the driven should stay closed (large) until you're moving well along in "low" gear.
sounds like the driven spring is spent.
my comet 30 had the same problem.
unsure if what i did to test my 30 will work on a 40 but i think it would:
find some sockets or similar objects and ziptie them into the driven pulley under the cams so it's unable to open/shrink it's effective OD.
test drive.
if it now accelerates and climbs hills w/o belt smoke replace the driven spring.
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