On Monday, Feb 1, 2021, 4:32:24 AM (6 days ago), Peter Wieck wrote,
Have you _ever_ changed the fuel filter in 30 years?

My mother was the original owner and drove it until a few years ago.
The fuel filter could have been changed; I don't know. The user
manual calls for replacement every 96 Mm (megametres). Will change it
now and test again.

So we have established that it is not ignition, but fuel related?

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.

Feb 6, 2021, 9:08:20 PM (10 hours ago) Dave Garrett wrote,
Sorry for the delayed response. Should be under the dash near the
fusebox. Here's a thread with pictures that is specific to the CRX but
I'm fairly certain that most/all of the info contained within is
applicable to the Civic Si as well.

https://www.crxcommunity.com/viewtopic.php?f=3&t=56535

Good thanks. After installing a new filter and testing, will resolder
the relay. This car might still be running in 2050. Electric drive
conversion kit? Run on ethanol then? =8~)

Thx, ... P.