The question came up about ignition problems. There seemed to be more than one Porsche owner that had to replace their ignition distributor because the advance was not working. I was asked is this normal and was there any way to fix the distributor.
It seems to be a normal practice to replace ignition distributors when the internal parts do not work right. The advance mechanism is usually suspect as it is subject to a great deal of heat and high frequency vibration. The requirements of a ignition distributor are very demanding and the least little wear and imperfection will cause odd behaviors.
The installation of, a new distributor will usually return your Porsche to crisp performance, but they are not cheap. A new distributor for a 911 SC costing $800- 1,100 and the 1980 928 over 1,300! The re-built ones cost about a third of that.
So what usually goes wrong? I have removed, disassembled, cleaned, inspected, lubricated, reassembled, and reinstalled and timed many ignition distributors. My observation is that the grease in the rotor and stator plate becomes old, dries up, and the stator plate seizes. This will cause the vacuum retard-or vacuum advance (for cold start-up) to be ineffective as the distributor will be stuck in one position.
I did a major service on a 930 Turbo with this problem. When the engine first started it wouldn't idle at all. Finally the engine would burp and burbble up to something near running speed and as I accelerated the throttle a couple of times all of a sudden the engine accelerated to a high idle speed and stayed there with the throttle closed. Driving it was even more exciting as on one run it ran like a dog and the next run it accelerated very strong and "pinged" on boost. The culprit was a sticking rotor/stator plate. The normal cleaning and relubrication restored the engine to it's normal silky smooth running. This particular problem with the sticking rotor/stator plate seems to be common in the 1978 and later 911/930. Maybe the lubrication wasn't up to the demand of the emission requirements and the turbo heat.
Once in a great while one of, the two, mechanical advance fly weight return springs breaks. The small and delicate bushings that fit over the fly weight spring retainers will break-thus reducing the springs tension and the advance will be too much for a given rpm. These problems can be repaired if you have the parts.
The main reason to replace a distributor is for worn distributor shaft bushings. When the bushing wears out the rotor will fly out of it's track and hit the distributor cap contractors. This can and eventually will break the distributor cap. The bushings can be replaced-if you can find them. If you are driving an old Porsche it would be a good idea to keep en eye open for a used distributor at the swap meets. You may find a perfectly good distributor but at least you will have some serviceable Darts.
The choices for replacing or repairing are one of time and costs. I prefer the warranty re-built as long as the part numbers are the correct match. If the vacuum advance-retard units work correctly and the problem is old grease causing a sticky vacuum advance/retard, I wold have your distributor repaired, this shouldn't cost much more than a hundred bucks.
930 Turbos: When making timing adjustments to the 930 Turbo you should be aware that the large vacuum unit on the ignition distributor do three things that all need to work together. All turbos have a vacuum retard feature. This retards timing at idle only with the throttle fully closed. As soon as the throttle is opened this retard is gone and the timing advances.
The second vacuum feature is on the opposite side of the vacuum unit. This vacuum is used to advance the timing for cold start and warm-up. This vacuum supply is controlled be a electric switch for ('77-82) and by a thermal switch on the breather cover('83-95). The third source is boost pressure. The 1978 California specification Turbo and all later ones require the high rpm timing check done at 4,000 rpm with the vacuum retard hose disconnected. This timing at 4,000 is taking into consideration that the vacuum unit is fully operational and at 0.5 bar boost it will retard the timing by 4-6 degrees. The workshop manuals do not give you a performance check of this, but I would recommend that this be checked. Especially if you encounter roughness and pinging when on boost. This will melt expensive things!
Lee Rice writes the monthly Technical & Safety column for the Orange Coast PCA (zone 8) Newsletter. He has generously allowed Pelican Parts to republish these articles here for the benefit of everyone who visits the site.