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just for some background info the car is a 76 corvette witha weiand 177 msd 6al and a holley 750 dp
i took it out drove it about 20 min and it was fine parked it and visited a friend for a while then i got back in the car and drove a little it started over heating and then shortly after it started breaking up drove it home and parked in the garage the next day i decided to read the plugs and saw two wet gas smelling plug numbers 7 and 6 at this time the car had autolite 3924 gapped to 45 thous
i wanted to use a colder plug gapped down a little to see if i would benefit
i fixed the overheating problem (electric fans not turning on)
so i changed out the plugs for 3923 gapped to 35 thous and took the car out it was missing like crazy when i go to wot the car misses a lot so i took the car home checked the timing it was about 20 deg at 950 so i took it down to about 14 at 950 and the car does exactly the same thing now here what makes me thing it is the timing with now load i.e. when the car is parked idling and i take it to wot it is fine no miss no backfire its perfect when i drive the car down the block and do exactly the same thing it misses and buck and backfires
i have spent so much time and made so many change on this car i think i am unable to keep up with everything please help me make it right
 

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Sounds like you have a pickup coil wire broken in the HEI. Have seen many times where when the vacuum advance is working and it reaches a certain point the ignition cuts out, vacuum advance falls back and ignition regains pulling the vacuum can back in cutting it out. Sounds kinda like hitting a rev limiter except at 2500 RPM. Pull the cap off and give the skinney little wires a tug. the wire will break inside the insulation so the one that streaches is the broken one and the car won't run anymore!

OR pull off the vacuum line to the vacuum advance and see if it still does it.
 
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