, intake manifold vacuum is a "free" indicator of engine load, which correlates nicely with fuel mixture being supplied - lean mixture at high vacuum, and rich mixture at low vacuum.
At idle, the engine needs additional spark advance in order to fire the lean (and exhaust-diluted) idle fuel/air mixture earlier in the cycle in order to develop maximum cylinder pressure at the proper point after TDC for efficiency, so the vacuum advance unit is activated by the high manifold vacuum, and adds another 15 degrees of spark advance on top of the fixed initial timing setting. For example, if your initial timing is set at 10 degrees, at idle it’s actually 25 degrees with the vacuum advance connected (a properly calibrated centrifugal advance mechanism will not have started to move yet at idle rpm).
The same thing occurs under steady highway cruise conditions. The mixture is lean, takes longer to burn, the load on the engine is low (it takes only about 40 horsepower to cruise at 80 km/h) and the manifold vacuum is high, so the vacuum advance unit is again deployed and adds 15 degrees of spark advance over and above whatever the distributor centrifugal advance mechanism is providing at that engine rpm. If you had a timing light connected so you could see it as you cruise down the highway, you’d see about 45-50 degrees of spark advance - your fixed initial advance of 10 degrees, 20-25 degrees provided by the centrifugal advance mechanism, and the 15 degrees added by the vacuum advance unit.
When you accelerate, the fuel/air mixture is immediately enriched (by the accelerator pump, power valve, metering rod piston, etc.), and that rich mixture now burns faster and doesn’t need the additional spark advance anymore.
When the throttle plates open, the manifold vacuum drops, and the vacuum advance unit diaphragm retracts to its zero position, "retarding" the spark timing back to what is being provided at that moment by the fixed initial timing and the centrifugal advance mechanism. The vacuum advance doesn’t come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean and needs more spark advance for fuel efficiency.
That little unit on the side of the distributor sounds pretty important now! But it is very easy to check if it is working properly:
Step 1: Remove the vacuum hose from the vacuum advance unit and attach a test hose to vacuum advance unit (you'll use this hose to apply a test vacuum to the diaphragm - alternatively you can just disconnect the hose from the manifold end).
Step 2: Remove the distributor cap and if fitted, remove the rotor button and plastic cover so you can view the distributor workings (you don't need to disconnect any wires from the distributor cap).
Step 3: Use a vacuum pump, or just suck on a clean piece of hose attached to the vacuum advance canister to apply a vacuum to the vacuum advance.
The unit should hold a vacuum. If it does not, you need to replace the vacuum advance unit. A bad diaphragm will produce little or no advance, and it will introduce unfiltered air into the carburetor’s throttle bore. Look at the inside of the distributor - you should see that the arm coming from the vacuum advance unit should pull on the base plate when vacuum is applied, causing the base plate to rotate around the distributor shaft.
A faulty unit is easy to replace - they are usually only held on to the distributor by one or two screws and then once loosened the shaft just slides off the breaker plate.
Testing your vacuum advance literally takes less than 5 minutes (and about the same time to replace it). I found that the vacuum advance unit on my Mitsubishi Van was no longer holding vacuum and went to look for a replacement at the wreckers - between two separate wrecking yards I found 8 vans that had the same distributor - and on every single one of them the vacuum advance unit was leaking!
Speaking to another car enthusiast the other month, he mentioned that a colleague of his who rebuilds distributors will collect as many spare vacuum advance units as he can find - apparently he is constantly replacing them and made the comment that ‘you wouldn’t believe how many old cars are driving around with leaking vacuum advance units’. Well, I would now.
Go and check that vacuum advance unit - I’d be willing to bet that of the several hundred pre-90s Benzes owned by our members, there would be at least a dozen or so of them with failed or leaking vacuum advance units, sending their owners around the bend trying to figure out why their car won’t run properly.