I have a 2002 1200 custom sportster, have had issues with 1st gear popping in and out of gear when accelerating hard, the other gears do not do this. Took it to the local HD dealer $800 later told me it was fixed, well its doing it again. Anyone have similar problems with the transmission? The bike has about 19k miles on it.

 

Either your:
not shifing into first completely
the heat treating was wrong and the ears are rounded
the shift fork is bent
the gear spacing is incorrect.

 

 

The typical reason for the 1st gear "jump" is worn edges on the dogs of the gears that transmit power in 1st gear.
It's been a while since I did a set, but if I recall the main culprit is 3rd gear on the counter shaft as that's the sliding gear that moves when engaging 1st.



What happens is the engaging dogs slip over one another and re-engage with the next dog and that gives the hiccup kinda jump you are getting when pulling away in 1st.

How many miles have you done since the repair work?

 

Mine does that in 3rd gear occasionally, also when getting on it.

John