Problem: '29 coupe suddenly started running very rough, not drivable
Notable symptoms:
- Sounds like the cylinders are only occasionally firing, engine rotation very irregular
- Sparkplugs show a lot of black residue
- Exhaust seen coming from somewhere at the back of the engine compartment, maybe from the point where the manifold meets the muffler? Hard to tell.
- At idle, ammeter wobbles constantly
- If left at idle, engine will stall out within 30-60 seconds
What I already tried/examined:
- Lights and starter work fine, so no problem with electrical grounding
- Re-did the timing, checked clearance at rotor and points
- With ignition on, opened points. Spark was seen at the points and a spark jumped from the coil wire to a plug terminal when held nearby. Worked every time I opened the points. (Modern distributor plates installed last month, older coil)
- With engine running, grounded each plug terminal to the block with a screwdriver. Saw spark at each plug.
- Using Nu-Rex "C" spark tool, saw good spark between coil and distributor up to 10K volts
- Fuel line is clear
- Removed and inspected carburetor, seems fine, no deposits, all parts move smoothly (balanced carb is less than a year old)
- Tried various settings for GAV and spark advance, no notable change in behavior
- Tried with air filter off, no change
Anyone have any ideas? I don't have any spare parts to swap in unfortunately. It's bewildering because I just drove it a couple weeks ago and it was fine. I've worked on a few components since then, but nothing in the fuel-air-engine block system. The only system I've messed with is the distributor and coil, but I feel like I've tested them thoroughly.