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Unexpected source of a misfire - incorrect rotor spring contact setting


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Hi All,


I thought it might be useful to share an experience I've had with my Mk3 GT6 and an annoying "soft" misfire which only appeared when accelerating hard in 3rd or 4th gear  I changed all the usual things: Plugs, HT leads, rotor, distributor cap and checked the inlet side of things for air leaks but to no avail (the car has Lumenition Optronic ignition). When I went back round the various components I noticed that the distributor cap centre electrode was looking very worn out, which given that it had done less than 200 miles looked like a clue to the problem. When I checked the rotor spring contact setting I found that it was~2mm too low and my guess is that the rotor arm spring wasn't making firm enough contact with the centre electrode of the distributor cap, giving rise to arcing and robbing the plugs of some volts. With the correct rotor spring setting the distributor cap now needs a positive push down before the clips can be fastened up and the misfire has gone.


In nearly 40 years of motoring I've never checked a new rotor arm spring, assuming that since it was manufactured for a specific distributor that it must come set to the correct dimensions. Is this another example of QC issues with spare parts or have I been missing a trick all these years?


The attached file gives the correct dimensions for an AC Delco 204 distributor.





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