273 cylinder out

The late 273's had the same heads as 318's; the last 3 digits on those head casting numbers are 675. What year is your car/engine? (Of course, heads/engine could have been swapped out regardless of year.)

So how low is the 'low' compression on that cylinder? The rings can be very poorand if the rings are shot, oil will get up on the top ring and make it seal better at the low piston speed when cranking. And, the ring grooves can be shot and the ring will flutter more at RPM and not seal much at all.

As in post #3, if there is a valve clatter in that cylinder, then a valve adjustment may do wonders.

Also, get a vacuum gauge and hook it up to a port under the carb to read manifold vacuum at idle. See what the idle vacuum rads, and if there is any steady fluttering/pulsing of the vacuum gauge needle.

And, get a voltmeter, put it on ohms range, and measure end-to-end resistance of each spark wire. Standard resistance wires will typically be in the 3,000 to 7,000 ohm range and should not be any higher than 10,000 ohms.

BTW, a spark jumping the small gap of a spark plug in open air is totally meaningless. It takes only 2-3,000 volts to jump that gap in air, but will take up near 20,000 volts to jump the same small gap in a compressed fule-air mixture. A good spark can easily jump a 1/4" gap in open air.