Loosen those big round clamps holding the rubber air pipes onto the front and the back of the carb, remove the needle and carb piston that the throttle cable attaches to at the top of the carb, it just unscrews...it may be tight and may need a pliers or something, the carb should come out then. You will now see for 10mm hex bolts holding the rubber pipe as it flares into a square onto the barrel. loosen them and take it off, your reed cage will come out with that, there should be 4 petals (fibreglass flaps) if one is gone, its snapped off and thats your problem.
Be careful not to damage any gaskets and dont lever the inlet off the block you may hurt the metal mating surfaces, otherwise its really simple, try keep things clean and get as much dirt off the parts before you start, you dont want that much in your carb!
good luck