Auto Parts: car hesitates., ac delco parts, vacuum advance

Question
Hi, I own a 1975 chev. nova which I've own for over 30 yrs. 4.1L 250 cid L6 1BBL (D). 3 speed manual. Lately as I shift from 2nd to 3rd I have notice some hesitation as I accelerate, I have replaced the dist. cap & dist. rotor w/ orig. ac delco parts which for a couple of weeks solve the problem but now the hesitation is back. I have checked spark plug cables they seem to be ok as well as spark plugs. Please advise. Thanks.

Answer
    The problem is almost certainly in the ignition.  The hesitation is a kind of give-away as to why.  When you step on the gas, the vacuum advance moves the ignition pick-up, thereby advancing the timing to give more power and crisper throttle response.  After thirty-four years of moving back and forth (usually a lot sooner than that, actually) the insulation on those wires cracks and splits.  
With the insulation gone, the copper stranded wire is all that holds it together, and it starts to perish.  So, when the advance tries to move the pickup, the signal from the pickup is interrupted or degraded.  Replacing the pickup is a tricky job, the distributor needs to be disassembled completely and the shaft removed.  Given the fact that the centrifugal advance is often worn, as well, I would suggest replacing the whole distributor (you may as well keep the new cap and rotor you bought, distributors usually don't come with new ones) with a rebuilt unit.  While you're at it, get new wires, as well, and take the coil off and examine it for signs of leaking or cracking on the tower.  After the entire ignition system is in first-class tune (timing set correctly), only then can you diagnose the possibility of something else being wrong.  I can't emphasize this enough: do not try to diagnose a non-computerized engine before you completely tune-up the ignition; you can't get accurate readings on the other components and it's probably the ignition that's at fault, anyway.