Bought the car 2 years a go very ruff shape started to sort it out it had a sligth tvitsch at all rpm levels under load and off load !So i started to mesure things up .It had 4 bad cylinders leaking valves the carbs baseplates were varped do to overtorqe and leaking and so on .Hawe now rebuilt the engine new pistons full head restauration the works !Hawe also installd new carbs stock setup new fuellines all the way to tank new fuelpump .Replased all ignition hardvere exept the ditributor itself .Chekt all connektions new battery full voltage att all points messured everything !New ignition box today allso bypased the ignition key part and bulhead conektor by wire from battery to resistor and the darn thing still has the the twitsh att all rpm levels !What to check next ? Running out off options here ?
A real head scratcher. Here is what I would do and almost all have already been suggested:
1) check output of alternator.
2) clean all electrical contacts and grounds for the alternator, voltage regulator, coil, and the small ground wire on the negative cable at the battery. This small wire grounds to the radiator support.
3) swap out the coil with a known good one.
4) Replace the rotor. They're cheap. Keep the distributor cap for now.
4) recheck compression on each cylinder.
5) check both ends of each ignition wires and coil wire...check the electrodes for fit (snap/click) and visually check the wire core connection to the electrodes. Also, you could put an ohmmeter across each wire electrode and gently move/flex the wire ...watch for an open. It would probably be easier to swap out the ignition wires with a fresh set.
6) Curious, what brand of plugs are you using? Not to slam NGK, but I have had issues with NGK in the past. Fortunately, my car's diagnostic computer told me which plug/coil to check. I still use NGK and have had no recent issues.
7) Curious, what is the cam degreed at?
8) Curious, do you have a non-stock valvetrain?
Good luck and let us know what you find.