Float style carburetors are common on small engines used on gas powered outdoor equipment, in particular the lawn and garden tractor. Many times when the engine is running rough or is hunting badly (engine speed not constant) it is because the carburetor needs a good cleaning and service.
Preparation
Record the engine type and serial number then obtain the relevant carburetor kit, engine and filter gaskets. Prepare a clean work area, an area of NO SMOKING or NAKED FLAME (gasoline vapors ignite very easily). Basic tools required are set of wrenches (socket or open ended), needle nose pliers, can of carburetor cleaner, a container to clean parts and a container to store parts.
Carburetor Removal
Take a photograph or make a pencil sketch of the choke linkage, governor linkage and springs connected to the carburetor. This will save a great deal of heartache when you come to reassemble. Shut off fuel to carburetor before removing the fuel line to the carburetor. Some have a fuel shut off valve, if not clamp the rubber fuel hose with a G clamp, or drain into a clean container.
Remove the air filter, the carburetor fixing nuts (screws) and governor spring. Ease the carburetor away from the engine, twisting as you do so to disengage the Z shape linkages to the choke and throttle valves. Note that there will be fuel still in the bowl at the base of the carburetor. Tip carburetor upside down to let the fuel drain out into a container.
Carburetor Disassembly
Now that you have the complete carburetor in your hand and before attempting to take it apart, look for the two screw (some have only one) with springs under the screw head. These will be the idle and high speed adjustment screw. Count the number of turns required to screw home the needle, about one & one and a half for the main jet and 1 time for the idle jet. Make a note as this will be the setting will to be used later.
Remove the high speed and idle adjusting screw, clean and place in a container ready for reassembling. Remove the bowl fixing nut, on some carburetors this nut is where the high speed adjustment screw is located. Clean the nut and replace washer if required. Those that house the high speed needle will have one or two small fuel transfer holes. These do get clogged - clean out with thin wire (obtain thin floral arranging wire from a craft shop). The float is next, watch out for the fuel inlet needle and how it is attached to the float hinge. Some of the brass floats do leak after a time, check by shaking float to hear if fuel is sloshing about, if so replace the float.
Use carburetor cleaner and spray outside, then the inside of the carburetor, the air and needle orifices. WATCH YOUR EYES - USE GOGGLES. Ensure that the choke and butterfly valves move freely and that the throttle and choke return springs are closing the valves.
If they are weak or broken then replace them by unscrewing the butterfly valve and removing spindle, then reassemble with new springs. Inside the venturi at the throttle valve end, there are 2 or 3 small holes (need to move the valve to open position to see them). I use a staple with one leg bent straight and using needle pliers cleaned out those ports. DO NOT FORCE IF STAPLE IS LARGER IN DIAMETER THAN THE PORT DIAMETER.
Reassemble
Insert fuel needle and float. Fit the bowl to the carburetor with a new 'O' ring. Screw in the high speed and idle needles (don't forget the tension springs, 'O' ring and washer) until they just seat, then back off the screws by the number of turns you recorded when disassembling. These settings should enable the engine to start, if not, screw both needles back in until they just seat. Back off the idle screw one turn and the high speed by 1&1/2 turns. Final needle adjustment is done when the engine tests are conducted.
Using the photos taken or your sketch, insert the governor and choke linkage, then with a new gasket fit the carburetor to the engine body. Reconnect the governor spring into the hole on the throttle cam, usually the same hole as the governor linkage. Reconnect fuel line and filter cover (with new gasket) to the body of the carburetor. Replace worn or dirty air filters
Adjustment
Start engine and allow it to warm up to normal operating temperature (3-5 minutes). Set the throttle control to the maximum speed position, then turn the high speed mixture control in (clockwise) slowly until the engine runs erratic. Note that screw position. Now back off the screw (counter clockwise) until the engine again begins to run erratic. Note that screw position, then screw back in to the mid position at which point the engine should be running smoothly.
Set the throttle control to idle or slow position and adjust the idle mixture screw in the same manner as you did with the high speed mixture control. Some times after setting the idle screw you may need to go back and readjust the high speed setting.
Disclaimer - This article is intended as a guide and is generic and therefore I cannot be held liable for injury or damage. There are many types of carburetors used on small engines, each with their own means of controlling the fuel/air mixture to the engine. However the above is typical of many carburetors used on small engines that I've serviced and repaired during the years I owned an outdoor power equipment sales and service business.
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