How many miles since new or rebuilt?
A 67 is pretty old one may assume that many internals are worn out.
67’s didn’t have HEI out of the factory, thus how is this wired ?
Bucking as the crankshaft takes load can indicate:
- Detonation after all the gas in 67 was a different blend let alone changes as to how we get to the octane rating.
- HEI is a 12 volt system check the volts going into the distributor. Assuming this is a GM style coil in cap. Keep in mind that for the old points system the Run voltage is different from the Start voltage the former being reduced 6 to 9 volts through either an external engine compartment mounted resistor or a resistive wire between the ignition switch or potentially off an “I“ terminal of the starter solenoid. This opposed in the 67 time frame during cranking only did the ignition receive full B+ voltage.
- Too much spark plug gap or a conductive coating having developed on the internal insulators .
- Secondary wiring to the plugs either leaking voltage, not the proper resistive 8mm wires.
- Too much or too little advance for the engine RPM. As you go up in gears the crankshaft load increases. The carb mixture the advances have to track this. You need to be sure the vacuum and mechanical advance systems are functional.
- If PCV is present check it for function.
- Cam drive falling out of time, hard to check on these engines it’s a gear drive probably with GM’s fiber reinforced plastic cam gear, they wear out. Normal wear is to fall retarded this gets the cam to crank out of synch and takes the ignition with it since the distributor is using the cam as jack shaft it times independently of the cam to crank relationship, which is to say you can correct the ignition timing to the crank but that doesn’t fix the cam timing to the crank.
- In this age of crap parts you cannot assume the new equals functional.
- Mixture falling lean. It could be carb with floats too low or insufficient pump delivery which can be caused by the tank pick up, lines or filters not permitting sufficient flow. It can be with jetting i’d r the enrichment system of the carb, check manifold vacuum with a gauge. my builds always include a vacuum gauge a misunderstood instrument that tells so much about engine health. Follow the link for all the data that gauge tells you, yes it’s a Ford forum but here we’re talking the basic physics of engines not who did the engineering solution. Keep in mind when transferring this info to performance motors that lower idle readings and some rhythmic wandering of the readings is normal as cams get big to bigger.
Keep in mind the old saying “most carb problems are electrical”. “Most” doesn’t mean all but more frequently than not the root cause is in the ignition system. This isn’t a directive but it is a guide to your thinking and processes of elimination.
Not spell or grammar checked, the spell checker on this form is junk and it has no grammar checker and I’m too time pressed to go back and read this.
Bogie
