Actually it doesn't make sense that installing a spark plug would make the engine run worse UNLESS that plug wire was screwed into the distributor cap at the wrong location... BUT... if that was the case, that means that at least one other spark plug wire would have to be screwed in to the distributor cap also... which in turn would make it an absolute truth that the engine would be running on two (2) cylinders.
I see above that "Tim" has requested that you perform a compression test... In the trouble-shooting procedure, regardless of what the problem is, the first thing to do is to take a compression test, then a spark test, neither of which has been done yet according to the above.
Remove all of the spark plugs, then do a compression test. What is the compression psi of all the individual cylinders.
With the spark plugs still removed, do a spark test. On that model the "Air Gap" for the spark to jump should be set at 1/4" and should be a strong blue lightning like flame... a real SNAP!
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(Spark Tester - Home Made)
(J. Reeves)
Most auto part stores have "air gap" spark testers... OR..........
You can use a medium size philips screwdriver (#2 I believe) inserted into the spark plug boot spring connector, then hold the screwdriver shank approximately 7/16" away from the block to check the spark or build the following:
A spark tester can be made with a piece of 1x4 or 1x6, drive a few finishing nails through it, then bend the pointed ends at a right angle. You can then adjust the gap by simply twisting the nail(s). Solder a spark plug wire to one which you can connect to the spark plug boots, and a ground wire of some kind to the other to connect to the powerhead somewhere. Use small alligator clips on the other end of the wires to connect to ground and to the spark plug connector that exists inside of the rubber plug boot.
Using the above, one could easily build a spark tester whereas they could connect 2, 4, 6, or 8 cylinders all at one time. The ground nail being straight up, the others being bent, aimed at the ground nail. A typical 4 cylinder tester follows:
..........X1..........X2
.................X..(grd)
..........X3..........X4
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Let us know what you find.