That very same thing was happening to my 8n side mount distributor tractor. All of the electrical checks were OK.
It turned out that that terminal stud located on the exterior of the distributor where the wire is hooked from the coil was shorting to ground. When the terminal nut gets torqued, the insulation gets sheared. The insulation is made from gasket like material. I made one from plastic from the top of an aerosol can.
The tractor now starts Oh so much better with a rich spark.