Yeah. ...your idle is way too high. ...so the clutch is fully engaged. ...that's why your brake is trying to kill your engine.....
The chain doesn't look that bad for tightness.....get all the bolts in before you do anything else. .....
Take the linkage and air filter off and make sure the butterfly in the carb is almost closed.....then make sure it doesn't change when you hook up new linkage.......