MF65 stalling

I have a MF65 giving me fits. Starts ok but runs rough, sputters and coughs. Give it any throttle and it backfires. Now and then runs fine. Not very often though. Recoated gas tank. Replaced bowl and fuel line. Added in line filter. Rebuilt Marvel carb best as I could. Tore down distributor and cleaned weights and springs. New plugs, wires, cap. points. Checked timing. Air mixture needle looks minutely bent. No ammount of adjusting seems to help. Can anyone help?

Just because you have all those new parts, don't assume that the ignition system is functioning properly. Test it to make sure.

Have you tried checking the spark by pulling the center wire out of the distibutor cap, holding it near a good ground and trying to start? You should have a FAT, BLUISH-WHITE SPARK, THE COLOR OF LIGHTNING ABOUT 1/4" LONG. If it's blue, reddish blue, or yellowish you have a weak spark. Check all the connection, make sure the points are clean and properly gapped, that you have battery voltage to the battery side of the coil with the points closed. If it starts running ratty, put a jumper across the ignition switch and see if that cures it. Ignition switches get high resisitance in them from dirty contacts and that can cause a weak spark. Make sure that you have the correct firing order. Check the timing with a timing light and verify that the advance is working. Are your wires copper core? They should be. Carbon core wires don't work well on tractors. Do you have the correct coil? If you have a 12 V system, you should be using a real 12V coil like a NAPA IC-14SB coil(~$15). It needs no separate resistor. If you have 6V coil on a 12 V sytem, you'll need a resistor to drop the voltage at the coil terminal to 6V to limit the primary current to 4 amps max or the coil will overheat and eventually burn out.

Make these checks sytematically, checking for effect any time you change anything.