I have a similar problem and have given up solving it. My FZ, which is equipped with a 4-1 one-off exhaust system, doesn’t go forward smoothly under 3500rpm when I turn the accelerator. The bike is okay over 3500rpm and runs at gathering speed over 6000rpm. I tried another 4-1 exhaust system (Over Racing) but in vain. The FZ had the same kind of problem. Though I thought a 4-1 system was not good for the FZ, I realized my mistake when I rode the 2MG equipped with an original 4-1 system. The bike accelerated smoothly even when I turn the accelerator at 2000rpm.