This is a tutorial containing a ten step process to rebuild a small engine carburetor that may have become dirty or clogged, causing the engine to fail to run. In order to successfully complete this task with the given directions, it is assumed that you (the reader/todays mechanic) will have adequate knowledge on the tools and ideas presented in these steps. Prior to starting this project, I would visit your local hardware store and purchase a carburetor rebuild kit (make sure to have engine's serial #, as well as its make and model) also grab some carburetor cleaner if you don’t already have some. Always make sure to wear adequate safety gear when working with various tools and chemicals. Good luck, and good fixing.
Identity the location of the carburetor on the engine and remove bolts that are fastening the carburetor to the engine block. When pulling off the carb, also remove the throttle and choke cables. (When doing this it is important to either mark or at least note where the choke and throttle cables are attached.) In addition to these members, it is important to carefully remove the gas supply hose.
Once the carburetor is completely disconnected, place it into a pan or other workspace that will help catch any springs or other small pieces that might come off when disassembling. At this point I always like to take some of my carburetor cleaner and an old toothbrush and clean the outside of the carb. Often times there is a large buildup of grease and dirt that can hinder moving pieces when the engine is running.
As shown in the diagram above, there will be (normally three) screws on the outside of the carb. One not shown above is the idle screw that keeps the throttle at a minimum closed position to ensure that the engine does not stall. The other two screws act as valves for in the fuel inlets for high and low throttle position. Take note of the position of these screws when removing to ensure that when reinserting them after cleaning the "valves" are open the same amount.
Remove the bowl on the bottom of the carb as well as the screws mentioned in the step above. Inside of the bowl you will see a device called a float that acts as a valve for gas coming into the carb from the gas tank. Remove the pin that allows this piece to hinge and set it and the needle off to the side (this needle should be replaced with the new one from the rebuild kit).
At this point I like to remove all of the existing seals on the carburetor. Before doing this though it is always a good idea to check to make sure that the pieces in the rebuild kit are a direct match to the existing components. Items such as the engine block gasket, air cleaner gasket, bowl O-rings, bowl needle, and any other replaceable components should all be direct matches in size, shape, and thickness. (All of these within reason.)
Begin cleaning the internal parts of the carb with your carb cleaner and an old rag. It is vital to remove any remaining pieces of gasket or other foreign objects to make sure that the new gaskets and seals will seat properly.
One of the largest causes for a failed carburetor will lie in the jets and ports. When using the carb cleaner make sure to allow a steady stream to flow through the jets and ports to flush any foreign particles that will cause limited flow of fuel causing stalling or a failure to start.
Once the entire carburetor has been cleaned and flushed out, it’s time to start the reassembly. I normally start with the needle and the float components of the bowl. When rebuilding, made sure to use the new seals and gaskets provided in the rebuild kit. If you happen to be in a situation where you are reusing a seal or gasket I like to coat and rub with a thin layer of engine oil before putting on the carb. As you install components such as the bowl it is important to work slowly and to make sure that when tightening bolts, they are not over tightened (use torque specs if available). When installing the valve screws make sure to tighten and then back out to original factory threading (measured during disassembly)
Now that the carb is completely reassembled, cleaned and ready for action, reinstall the entire unit back onto the engine block. Be sure to reconnect choke and throttle cables before tightening it down as the carb must be free to remove/refasten many model's cables. I like to wait to reconnect the fuel line until last just in case you need to pull the carb back off for any reason. Once all of carburetor components are reinstalled, take a look at the air box. It is always a good idea to completely clean and sometimes replace the air filter. (A good test is to hold the filter up to a light source like the sun or a light bulb and look to see if any light passes through. If there is none or it is very faint, it is time to change the filter.)
The final step is the long awaited reveal. STARTING THE MOTOR. When doing this, it is important to ensure that any gas valves are in the on position and that the choke is engaged. Once the engine starts up it may be necessary to make minor adjustments to the idol and high/low jet screws. It is important to realize that the efficiency and over-all performance of your carburetor will be greatly improved. Adjustments like the idol set and the main adjustment screws (high/low) should be made only after the engine has been warmed up to running temperature. After these few minor adjustments you are good to go. Your newly rebuilt carburetor should be ready for the long haul now.