I have a 1987 Honda Accord carburated.

I have a 1987 Honda Accord carburated, automatic. About 3 months ago it would misfire occasionally while driving, then it started to do it even when I was stopped, then about 2 months ago it just died and wouldnt start. It would backfire and spit puel out of the carburator. I had a mechanic check the timing and the timing is good. I changed the distributor with a used one from the junk yard, after that we started the engine and it ran good, so we put all the parts back on and then it would not start again.
I now know it is the 4th cylindar causing the issue. The car will sometimes start with all 4 spark plug wires on it, but will die quickly. The car will start and run on 3 cylindars. If I connect the 4th spark plug wire the engine will stop running. We replaced the head gasket and valve seals and cleaned all the valves.
My mechanic tested for spark with a spark tester and when the 4th cylindar spark plug wire is not connected to the spark plug the tester light will pulsate normally, however when the wire is connected to the spark plug the tester light stays on constantly.
I have changed the spark plugs, spark plug wires, distributor cap and rotor.
What could be the problem?
Thank you