Caring for your carburetor
With your 4-cycle engine asked to do more work with less downtime, why would you want to wade through volumes of manuals to make a few minor repairs? You can make the process more simple by focusing on the most common problems. I divide carburetor repairs into four major categories: flooding, fuel quality, dirt and misadjustments. I'll focus here on the most common causes and offer some simple solutions for these problems. Refer to Figures 1 and 2 (pages C 44 and C 46) for views of the parts mentioned in the following text.
Carburetor flooding The function of the carburetor is to mix fuel and air at the proper ratio to provide optimal horsepower and efficiency. When the engine compresses this mix of fuel and air and ignites it at the proper time, you get an explosion. This explosion pushes on the piston and crankshaft to develop power.
Many things can interfere with this process, carburetor flooding being one of the main ones. This problem often is obvious because the engine runs with heavy exhaust fumes or even leaves puddles of fuel on the shop floor under the engine. The carburetor is not the only cause of flooding, however. The single-most common cause is dirt in the fuel. Dirt usually gets into the fuel system via the fuel tank. If the area around the filler neck is left dirty, you can easily knock it in when you're refueling. Typically, the fuel filter traps the dirt. However, if the fuel line doesn't have a filter, or it fits improperly, the dirt settles to the bottom of the fuel bowl. As you use the equipment, the dirt bounces around until it is caught between the inlet valve and the seat. This will cause flooding because the valve won't seat to shut the fuel off. Therefore, it is important to install fuel filters and to keep the fuel clean.
The carburetor itself can cause flooding problems, too--particularly the float valve (needle) and seat. As you operate a piece of equipment, it transmits a lot of vibration to these parts. The vibration causes small ridges to form on the float valve. Then when the fuel level raises the float to shut the fuel off, the float valve will not seat properly. When this happens, fuel continues to flow and floods the engine. For this reason, many companies today use rubber-tipped valves. These resist wear better, but vibration still affects them--it simply takes longer. Thus, when performing routine service on the fuel system, it is important to inspect these parts for wear and replace them as needed.
Another common cause of flooding relates directly to the float assembly. This component can develop a leak and lose its ability to float. In cold climates, if water gets into the fuel bowl and freezes, it can crush the float and cause it to leak. You can also accidentally crush the float assembly if you use compressed air to blow an obstruction clear from the fuel inlet. This can happen because the float bowl has a small atmospheric vent that often will not allow the high-pressure air to leave as fast as it enters. To check the float for a leak, simply remove it and shake it to see if there is any liquid inside. Or place the entire float assembly in a cup of hot water and watch for air bubbles to rise from it. These bubbles form because you are warming up the air inside the float, causing it to expand. Note: Never attempt to warm the float assembly with any kind of open flame. Doing so can cause the gas vapors that form inside of a leaking float to explode.
It is not uncommon for your float assembly to need some adjustment from time to time. After bouncing around the lawn or work site for an entire season, the small tang (also called the float arm or float tab) that closes the float valve against the seat can get bent out of adjustment. By adjusting the tang, you can prevent carburetor flooding, hard starting and poor fuel economy.
To make this adjustment, drain the fuel from the carburetor and remove it from the engine. Turn the carburetor over so that the weight of the float rests on the float valve and seat (see Figure 3, page C 46). With a ruler or scale, measure the height of the float opposite the pivot pin. This measurement should be the same on all sides. It's not always necessary to purchase a float-adjusting tool from the manufacturer to perform this task. However, a carburetor will function fine as long as you make sure that the float is parallel with the gasket surface. In most cases, to make this adjustment you simply bend the small tang that activates the float valve until the float is parallel to the gasket surface. If your float tang is plastic, you will need to purchase a new inlet needle and seat; the plastic tangs are not adjustable.
Fuel-related problems Modern fuels such as reformulated gasoline (RFG) and other oxygenated fuels (required in some parts of the United States) can cause a lean carburetor condition. RFG production varies, but the result is basically the same: gasoline with a percentage of alcohol, or oxygenate. The problem with RFG is that it causes some elastomers to swell. In a carburetor with a rubber-tipped float valve, this is a problem because swollen float valves can cut off th e flow of fuel. In addition, fuels with higher oxygen ratios cause problems because they lean the air:fuel mix too much and can cause overheating. First, try a different brand of fuel to cure the problem. However, if overheating persists, contact the carburetor manufacturer to see if a kit to upgrade your fuel system to handle RFG is available.
