You might just be describing the difference between 9.0:1 CR (I think that is what a '77 R100S has?) and 8.2:1 in the '81? If the S is single plugged there's a good chance that if you jetted the '77 to where you needed the choke more it would return the favor by pinging. Our shop had a lot of owners of then new 9.0:1 CR bikes lower their compression for pinging in the late '70's. You could always raise the compression in the '81. If I wanted more power than the '77, I would much rather start with the '81 but that's just some of us out there. But then again, you could have a problem like weak coils or any number of other possibilities. A blue spark in thin air doesn't really tell you. Good luck 00 Green. This is just the type of thing that is you just have to figure out.