I'm having trouble lately with my 4 speed stick transmission from a 73 Datsun A12 engine. The shift lever and the detent are still normal but the previous gear is still energized when shifting back to neutral. What does anyone think could be the problem? Also what years are the same for this transmission.

Are you saying it is sticking gear?

No. The lever when moved to the neutral position doesn't take it out of gear that it was in.

Is this the scenario?

* when the engine is off, the gear lever can move to all the gears, including reverse, and it feels normal

* when the engine is running, with clutch pedal pushed down, ditto

* engine running, shift to NEUTRAL, then let up the clutch pedal and car tries to move (i.e. lever in neutral position but gearbox is actually in one of the gears)

if so, it may be that one of the internal shift fork/pins has come loose or is cracked, so that when the lever moves it doesn't actually take it out of gear. This is a rare problem but not unheard of

especially if it only occurs between NEUTRAL and 1st/2nd (i.e. the 1st/2nd shift fork) while 3rd/4th has no problem

Yes, mine does the same as you described

Is the clutch adjusted correctly? The pedal requires about 1" play. Not enough play can cause trouble shifting

If adjustment is good, you could disassemble the transmission and inspect the shift forks. Or just buy a good used transmission for $50 to $150 dollars. See the reply to your question about buying transmissions