Things You'll Need
Small set of jewelers type screwdrivers Philips and slotted
Small pair of needle nose thin jawed pliers
Walbro carburetors are one of the most popular brand of carburetors on the market for small gasoline engines used in weed eaters, leaf blowers and some chainsaws. Over time, almost all carburetors will need rebuilding. The small rubber parts become brittle and can break, combined with small orifices becoming clogged from debris. By following a series of processes you can rebuild the small carburetor in a about an hour and keep hard earned cash in your pocket by doing it yourself.
Remove the carburetor from the small engine using the set of properly-sized Allen wrenches. Place the carburetor on a firm flat surface that is covered with a few layers of the newspaper. Some gas may leak from the gas inlet tube and the newspaper will absorb the errant liquid.
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Observe that there are two removable covers on opposite sides of the housing. One cover has four small screws and the other has only one large screw, centered in the middle of the thick aluminum cover.
Remove the four small screws from the housing of the carburetor, using the small jewelers screwdrivers. Set the small cover along with the screws to the side of the work area. Employ the sharp edge of the pock knife and carefully pry the rubber fuel pump gasket from the surface. You can set this part aside for the moment.
Turn the carburetor over and remove the large center screw on the thicker cover. Again use the pocketknife to pry the rubber gasket from the housing. Note that under the gasket lays a small needle valve held in place with a small plate and an even smaller Philips head screw.
Use the small jewelers screwdriver and remove the holding screw that secures the needle valve in place. As you pull the plate from the housing using the small needle nose thin jawed pliers, be aware that a very small spring resides under the plate. This spring is not contained in the rebuild kit so do not loose it. Remove the plate, small metal rod, small spring and the needle valve from the housing. Note that under this area is a small diameter screen , approximately 3/8 inch round. Pry the screen loose with the sharp end of the pocketknife.
Remove the small fuel control mixing screws marked "Hi" and "Lo" from the housing. Set these screws aside and be sure that the small springs enclosing the screws stay with the correct screw.
Use the carburetor cleaner and thoroughly spray the metal housing down. Pay particular attention to the small orifices and drilled holes. Attach the small diameter tube to the spray nozzle of the cleaner can and flush all openings in the metal housing. Allow the carburetor to completely dry before assembling the rebuild kit back into the carburetor.
Open the plastic bag of the rebuild kit and lay out all the parts that are contained in the sealed baggie. You will note that not all the new parts will match up to the ones you removed. The rebuild kits that are sold contain all parts for all Walbro carburetors. Match up the parts that you have removed and lay them out on the newspaper. Place the extra parts back in the plastic bag so they will not interfere with the rebuild.
Replace the new parts in exactly the order that you removed them. Insert the new small filter screen and push it down with your finger to properly seat the screen against the raised edge of the round housing hole.
Replace the new plate, lever arm and needle valve into the float area. The insertion of the small spring can be a little tricky but take your time. Lay the housing on the newspaper and then insert the screen into the machined hole. Lay the plate on top of the spring and then fit the needle valve into the catch part of the plate. The thin nosed pliers can help in holding the spring in place while starting the small Philips head screw.
Install the new gasket over the needle valve area and tighten the thick aluminum cover in place with the single screw. Assemble the "Hi" and "Lo" mixing screws back into their appropriate openings. Fully close the screw valves and then open them to 1 ½ turns for a rough adjustment.
Turn the housing over and install the new diaphragm fuel pump. The new pump may be a plastic composite instead of the rubber one that you removed. Walbro has changed this part and the new one works fine. Be sure to line up the two small placement peg holes with the openings in the diaphragm. Place the cover over the fuel pump area and tighten the small screws.
Install the rebuilt carburetor on the small engine and give it a try. You may have to readjust the "Hi" and "Lo" speed mixing valves on the side of the carburetor for proper operation.
Rebuild the carburetor in an area that is well ventilated and well lighted.
The fumes from the gasoline and cleaner can be highly explosive. Be sure to keep track of all the removed parts until the carburetor performs correctly.