Mercedes-Benz Ponton
Distributor Point Gap & Vacuum Advance

An adjustment process including theory, technical specifications, photos, and some hints on technique.

Scott Gordon / Jeff Miller / December 24, 2001 - February 9, 2002

Theory

As the crankshaft rotates and pistons go through their cycles, the distributor acts as the brains of the operation. It precisely controls the timing of the spark. Each cylinder's spark must occur at a precise time during the rotation of the engine. The distributor is driven off an idler pulley in the timing chain, such that it rotates at half the speed of the crankshaft. The engine makes two complete revolutions in the same time that the distributor makes one 360 degree revolution.

As the distributor rotates, the lobes on its shaft displace the breaker points to open and close them. One point is stationary and the other is movable. There is a small follower made of fiber filled phenolic resin, or nylon, or Teflon, which rides the cam on the distributor shaft, opening and closing as each lobe passes by. The number of lobes is the same as the number of cylinders in your engine. Each time a lobe passes under the follower, the points separate, breaking the circuit and firing one cylinder in the engine. Finally, the distributor earns its name by sequentially distributing that spark to each cylinder in the firing order. By adjusting the distributor and points, the timing of the spark can be set precisely. This is important because if the spark is too early, pre-ignition (knocking) is the result, which can be detrimental over time. If the spark is too late, the engine makes much reduced power and will seem sluggish.

Distributor Kung Fu

Scott Gordon / Tue Apr 29, 2008 8:39 pm

International Ponton Owners Group (IPOG) forum message Nr. 59102.
http://groups.yahoo.com/group/ponton/message/59102

Doug Broome wrote:

I have never removed the distributor or even closely examined it. Imagine my surprise to discover that someone before me must have installed the distributor 180 degrees off. I have never removed a distributor. Is it a difficult task? FYI, the engine timing with a strobe light is within specifications. The dwell-closure angle is at 36 degrees (exact middle of specified range) but the ignition points are at .019 thousands, not .027 which the book calls for. When I set the points to .027, the engine runs, but poorly. Is this a consequence of the distributor being off by 180 degrees? The car is a 1959 Mercedes-Benz Type W180 220S cabriolet.

Scott Gordon replied:

Ah, Grasshopper you ask so many questions. The situation is complex, partially because the distributor is only turning at half the speed of the engine. The spark only fires in each cylinder every other revolution. The distributor only turns once for every two turns of the crankshaft. This means that many configuration are possible. But none are correct except one. Rather than fixing the situation, let's give you whole knowledge and comfort that it's just a simple machine with lots of room for error. Only then will you be comfortable enough to achieve true performance, economy and enlightenment.

Let's take it from the top. First of all you are right to measure dwell. Dwell is the important number. Never mind ignition point gap, get the dwell right. Do this first.

Next, the distributor can be removed and replaced with impunity. All the spark plug wires can be removed at the same time. Go ahead do it, take all the spark plug wires off at once. Feels naughty, doesn't it?

Now disconnect the ground strap if you have one, the wire to the coil and the "collar" screw which retains the distributor, and pull the distributor straight up and out. Whatever will we do now?

First, as you mentioned, we have to find TDC (Top Dead Center). On the Mercedes-Benz Ponton series there are many ways to do this: flywheel, camshaft, crankshaft pulley. They should all agree. Maybe you want to do that first! But if you know where TDC is, go ahead and go there. It's Top Dead Center on the #1 cylinder. I usually jack my driver's side rear wheel slightly off the ground with the transmission in 4th, and rotate the wheel by hand with a helper finding the pointer and the mark. Once you have this mark, why not put some paint on it, so you can find it again later? Silver or white are best, clean the area first, and then just dab it on. you want to be able to find it in the dark.

Now take the distributor, and you have renewed the O-rings if necessary, right? You have spun it in your hand to check the bearings, right? You have seen if you have excessive bearing end play along the shaft, right?

Like I said, take the distributor and remove the cap and do these things if you need to. Don't disturb the points if you can help it, because they are still set at the magic dwell number. Oh and for the next step you'll need the rotor installed.

Now is the trial and error distributor insertion. Because of the splines and the helical cut of distributor gears, it's a little hard to predict where the rotor will end up as you insert the distributor. But do it a few times if you have to, the object is to get the rotor to end up pointing to the mark on the lip of the distributor. Note that you only have to get close, the distributor can then be turned to line it up exactly.

Take a minute to check out the distributor once you have it in place. There are many places where it can be and work perfectly; choose the best one. Does your current spot mean the vacuum advance line will lead correctly? If not, change it. There is no real right or wrong. As long as the rotor points to the line on the rim at TDC, it's right. You also want to make sure that you have enough space to turn it a little bit in either direction to set the timing later.

Go ahead and tighten the collar screw and get your helper again. Rotate the engine one more RPM in the forward direction (from this point you will not ever go backwards, go forward again if you go too far, and get close to TDC again.) Loosen the collar screw and retard the timing a few degrees (5-10 will be fine) by turning distributor in the direction the rotor moves. Remember that the distributor is all about time; moving in the same direction as the rotor delays (retards) time and moving in the opposite direction accelerates time.

