JD-40S timing issues?

What is the required points gap and timing info, and how do I set it? (Ideally without a strobe.)

I've been working on an old 40S that has been having problems starting etc. (though it worked once it was going). Thought for a while it was gas, but cleaned the whole thing out. Then changed spark plugs and cleaned points, and it started, but fired very irregularly. Moving the throttle, I could find one or two locations in which it would fire enough times (half, maybe) to get the tractor moving, but any steep incline or PTO work would shift the rhythm of firing and I'd have to scramble to shift the throttle to the right place or the engine would cut out.

This leads me to think the problem is with the timing. Since it seems to have got worse since cleaning the points, I suspect it may the points gap; or it might be the timing in general.

Further ideas? This board has been incredibly helpful so far, so deep thanks in advance.

Be well,
John

CPeter