SP-1 interrupter not holding

[halfmoonclip]

A buddy just acquired an absolutely mint SP-1 at a local show; it looks virtually unfired.
The bolt carrier is unshrouded around the firing pin, and it has a notched hammer.
If you dry fire the piece and hold the trigger back, the hammer fouls the bolt carrier if you attempt to cycle the action. If you dry snap and release the trigger, the action cycles normally....
...except occasionally the hammer wants to follow the BCG home.

I've replacement, standard, FCG parts in my stash, and in the short term I told him we'd replace the OEM stuff and see if it solves the problem. My hammer is notchless, tho' I think there is a notched one in there as well.
What do you think? Out of spec disconnector? Other ideas?
Thanks,
Moon

[Robb Jensen]

Personally I'd get rid of the unshrouded carrier (I despise those) and would add a new un-notched AR15 hammer. If he wants to keep the gun original for resale just maintain possession of the original carrier but use a Colt or other quality M16 carrier.

[halfmoonclip]

Concur on the unshrouded bolt carrier; my first AR after the ban had one, and I was no fan of it.
As noted, the short term fix is to replace the FCG parts and see what happens. The carrier may be in the long term mix.
Moon

[halfmoonclip]

Okay, my friend brought the rifle over and I detail stripped the lower. Someone had managed to force the disconnector spring into the groove above the trigger rather than in the round hole further back the trigger tail.
Problem is now solved, function checked and a quick range run for verification. Thanks for any comebacks.
Moon

[halfmoonclip]

...and let me add, I'd gladly give a case of beer to know who in hell was in there fiddling around with the spring. It had to have been really forced into that groove above the trigger....
Proving once again that nothing is stupid-proof, but I'm guessing that the function issues are what kept this gun unfired for all those years.
Moon