I feel for you. Have you checked plug lead impedance? If a lead or leads are marginal they can break down with temp change. A trick I learnt, Try a carb starter spray when looking for air leaks instead of water. It's very effective, but just remember it's flammable.

With that much compression and that little vacuum,you've got valves open when they shouldn't be,or you have a substantial vacuum leak.I'm betting you have a timing chain that's jumped a couple of teeth.I don't know why it gets so much worse when warm.

 

Paul,

Boonville,MO

Slick, i have come across that cracked floor in intakes over the years and generally gives backfire result , but changing intakes could be worth a try, as its the one thing that hasnt been changed?

  AussieBill            YYYY    Forever Y Block     YYYY

 Down Under, Australia

PWH42, just to be clear, that low vacuum reading was the number when the engine was being spun by the starter with the ignition disabled. When it is actually running the number is 19-21" depending on mixture, timing, etc.

ian57, all the wires are new, and it's true I haven't gone back through them and measured resistance since installation. I have avoided using carb spray because I didn't want to ruin the paint on the intake but I suppose at this point having to repaint it is worth getting that confirmation. I would think that if I were dealing with a vacuum leak that intentionally creating one as I have done would make all the symptoms worse, but it didn't seem to do that. I tried creating a vacuum leak on my other old car to see how it behaved and it did not cause the same trouble. Perhaps it would be worth running the valvetrain-related tests on the other car for a comparison.

Slick, the butterfly has been removed. I have checked for a leak in the manifold as best I knew how, both visually and plugging one pipe while blowing air into the other and listening at the intake, but there was no evidence of leakage. Who knows, though, maybe it cooled down enough to close in the time it took me to get that test together.

I have a couple questions you guys might be able to answer that would help out, all based on assumptions I'm making that I'm not totally certain are valid.

1. I assume that if I see a steady flash from my timing light at the coil to distributor wire, that means that a spark is jumping the gap at each spark plug consistently OR ELSE leaking out of a plug wire to ground consistently. In other words, I can take such a steady signal at that wire to indicate that my ignition system is reliably generating and distributing a spark and therefore any misfiring that occurs while I see a steady signal must be caused by a mechanical or fuel-related issue. Is that a valid assumption?

2. I assume that consistently getting the compression numbers I'm getting means that, at least while conducting the test, the valves ARE fully seating. Is that a valid assumption?

3. I assume that in the course of trying 2 different '57 AFBs with bone-stock correct calibration and 1 modern, quasi-universal, never-been-used-new carburetor, and adjusting float levels and mixture settings and checking passages and replacing gaskets in each, that if I was dealing with a carburetor problem here I should've been able to resolve it by now. Do you think it's fair to rule out carburetion?

If we agree that all signs point to properly functioning fuel and ignition systems, then mechanical is the only alternative. I've tried what I know to actually FIND a problem in the valvetrain without getting a conclusive result one way or the other. I could remove the intake and test it for cracks. I could also remove the heads and go through the valvetrain, have them rebuilt, whatever. My goal has been to find concrete evidence that there is a problem in either area before digging in, but as has been the case with this whole problem, I haven't seen enough to make me sure.

What would you do at this point if a full teardown and rebuild was not an option?

1954 Crestline Victoria 312 4-bbl, 3-speed overdrive

I wouldn't rule out ignition yet. Coils can give intermittent faults that will kill an engine, earthing internally when warmed up then appearing perfect a short time later as they cool slightly. Not sure what you would see with timing light direct from coil being 8x that of a plug lead. The light might not dull fast enough and for that matter the brain might not be able to register it.

peeeot

It could be that the 9 volts that your coil sees with the engine running is TOO MUCH and the coil is overheating. Try another ballast resistor and get the running coil voltage below 6.9 volts ( Fords 1966 car limit ) and see what that does. Pete

From my understanding coils aren't specific to a particular impedance resistor. I recently purchased one and tried to match it to the resistor for my car but they all just specified, (use with resistor). If the coil is the problem then it would be faulty and it would be unlikely that changing the resistor would fix it. If the coil is designed for a lower voltage and is now breaking down then it still has something wrong with it now.

ian, Go back and read the entire post from the beginning. He's tried 3 or 4 coils and almost the entire ignition system.

Frank/Rebop

Bristol, In ( by Elkhart) 

Sorry, might of missed it. I did read all that is there quickly and the piece written on latest update by peeeot. Could see the testing of voltages but not swapping out of coils. When I go back to page 1 the subject seems to have already started. I don't see the subject from the beginning for some reason, maybe it was in there.

I didn't read anything on road test. Has it been driven? If so is it doing anything unusual?