I've often read the question here 'what is the best affordable power upgrade is for a relatively stock XR100?'. Many different answers are usually generated. Very few have a verified answers (not to say that the response is incorrect, just not verified); BBR notably does acknowledge a modest 2 HP gain over stock with their 120 BBK though if I recall it's not stated as to what pipe and carb are used in concert with the BBK to generate the extra 2 ponies. Aside from uncorking the intake & exhaust/jetting (yawn) I'm betting the best bang for the buck on a stock XR 100 is a lightened flywheel. While this won't show an improvement in horsepower generated by the engine, it does reduce the weight on the crank and provides a great improvement on acceleration. Inner rotor kits for the venerable 100 are currently anything but affordable. I'd also be concerned that they sacrifice flywheel effect much more than a lightened stock flywheel, making the bike more difficult to drive. In the coming days I will be putting together a comparison of my stock, uncorked (and very tired) engine, then with the lightened flywheel only. I thought I'd set a steady idle speed with the engine hot with stock flywheel, then see what the change in engine idle rpm is with the lightened flywheel. Yup, I do have an inductive tach, though it's for 4cyl engines. The reading will not be accurate to true engine rpm, but for the purposes of comparison reliable. Lightened flywheel is the first step; I have also laid hands on a 120 BBK, camshaft and pipe. I'd like your input as to how comparisons or 'real' numbers could be done to show the results (sorry folks, no access to a dyno) of each of these items as individuals. I do have access to an array of tools / equipment, so don't be afraid to ask for a technical result.