Pull the wire from the coil, at the distributor end. Have someone crank the tractor while you try to note the spark here. Is it a nice deep blue spark? Or is it some lame yellow spark? If you have a nice blue spark, check your timing. By the way, get rid of those resistor plugs - and resistor wires too, if you are using them.
If you have a yellow spark, it could be from several things. A poor ground (dirty/rusty inside distributor) could do it. A bad capacitor (new ones can be bad) can cause it too. Make sure your distributor cap is clean and dry. If the tractor ran fine before you tuned it up, the odds are that your coil was wired correctly. This means I wouldn't use a resistor - or leave one in if it had one.