Don't change anything while you're sorting things out! Just leave jets, weights, etc exactly as you found them. Then change/clean one thing at a time, carefully. The idle speed will change after startup since it has an electric "choke" that adjusts after a minute or so. The weights and belt won't affect anything at this time. A fresh sparkplug and coil helps. Most likely the ratty behavior is coming from the carburetor, and I'm guessing the pilot jet (not the main jet) may have a speck of gonk in it somewhere. It's a very fine jet, maybe .3mm in size, and the tiniest speck of fly-sh** will be a nuisance. If it has a diaphragm carb the diaphragm must be assembled carefully. The Keihin carb is an easy assemble, but still care must be taken with the diaphragm, the accelerator pump, etc. and the bowl.