With the remark: "Left key on battery up lights horn work ,no juice to coil cranks ok"; like often said above, could be almost anything; however, some people's questions are directed at just seeking a very common "single" cause & response for "Wont Start"
With the riddle question above here is my serious single interpretation that I witnessed most often ...... experienced often because of our humid area.
1. Tried starting, but "Wont Start". (But, starter cranks engine rapidly)
2. So, leave keyed switch ON; and next try,
3. Lights work, horn works; and,
4. Battery up ..... i.e., to par. (Ref: 1. because of rapid cranking)
5. No juice to coil .... see no plugs firing, see no spark from distributor end of coil wire ....... Hmmmmmmmm
6. Cranks OK ..... (Ref: 1. above)
7. One simple single fix with the very unique Model A "open" to moisture distributor body:
With a point file, clean the metal oxide off of points ..... spark now jumps at end of coil wire; hence, juice at coil.
8. Get back in car ....... crank ..... it starts.
Points hardly ever corroded with our old tractors with "closed" magneto & distributor bodies .... Model A & B points are constantly exposed to temperature and humidity changes with an wide opening in the side of the distributor body.
Hope this single, simple serious often occurring "Guess" helps.