Firstly, check the carbs are Synchronized, if not they backfire on tick over. Causes damage to your fuel pump diaphragms and will melt the plastic pulse pipe outlet from the block.
Each carb has a adjustment srew (philips head) with the engine at tick over or siwtched off, undo each screw so the butterfly is loose. Then push the link rod down so all the butterflys are shut, then do the three screws up tight again. Make sure the center carb has a very small gap between the roller and the cam, as this is your pick up timimg.
Easy as that.