Clunking noise HELP

[Bonejumper]

When son takes off and puts it under torque their is a clunking noise,and slipping? Changed torque spring to see if it had got weak but,no help. Any Insight would be helpful. THX

[jake55]

check chain tension, if not tight enough it could be jumping teeth on the sprockets.

[Bens Dad]

check chain tension, if not tight enough it could be jumping teeth on the sprockets.

Rob Check Trans. gears sounds like teeth are broke in Trans.