I didn't change the spring. I made a button bearing (photo below) that just tightens the original a little but more importantly allows the spring to twist freely as it cycles.
[IMG]

[/IMG]
The only other thing wrong with the B-4 is the trigger. My fix is found in the link below.
http://www.getphpbb.com/phpbb/viewtopic.php?t=930&mforum=drfaf