buy a S&S crank/flywheel then you will have no problems I put several thousand miles on my 07 SG with a 120 zippers kit that includes a S&S crank not one problem the bike has since been totalled but I bought it back from the Insurance co. so I could get my 120 engine for my new 07 SG
Your Zipper 120 kit I am sure it rocks and rolls but gas wise how is it did you lose alot on fuel mileage. Not riding One Hundred but out of town cruising between 75 and 85