660 blew belt with 1200 miles

Bought the bike a month or so ago with 950 miles. Owner claimed it had a clutch kit, couldn’t remember whos. Serviced primary, belt looked fine, possibly glazed some no cracks. Was told this 2004 bike had sat approximately 6 years of its life (belt dry rot?) Roughed up the pulleys with 120 grit flap wheel on dremel. Had Post office weigh a roller for me, they showed 8.5g with cover. I was thinking running in high on slow technical trails would be fine since the rollers were so light. Belt cover got super hot the other night. The next night blew the belt running 40mph or so after dark. Luckily someone came along and tow’d the bike thank God. New to cvts. Thought I read here that covers will get hot and it’s normal, that’s why I wasn’t concerned. Was that an indication of belt slippage? I’d installed an orange spring when I bought the bike even though EPI tech recommended blue. Bought a scale at Harbor Freight yesterday, the rollers are right around 17g with cover. Close to oem I think. Have dyno jet 4th notch needle, 155 main, 3.25 out mixture, Uni and pipe nozzle. Runs real well for what it is.

Would this have been prevented by running low on slow technical trails and should I go ahead and shim since I’m running 27" Bighorns? The bike pulls wheelys in high and will flip in low if there’s enough traction.

Tia, Jerry