Don't know what's wrong.
No visibly damaged mechanical bits in the engine bay.
No water in the oil or oil in the coolant.
Fuel pressure shows normal.
Oil pressure shows normal.
No visible unusual leaks.
No visible damaged wiring.
Basically, nothing visibly wrong.
How to diagnose?
I started with the easiest items.
The Ignition system.
I pulled each plug cable while the engine was running.
Each cylinder is firing and is getting spark (engine runs on 3 cylinders whenever one of the plug wires is pulled).
Checked the distributor.
An electronic ignition conversion was previously installed (Ignitor by Pertronix).
My understanding of how it works is that the module picks up the location of the cam via the plastic piece on the distributor shaft having magnets that are detected by the module and collapse the coil to ignite the spark.
The original coil, cap and rotor is still used to channel the spark to the plugs)
Took the cap & rotor off to take a look.
Also pulled the magnetic ring that goes over the distributor cam to take a look.
Nothing initially looked awry.
The rotor has some corrosion on the inner contact but not terrible.
I did notice that this grounding wire between the base plate that the ignitor is mounted to and the distributor housing was broken.
The left arrow points to the broken wire/insulation.
The right arrow points to where it was located by a metal tab that's bent over.
Doesn't seem like the most secure way to connect a wire to the base plate...
Another view of the broken wire (from corrosion it looks like).
Removed the broken ground wire.
Bent this tab up to remove the insulation that is under the bent over tab.
Stripped some of the insulation at the end and twisted the wire together.
The wire strands are very thin and seem pretty corroded.
I placed the wire under the tab and bent it back down.
Now that I'm looking at the picture closely, I think the wire was originally soldered to the little plate with the tab on it.
The tab was probably just used to hold the wire/insulation in place, and not to make the actual connection to the little plate.
This is after I placed the stripped wire under the tab.
I don't know if the contact/ground is very good.
May not be such a good contact...
The rotor.
Lucas brand.
There's a lot of erosion and charring on the contact edge.
Used a fine file to clean up the rotor.
Put everything back together with the rotor cleaned, the cap contacts also lightly cleaned (the cap contacts were pretty worn/pitted also) & the ground wire kinda fixed (poorly).
There was no change to how poorly the car is running.
I ordered a new "heavy duty" rotor, cap & new ground wire from Northwest Import Parts (my MGB parts supplier, based in Washington). It should come in a few days.
When it arrives, I'll replace all three parts and see if it makes any difference. I'm not too hopeful...
