Henry Magno / email@example.com / September 5, 2001
This article originally appeared in the International Ponton Owners Group (IPOG) archives on September 5, 2001. Henry Magno runs a Mercedes-Benz restoration shop and mechanical service garage in Massachusetts. See Links for details.
Question from an IPOG Member
The dwell angle seems to vary over 5° when I rev. the engine in my Mercedes-Benz Ponton. Is this due to a worn distributor or faulty mechanical advance?
Henry Magno's Reply
Distributor rebuilding is not mysterious. A change in dwell would be one symptom indicating a problem, but I do not see how the cause would be the mechanical advance, but most likely it is play in the shaft or wear in the breaker plate. So, without tearing one down, the symptoms could be: misfiring, variable dwell, not meeting specs on advance.
Centrifugal advance can be tested with the strobe and tachometer with the vacuum advance disconnected, but testing the vacuum advance I think a distributor bench tester would be needed because you need to measure advance as a function of vacuum, and in the car, you are reading the sum of the vacuum and centrifugal advance on the timing scale. So you would need to have a vacuum meter attached and do some calculations.
For practical purposes, you can disconnect the vacuum line to the diaphragm and apply vacuum from a hand pump, or suck on it to see if the breaker plate moves. If the adjustments on the pull rod have not been tampered with and the diaphragm is OK, this is good enough.
Even if you have no symptoms, it is a good idea to take your distributor apart and clean it up and lube where necessary. It probably has not been done in ages, if ever. First, take it out of the car. Then after taking the points and the bolt for the wiring connections out, remove the vacuum rod then the breaker plate. Then remove the cam section by carefully detaching the advance springs from the pins on the base plate attached to the shaft. Keep track of all the various fiber and shim washers.
Things to Check
Play of shaft in housing. Usually OK in these cast steel housings. If the play is OK, you do not have to remove the drive coupling and shaft. If it is not OK, do not bother to remove the shaft unless you are going to re-bush the housing. I would look for a good used distributor at this point, instead.
Fiber "plate" under the weights. They can be broken and cause the weights to hang up. Clean up the weights and lightly oil or grease where they rub on the pivot pins. Observe the position of the weights. The two pins on the cam that engage the weights are different lengths. It only goes together one way. It is hard to describe without one in front of me at the moment.
Advance springs to see whether they are stretched or deformed.
Clean up center pin that the cam slides over and the bore of the cam itself. If it is not clean and lubed, the cam can hang up or rust to the pin and you have no centrifugal advance. That is what the little felt plugs are for, that you are supposed to oil when you remove the rotor. This recently happened to me on a car I restored. It was only a few years since the distributor was apart. The customer complained that the car had no power and he was right. Of course I was fearing the worst, but when I pulled the distributor cap and tried to turn the rotor I felt resistance and then it snapped free. I had not put any felt plugs in, and since the car is not driven much, that, and neglecting to use a few drops of oil was enough to hang it up.
The breaker plate assembly. Look at where the top plate fits into the bottom plate at the center hole. If the boss is worn, the top plate can shift sideways and cause a variation in dwell. When reassembling, make sure the screws that hold the condenser are short (4 mm x 5 mm or 4 mm x 6 mm) and do not protrude into the case. They are right on the level where the pins for the advance springs are. If they are too long they will either rub the eye of the spring off or shear the pin off completely. This happens! Of course, every time a condenser is replaced the correct screws are necessary. I am sure many times slightly longer screws are put in after the correct one was lost, without a second thought about what they might hit inside.
Also, make sure the fiber washers on the through bolt for the wire connections are in good shape. It is easy for the bolt to short against the case, causing a misfire or no spark at all.
Check that all is in order with the aluminum mount for the distributor. Cars with the octane compensator can be a problem if parts are missing on the mount and the distributor is not locked down. If in doubt, modify it to lock in one position and eliminate the dash control cable.
And of course, make sure there is a good cap, rotor, points and condenser.
- Henry Magno
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