The last few roller timer problems I have had have been due to dirty oil shorting out the timer terminal in the timer case hole. When this gets bad enough the car pops under load but runs somewhat better at higher speed or in low. It also gives one coil a constant faint buzz when cranking. Today I cleaned one timer that had this problem. Taking the nut and insulating washer off I could see there is no insulator in the hole. Putting one lead from the battery to the timer case and one to the terminal threads, you could see the dirty oil smoking. I cleaned the timer with solvent and hooked the battery up as before. There were faint sparks still making there way through the burned area in the timer insulator ring. A little scraping solved this problem. I counter bored the insulator ring slightly and made a nylon washer (pink in photo) to insulate inside the hole. The timer works well now.
I have only noticed this problem on the old repro Tiger timers and only since using Zinc in my oil. Could it be the Zinc? Also it happens only on the bottom two terminals as they get more oil in them than the top two. Using grease rather than oil may alleviate this problem.
I also noticed that some of the holes in the case are not centered well with the holes in the insulator ring.
I still maintain that this type of failure has left many otherwise good timers out of service.
Is Zinc magnetic or can it carry current? If it can then maybe too much can short the timer or magneto out like graphite.
Like other metals , zinc conducts electricity and can be formed into wires or sheets.
Been there too........my problem turned out to be some crud that had collected in the bottom of the timer body so I cleaned it out really well, tightened all the nuts and then bored it as tightening those nuts causes a bit of distortion.
Haven't had any trouble since.......
The compound found in some oil is ZDDP, a liquid. It is referred to in slang terms as "zinc". It isn't conductive and is not related to crappy timer performance.
Ford recommended Vaseline be used to grease the timer. Everyone in my family has always used ordinary wheel bearing grease instead. It works very well.
I think the problem shown here is from cheap plastic insulators in reproduction timers. If you take apart a Ford roller timer or any of the ones made prior to say 1970 they have phenolic insulators that don't break down over time.
More information on roller timer maintenance: