Lambretta Series 1-3 Air Filter
An air filter's job is simply to ensure that the air entering the combustion chamber is clean. These are paper type filters and can only be cleaned with compressed air about every 3000 miles. Every 6000 miles or so, the air filter should be replaced.
Air reaches the carburetor by a small air scoop under the seat just forward of the fuel flap. It travels down into an air filter box, through the filter, down a flexible rubber hose and into the carb.
To remove the filter, remove the left side panel of the bike and loosen the clamp which secures the rubber intake hose to the carb (shown with a red arrow below). The filter can now be removed by unclipping the large spring retaining clip from the metal air intake (shown with green arrows below).
The filter and intake will now pull out sideways from the air filter box. Sometimes the rubber gasket gets old and sticky and may need to be replaced (like it should be here). There is a wing nut on the inside face of the filter assembly which needs to be removed to change out the filter itself. Once this is done, the filter will slide off the central bolt and can be discarded. Check the flexible rubber hose for any cracks or breaks that might allow dirt into the clean air supply.
Replace the filter and reverse the removal process to complete the job.