I agree with Joe, if the intake noise doesn't increase, the throttle is not opening. You need to trace the throttle Bowden cable to the carburetor - you may need to remove the air cleaner to see where it connects to the throttle butterfly.
It appears the Bowden cable is disconnected either at the throttle lever or at the throttle butterfly on the carburetor. The carburetor end seems more likely. The cable inner is held there by a small screw, or it has a fixed fitting attached permanently to the end of the cable inner. When the throttle is released suddenly, occasionally the inner cable can get detached. Also, occasionally the cable outer can get out of position.
Another failure mode is if the whole throttle cable has become dislodged at the throttle lever end. The cable and its attachments are very simple, just trace it and inspect the two ends to see whether they are properly connected.
