On Sunday, February 7, 2021 at 11:03:41 AM UTC-5, Peter Easthope wrote:


Pulled the cable from plug 1 and connected an old plug. With just a
little cranking the engine started on 3 cylinders. Ran fairly well
for several minutes. The spark was absolutely visible. Yellow rather
than blue. As the engine heated it began to falter. Quit after about
10 minutes. Behavour entirely similar to what I reported originally.
Three cylinders quit a little sooner than four.



You didn't follow the instructions properly. The steps were to look at the spark when it was cold and running well, then looking again after it crapped out. By running the car with the plug in the cylinder disconnected, you were pumping unburned fuel into the cat - which is why I worded it as I did. Pumping unburned gas into the cat is a fine way to melt it down in minutes. Hopefully this didn't happen.

"As the engine heated it began to falter."

What began to falter?? The engine, the spark, both?