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I have a 350 engine with the large cap HEI distributor in my 66 El C. The distributor has about 140K miles on it, and until recently has worked problem free. It has an Accel coil on it, and does not have a vacuum advance setup on it. The engine recently developed an intermittant missfire at most highway speeds, especially under load, but also while rolling downhill under no load. It cranks over easily, and fires right up every time. It does not backfire out the carb, and it does not backfire out the exhaust, but it pings and spark knocks - all sounds like it is up under the hood. The missfire got so bad, I couldn't maintain 45 mph on the road. I replaced the plugs, plug wires, cap, and rotor. This got it to where it would at least get up to the speed limit, but still with the missfire. I can't floor it and accelerate without it sounding pretty ugly, and power is way off. In neutral, it doesn't miss too bad until I rev it up to 2000 RPM or more, then the tach shows an intermittant jump of 200 or 300 RPM to go with the miss. I next replaced the condensor unit (with the 3 leads), and that made it run a little better still, but it is still way off from where it used to be performancewise. I notice the pick-up coil(?) is completely coated with the rust oxide you typically see in an old HEI, don't know if that is related to problem or not. The counterweights are all free to move, and they are coated with the red dust too. The module and the pick-up coil are Delco parts, probably original. There is one other item, but it was pre-existing to the missfire problem - I have one intake lobe that is worn, but the rocker arm hasn't loosened up on it since I adjusted the clearance over a year ago. Any ideas?
Thanks,
Jim
 

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Jim. You have checked almost everything. Did you also check the wire underneath the pickup? This tends to break sometimes due to it moving all the time. Just a thought ...

I.m.o the rust oxide will not influence the magnetic field much.

Rob
 

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The coil could be intermediately breaking down. I had this happen on my '69 a few years back and it acted the same way. There is also usually a ground strap under the pickup module plate that can break.
 
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