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The new Bosch coils right from a dealer for my 85 911 are junk. I don't know if that hold true for the older coils.
Again on my car the tach bouncing around a lot is often a alternator issue. Find a good 12 volt voltage gauge and see it it shows a solid charge or 13 to 13.5 volts but not past 14. On my car the tach bouncing was indication that the voltage was way too high. I finally broke down and but a voltage gauge in my 911. Mine is a custom tachometer with a volt gauge in the bottom of it.
I am sorry I can't help more with your 74 model.
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Glen
49 Year member of the Porsche Club of America
1985 911 Carrera; 2017 Macan
1986 El Camino with Fuel Injected 350 Crate Engine
My Motto: I will never be too old to have a happy childhood!
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