Originally Posted by
DArthurBrown
I'd say it's got to be the b-tension adjustment.... But, the OP said the bike was originally a 6 speed but is now an 8 speed. If they were spread the chain could be clipping the taper of the chainstay and jamming. I've had 126 mm spaced bikes where the chainstay protudes more to the inside than on most modern 130 spaced bikes.
That actually seems way more plausible than my explanation - at any rate the OP solved it with some common sense. Same problem arises with 5-6 speed conversion to 7sp+. Usually we just jam a spacer on the driveside to space it out. Depends on the bike/customer.