Well guys I have a weird one for you, working on a 57 T-Bird with a 312, rebuilt a few years ago with low miles. Cylinders 1-2 & 4 are not firing sort of. Pulling the wires makes no difference in the running, it's still rough, replaced those wires, swapped plugs, checked valve clearance, checked compression, 145 lbs. swapped carbs, it has a Pertronixs Ignitor II & matching coil, 12 volts at the coil air gap set, new plugs. When I said it's not firing sort of, it has spark at the plugs but to me it seems weak but still sparks and the timing is right on. There is a huge exhaust manifold temperature difference, the side that is the problem is around 300 degrees and the left side is around 500. I checked for a restriction but there is none. Out the tail pipes you have a definate fump fump fump witch to me typically indicates valves but again I checked them and I have a good steady vacuum reading. I'm at a loss, any ideas?
You’ll think this is crazy, but check the distributor cap sockets and plug wire connectors for corrosion. I assume you already did that at the inside of the distributor cap and rotor. John
Is it possible that you have bad intake lobes on the cam on those cylinders? Take a look at the valve action with the valve covers off. Better yet, measure the valve lift on the affected cylinders versus a good cylinder.
You replaced the plugs and wires for the cylinders in question. And the spark is still weak on only those cylinders. Bad distributor cap?
Are the exhaust valves opening ? You can have compression but no exhaust valve action will kill those cylinders. Was the intake manifold installed properly.
A worn out distributor can wobble, sometimes causing some cylinders to get weak spark due to fluctuating rotor gap. I have dealt with this in the past.
I'd have to agree with T McG in that the Pertronix usually either works or doesn't work with little in between. That is the same with most electronic ignitions though. They either work or you start figuring why they don't. Pertronix distributor that is reasonably new or a conversion kit on an old distributor of unknown quantity? It might be that if the distributor is an old original one that the bushings or the plate the module /points sets on is worn out. By sticking the distributor in a distributor machine or hooking the rig to a scope you should be able see if the distributor it's self is sloppy inside and not maintaining the correct air gap on all eight cylinders.
From what I'm reading, shouldn't be anything wrong with it. Maybe your ears and thermometer are broke
Had a similar problem with my 292, installed a new cap and thought the fit was sloppy. Ran ok to begin with, the I started to notice a random miss, changed back to the old cap and miss went away
I have since gone back to the” new” cap but I cut a narrow strip of electrical tape and applied it to the inside of the outer mounting lip to take up the slop, as in lateral movement, seems to be working ok so far.
If the engine has a miss the engine manifold vacuum will most definitely not be steady. This would be very evident. Ordinarily I'd mention to double check that there aren't a couple spark plug wires out of firing order. Since you've a steady vacuum (how much?) this is unlikely. A 57 T-Bird 312 should have the optimum Y-Block camshaft installed. Do you happen to know which one it is? Much wanking around can be completely avoided by utilizing an ignition oscilloscope.
Like Truck64 said, a scope check of the secondary will quickly tell you if the problem is ignition related. The problem is related to one cylinder head. I thought about a problem with the intake, but the fuel distribution doesn't work out to 1, 2 , and 4. Cylinders 2 and 4 are fed by the same side of the carburetor, but 1 is fed by the other side. You would have to have a blockage to two separate passages. I have seen valves with heavy carbon deposits that block air flow on some cylinders much like bad intake lobes. If a scope check doesn't show an ignition problem, I might think about pulling the intake and maybe the head.
I'm feeling like I need to be certain of manifold restriction. How was this checked? I would be busting the rusty plumbing away from the head to see no evidence of Rodent invasion. Nasty work I know.
Same symptoms on a recent repair. Spark plug wires were reversed on the 2 problem cylinders. Double check firing order and maybe switch the 2 cylinder plug wires and see if problem is solved. Really quick fix if it works.
a friend of mine had a 65 riviera that acted the same way. It had a pertronix, we put a points dist back in it and it ran great after that