Early 250S1 had 30hp and wacky handling, KH250 slowed as it got older but tamed most of the handling problems. 350 quite mad, 400 almost sane. Engines could be tough but eat main bearings & piston rings after as little as 15k, plus boxes of spark plugs. Also suspect ignition and crankshaft seals. Erratic running may be down to shot HT leads as they are poor quality. Engines need stock airbox for semblance of reliability. Poor gearbox, excess vibes normal.
