4 speed jumps into 1st gear at idle
 

My 70's era four speed was recently rebuilt as was the engine. Finally got the bike built and started. Good thing it was on a jack because it jumped out of neutral and into first gear. Killed the motor and put it back into neutral. Started again, same problem. What would cause the trans to jump out of neutral while at idle?

Does the bike shift into all four gears? It is necessary to time the shifter shaft to the shifter gear and the shifter gear tio the cam gear an the shifter cam. If not done correctly it will be impossible to shift into all gears.

If the bike shifts into all 4 gears, I would look at adjusting your shift linkage. The rod end that connects to your foot shift can be lengthened and shortened as needed. Seems that you might be sitting in a "false" neutral. There are notches on the shifter cam for each position (including neutral) and I am thinking that you are not in the "neutral" position on the cam but resting on the edge of it.

who ever did the unit forgot some of the parts or did not set the adjustments correctly

on the right side of the gear box to the right of the center post is the shift drum spring detent, it has a big bolt looking thing with a lock tab well it should - if you push the lock stays away and remove the bolt, it will have a pin and spring in side of it to see if they put the spring back in it - i can take pictures of what it looks like tomorrow it does come right out

Foot shift or hand shift?