Not wanting to reignite all that Zimmerman nonsense, but a new look at a possible American entry track:
Lets say the tracks runs from 10 to minus 10, starting at 0.
10 means USA declaring war on the CPs, minus 10 it declared war on the Allies. Right.
Factors:
Each full year of the war in Europe; (each 4 turns by my chronology): +1
Declaration of USW by the CPs: +2
Activation of Mexico by Germany (on my map): +3
Each neutral attacked by the CPs: +1
Each neutral attacked by the Allies: -1
Note: Attacks on an aligned neutral count the same, as long as it is not active. i.e. if Germany attacks Belgium G1 it moves the US +1 towards war; but if it attacks Belgium G2 after France has activated it there is no effect.
The USA can never invade neutrals. Except Mexico. The Germans can declare war then, if they want.
Overthrow of the Tzar: +2
American election: At the end of the American Winter 1916/7 turn the election is held (roll a die?); if the Republicans win: +2