be sure the motor is warm before you start adjusting carb.
be sure points are set right, new condenser,
clean fuel, I installed an inline fuel filter
I always use non-ethanol fuel, but I can still buy it in my area.
adjusting the carb to idle proper at 400 rpm's is only half the battle, you must adjust carb to take the fuel when you engage the plow or tool.
motor must be at normal operating temp, cannot adjust a cold motor to the finer specs.
with motor at normal operating temp, good gas, new tune-up and motor will idle at 400 rpm.
normally, I put on the disc, then go to a nice open area like a cut soybean field,
open throttle to 3/4 then take off, if engine starts to cut out and not take fuel under load, then I start opening up the large adjusting screw 1/8 turn,
back on and take off, you may have to do this a number of times until you get it to the sweet spot.
when you get it close, sometimes you can just stand beside tractor and open up throttle until you get it right.
not a 5 min job then go back in house and drink coffee.
but, again i say all things must be right, before you get to this point, or you will never get the motor properly adjusted.
I use autolite plugs, 386 or 3886, check with you dealer on those, good plugs are a must and you must have good compression!!!!!!!!!!!
a worn out motor will never run smooth and pull good.
hopes this gets you started.
I also got a larger or hotter coil for my tractor, so it would give it a bit more fire.
do you have wear in your distributor shaft, move it back and forth, you can tell if you have play,
this play will change your point settings
use top quality points, tune up parts, not cheap junk.
you have to pay to play, one of these days they may stop making old tractor truck parts.