You don't need to take your lawnmower to a repair shop for a spring tune-up. In fact, cleaning the carburettor in your backyard or open garage is a great way to spend an afternoon.
On most push and self-propelled lawnmowers, the carburettor is located directly under the engine's air filter. The air filter is typically a pancake-shaped, pressed steel piece, either to the left or right of the main engine dust cover, and relatively flush with its top. To get to the carburettor and to better see what you're doing, remove the air filter's top and bottom pieces (if applicable).
After removing the filter, you must remove and plug the fuel line from the mower's fuel tank to the carburettor. The black rubber fuel line will pull off of an inlet nipple on the carburettor. After loosening any hose clamps attached to the line on the carburettor side, you can usually pinch the line to avoid fuel spillage while pulling off, using pliers to work the line loose. Once the line is removed from the carburettor, plug it with the sharpened pencil to avoid fuel spillage.
Following your lawnmower's specifications (models vary greatly), remove throttle and choke cables from the carburettor body. Usually, pliers and/or a small Phillips-head screwdriver are all you'll need.
Most carburettors are attached to the engine's intake manifold with two nuts that sit on studs protruding from the manifold. Loosen these two nuts using wrenches or a socket set and remove the carburettor.
Lay newspapers out on a flat table. Make sure that you'll be able to keep track of the many small parts in the carburettor body. Rags will help with cleaning hands and parts, and a cheap digital camera or camera phone is ideal for taking pictures of the parts before they're all over the floor or you forget how they went together!
Create a game plan for organising parts. Many mechanics use snack-sized plastic bags labelled with permanent marker. Groups of parts that you photograph during disassembly can be put in bags labelled in the order they were removed. This is very helpful for reassembly. Other organizational aids include small bins, and having a parts diagram for your specific carburettor is always beneficial.
Make sure your work space is comfortable. You'll be there for a while. Set the carburettor in the middle of the table, and arrange your tools so they're easy to reach.
Take pictures before removing any parts and in between each removal step.
All of the gas that enters your carb goes through the nipple from which you removed the carb's fuel supply line. Use a wrench or socket to remove this nipple from the carburettor body.
Next, remove the large bulbous metal structure–the float bowl–at the bottom of the carb. This procedure varies with mower models, but it's usually a fairly obvious one. The float bowl collects most of the gunk that can hinder a carburettor's performance. Dump out all residual fuel and use the toothbrush and aerosol cleaner to scrub it. If the float bowl had a gasket between it and the carb body, be sure you don't break it, or you'll need a replacement. Many repair manuals suggest replacing all carburettor washers and gaskets at every cleaning, but with careful treatment they can be reused in a pinch.
Removing the carburettor's float bowl probably revealed at least two small passageways into the main tube of the carburettor. Sometimes, these are brass "jets" that can be removed with a flathead screwdriver and cleaned individually, but on some lawnmower carburettors, they're just holes in the carb body itself. Clean these using the aerosol and the paper clip, which you can fold out into a flat pick.
Make sure all moving parts inside the carburettor move freely. Directly under the float bowl, the carburettor's "float" should pivot on a pin inside the carb body. This rotation is caused by the changing fuel level inside the float bowl, and is used to regulate the flow of fuel from the inlet nipple to the bowl itself. The float can be removed by removing the pin it pivots around. Underneath the float, a small valve moves up and down for fuel regulation. Make sure the valve is clear and clean using the toothpick, paper clip and toothbrush. Use the aerosol cleaner liberally everywhere to make cleaning easier.
Look inside the main carburettor tube. Work the butterfly or slide (depending on model) that controls the flow of air into the carburettor with your hand, making sure these parts move freely without snagging, and clean them, if necessary, using the aerosol.
Use rags or paper towels to dry all the parts. If you are using brake cleaner, be extra sure that all parts are dry before reassembly. Brake cleaner is usually not flammable, and can cause your engine to be difficult to start after everything is put back together if the solution is lodged in carburettor passageways. Carburettor cleaner is flammable, and drying is not such a big deal. A small amount of flammable liquid in your carburettor will only help start-up the first time you try to fire up your engine.
Use your organizational method and digital pictures to work your way backwards through the disassembly process. Don't forget any parts. Also, if your carburettor has removable jets, be sure not to overtighten them during reassembly.
Once the carburettor is reassembled, it must be put back on the lawnmower. First, attach throttle and choke cables according to your mower model's specifications. Then, attach the carburettor to the engine's intake manifold first with the two nuts that you removed during disassembly. Next, reattach the fuel line to the carburettor.
Replace the air filter according to your mower's specifications.
Gasoline is extremely flammable. Do not smoke or work on your lawnmower near any source of open flame or spark. Also, be sure to provide adequate ventilation for your work. If you become light-headed, stop working immediately.