Now that you have the distributor installed, go ahead and fit the cap, as you do so, see which of the wire towers is the one over the mark on the rim of the distributor. That tower gets the wire that goes to cylinder #1. Now follow the Zündfolge marking on your head and insert the rest of the wires.

If you had just inserted the distributor randomly, you could then line it up with one of the wire towers and used that as wire #1, and gone from there. That's what the previous person did.

Now just get in and start the car (you want to take it out of gear, and get it down off the jack first!). It may not run perfectly at first, but it should start, even though the timing is pretty retarded. Go ahead and measure how much it's retarded, using your timing light with dial advance (You do have a timing light with a dial, don't you?). Then, set the dial for the factory setting, and loosen the collar screw and dial the distributor into the right timing setting. You can do this while the engine is running.

Our Mercedes-Benz Pontons have a few specifications for timing at different RPM levels, and some are done with the vacuum advance hose connected. Pay attention to this! Use the manual, Grasshopper. Recheck the dwell, and make sure it's stable at several different RPMs. Unstable dwell is one sign of a worn out distributor, with bearings going bad.

If you have an early Mercedes-Benz Ponton with the gasoline compensator, either timing advance on the dashboard, or under the hood like mine, you can use this to set the timing instead of turning the distributor. Nice!

If you don't have a timing light, you can start out retarded, and then advance the timing little by little, each time taking a test drive. Lugging the car just a little is a great way to induce knocking. As soon as you sense knocking, reset the timing back to the previous setting where there was none. Finished - the hard way!

Anyway, timing of any old car is like Kung Fu. You can master bits of it, but to receive the highest belt, go ahead and tear the system down, and start from scratch. You will should understand all phases to achieve enlightenment -- and transportation.

Best regards,
Scott

Dwell Angle

The degrees of distributor cam rotation during which the points remain closed is referred to as "dwell", or "dwell angle." The most accurate way to check the setting of the points is to check the dwell. Setting the points with a wide gap results in a low dwell reading and low voltage to the spark plugs. Setting the points with a narrow gap results in a high dwell reading and higher "high tension" voltage, but can also result in "point bounce" within the distributor at higher RPM levels which can result in the engine misfiring. In practice, "point bounce" does not occur too often. It may be based more in theory. It has to do with a physical limitation of the breaker arm not being able to respond to the lobe on the distributor shaft. Perhaps, with the closer gap, the tension (or spring action) in the breaker arm is lower and has lost some of its ability to follow the cam lobe. As the point gap is decreased, the engine performance increases but the points will wear prematurely. For "daily driver" vehicles, it is recommended that the points are set to the specifications outlined in the workshop manual (reference Table 1).

The proper setting for points is determined by dwell, and measuring the dwell with a meter on a running engine is much more accurate than simply measuring the static breaker point gap. However, static gap is usually reported as a starting point, so you can get an inoperable engine started.

Table 1. Static Breaker Point Gap and Dwell Angle
Mercedes-Benz Ponton models static breaker point gap (mm) dwell angle
180, 180a, 180b, 190, 190b, 190SL 0.4 - 0.5 50 +/- 3
220a, 219, 220S, 220SE 0.3 - 0.4 36 +/- 2

While the static gap can be used as a starting point, this measurement depends on new (unpitted) points and a little luck. Once the points are gapped, then you should fine tune using the dwell adjustment.

Go to the Beach, or Get Under a Shade Tree

The procedure unfortunately involves a special tool, the dwell meter (available at Sears for around $20) and a little bit of patience. On Mercedes-Benz Pontons, it is a trial and error procedure, and you should set aside about one hour to see the project through. Do the work in a nice place, get out of the garage and under a shade tree or go to the beach. You will be revving the engine, and you do not want to breathe the fumes, so go outside.

Bosch VJU 4 BR 22 distributor with cap removed. This distributor is original to the Type 180a/180b 4 cylinder sedans with the M121.923 engine

Photo 1.

If the cam follower on the breaker arm is worn, you might have the correct dwell angle, but the point gap will be too small. So to eliminate this, get a new set of points, install them in the distributor, and replace the condenser too. It is usually recommended to replace these two parts at the same time. Plus now is the time for routine lubrication. Take a small dab of grease and touch the cam in the distributor. Note that the distributor cam grease should be used extremely sparingly because too much may result in fouling the face of the contact points. This will lubricate the follower for the points and protect the shaft itself. Put a drop of light oil down the center of the shaft of the distributor, and another drop in the hole on the side of the distributor, after you slide back the tiny door.

Photo 2.

Set the static points setting first. Use a feeler gauge if you have one, or if not, then double a matchbook cover. Put the car in top (4th) gear, and roll the car (backwards is easier) turning the engine slowly until one lobe is directly under the follower block on the breaker arm. Take pains to make sure the follower is at the center top of the cam. The maximum distance you have to roll the car between cams is about 8-12 inches. With the car in 4th gear, you may also achieve the same results by jacking up and rotating a rear wheel, or use a 27 mm (1 1/16") deep socket to turn the crankshaft pulley bolt.

