You will need to remove the black grub screws holding the casing in position. There will be two to remove. Once they have been removed, put them in a safe place as per the picture below.
Once you have done this, slide the casing out of the way of the rest of the hub as shown in the picture below. Ignore the fact that the picture is of only of an axle and hub. With your repair, the axle will still be on the kart and the chain should still be in position on the sprocket.
Now slide the plastic switch component out of the way as shown below. The plastic component on your hub may be either black or white. That doesn’t make a difference. Notice in the picture the two teeth (circled) which engage the forward and reverse motion of the go kart.
At this stage you will need to make sure that the areas surrounding both of the teeth are clean. In most cases there will be a small piece of plastic which is inhibiting one of the teeth from operating. Remove any plastic and then test by pushing each tooth down. When released the teeth should spring back into position as shown in the picture below.
You now need to rebuild the hub. Ensure that the plastic switch mechanism is the correct way round as shown in the picture below. Position the top of the plastic over the teeth and then slide the bottom part of the mechanism into postion.
Check that the plastic switch is in the position as shown in the picture below.
Slide the casing into position as shown below. Replace the grubscrews and reassemble the chain guard. The forward, freewheel and reverse gearing should now be working fine.