Setting the Points

Once the cam is under the follower, it is time to set the points. You can find a way to do this yourself, but you need to be able to tighten the adjustment screw such that you leave the points apart the correct amount (an amount which changes during the process). Easy, huh? OK, let's go. Try the smallest feeler gauge you have which fits the range for your car (reference Table 1 or the owner's/workshop manual). Take it out of the holder, so you are holding just that gauge. Loosen the adjustment screw and put the gauge between the points. The points should create just enough drag on the gauge so that there is a tangible amount of effort to remove the gauge. Now, here comes the delicate part of the process. Try not to upset the new gap you have set, and at the same time, tighten the adjustment screw. Then, stand up straight (do not hit the back of your head on the grille) take a deep breath and gaze out from under your shade tree. After that, roll the car (still in 4th gear) to get to the next cam lobe on the shaft to verify the new setting. When the cam follower on the breaker arm reaches the apex of the next cam lobe, insert your feeler gauge (or matchbook cover) again. Does it seem like the setting is correct? If yes, then take the car out of gear and (attempt to) start the engine. If it starts, congratulate yourself on getting the gap set close enough to start on the first try. If the engine will not start, begin the adjustment process again. If the engine is now running, you will need to check it with the dwell meter to really see how it measures up. Follow the directions that came with your dwell meter and reference Table 1 or your workshop manual for the correct dwell values for your car.

Now, Check the Timing

Anytime the point gap is changed, check the timing because changing the gap changes the point in the shaft's rotation in which the spark plug fires. So get out your timing light, clip it to the #1 plug wire, smear a white crayon on that thin line on the harmonic balancer, disconnect and plug (a golf tee works well) the vacuum advance hose (Photo 3) between the carb and distributor, and fire it up. Set the timing by loosening the hex (Allen) head hold down bolt at the bottom of the distributor. Then gently rotate the distributor until the timing light shows the mark where it should be on the little scale on the harmonic balancer. Tighten it down, reconnect the vacuum advance.

References
Mercedes-Benz Service Manual S-1201-000 / Models 180 to 220SE / Job 01-3/6
Glenn's Repair and Tune-up Guide / ISBN 1 869826 329 / © 1987 / for Model 180c

BTDC = Before Top Dead Center
ATDC = After Top Dead Center

Table 2. Ignition Timing Settings at Starter Speed
Model Bosch distributor Strobe light setting at starter speed
180a / 190 VJU 4 BR 14
VJU 4 BR 22
VJUR 4 BR 27		 8° +/- 1° BTDC
180b VJUR 4 BR 28 4° +/- 1° BTDC
180c not covered in manual 3° +/- 1° BTDC
220a / 219 VJU 6 BR 24
VJUR 6 BR 24
VJUR 6 BR 38		 5° +/- 1° BTDC
219 VJUR 6 BR 38 1° +/- 1° ATDC
220S VJUR 6 BR 24
VJUR 6 BR 38		 8° +/- 1° BTDC
VJU 6 BR 38 2° +/- 1° BTDC
220SE VJUR 6 BR 32 2° +/- 1° BTDC
190SL VJUR 4 BR 11
VJ 4 BR 12		 1° +/- 1° BTDC
VJ 4 BR 11
VJR 4 BR 21		 9° +/- 1° BTDC

Vacuum Advance

Vacuum advance is a simple system which consists of the following components: A hose which goes from the carburetor (manifold) to a sealed diaphragm that is mounted on the side of the distributor. The diaphragm has a rod (Photo 1) that is connected to a plate in the distributor. When the manifold vacuum is high (at idle speeds), it pulls the plate and advances the timing.

Basically, when idling, the engine needs a little more time to burn the fuel. So, the spark must occur a little earlier. The vacuum developed by the engine is a fair indicator of the load on the engine. More vacuum is developed at low and mid engine load conditions. Less vacuum is developed under high load. The load is defined here as the work that the engine is doing, combined with the gas pedal position. It is an instantaneous thing, and going up a hill is more load, and throttle wide open is more load, and the two items together is max load. It is different than RPM. It is difficult to test this in the driveway, and you need a VERY sensitive vacuum gauge to do so. Look for one calibrated in inches of water rather than inches of mercury. WITH THE ENGINE STOPPED, you can test this mechanism by disconnecting the line at the manifold (or carburetor). Remove the distributor cap. Create a vacuum with a hand operated vacuum pump. The plate in the distributor will move if the system is working correctly.

Photo 3.

Typically, vacuum advance contributes MUCH less advance (measured in degrees) than the centrifugal mechanism of the distributor, which advances the spark due to engine RPM. The vacuum advance just "polishes the apple."

Notes

Photo 4.

A distributor adjustment cable on a 1956 Type W180 220S sedan. The cable was discontinued in 1957. It was superseded by a fine adjustment lever at the distributor itself (Reference Photo 2).

Photo 5.

According to the Type 220S picture parts catalogue, the dashboard switch for the distributor adjustment cable looks similar to the headlight switch.

Revision History

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