Another fuel-related problem is stale gas that has lost its ability to burn. This usually happens when a piece of equipment is in storage for more than a month. One way to prevent this problem is to close the fuel shut-off and drain the carburetor. Do not, however, drain the fuel tank because most tanks are not painted on the inside. This clean surface can rust and cause additional problems. Furthermore, if you close the fuel shut-off but don't drain the carburetor, the small amount of gas left in the bowl will evaporate. This will cause a layer of varnish to form on the internal parts, which will block fuel passages or prevent other moving parts from functioning (such as the float hinge or adjusting screws).
Another preventive measure is to add a fuel stabilizer to the tank on equipment that may sit for some time. If you do choose this route, make sure that you add the stabilizer and run the equipment for a few minutes before storing it. This is to make sure that the additive gets into the carburetor, not just the fuel tank.
The last fuel-related problem to consider is water in the fuel. Small beads of water in the fuel bowl can get into fuel passages and block them off. Adding a small amount of dry gas--basically an alcohol mix--can help eliminate the water, but too much can be just as harmful. Therefore, carefully adhere to the mix ratio listed on the bottle; most dry-gas mixes are packaged for a 15- to 20-gallon automobile fuel tank. Water in your fuel also can oxidize your carburetor's aluminum parts and rust its steel parts. Oxidation is the white powdery substance that you might find in the fuel bowl when you open the carburetor. If you encounter this condition, test your fuel for water using a litmus paste. Ask your dealer for help and materials.
Dirt in the fuel system As I previously described, dirt can cause many problems, getting caught in small passages and blocking fuel flow. Dirt can come from many sources: the environment, oxidized parts, rust and varnish build-up to name just a few. You can usually figure it's dirt in the system if your equipment runs fine one minute and quits the next. This happens because of equipment vibration during use that dislodges the particles. They then are caught in small fuel passages. Fuel filters help but don't always count on them to cover up for your careless mistakes when handling fuel in dirty conditions. Filters are designed to stop certain-sized dirt particles, and some fine silt can get past. After some time, this silt can accumulate and cause problems.
Eight steps to making adjustments You occasionally may need to make adjustments to accommodate changes in fuel quality and the engine's running condition. Here are a few steps to follow when you make adjustments:
* Remove the spark-plug lead from the engine's plug to prevent accidental starts.
* Locate the idle-mixture screw and the high-speed-mixture screw (if your carburetor has both). If you are not sure, consult the service manual.
* Lightly close the mixture screws until they contact their respective seating surfaces.
* Open the idle-mixture screw one full turn. Then open the high-speed-mixture screw one and one-quarter turn. This usually allows the engine to start and run.
The next steps of adjustment will have the greatest effect on your engine's performance:
* Reconnect the spark-plug lead to the plug and start the engine. With the engine at idle, first close the idle-mixture screw until the engine starts to run roughly. Then open it back up until the engine runs smoothly and continue to open it until it runs roughly again. Note where these positions are and set the adjuster screw to a mid-point between these two positions.
* Open the engine up to full-throttle position and perform the same adjustments to the high-speed-mixture screw.
* Close the throttle down to the idle position and perform Step 5 again. This is important because the two circuits in the carburetor have an effect on each other. If you make an adjustment to one circuit, you also add or subtract fuel from the other circuit.
* Again, open the throttle up and perform the full range of adjustments as before. Usually you will not have to turn the screws as far in either direction to get the best results.
It is important to complete these steps at least twice, always allowing the engine to stabilize itself at each throttle setting before making any adjustments. When you are satisfied that you have made all of the necessary adjustments, check that the engine starts easily, idles at the manufacturer's recommended rpm and accelerates to the recommended no-load rpm smoothly. If the engine's high rpm is off, you may have to adjust the governor linkages that connect to the throttle. You should consult the service manual for this because of the complexity of this step.
The table on page C 44 is a quick reference to help you to find some of the common causes for carburetor problems. Too many fuel systems exist to be too specific, so keep in mind that this is only a rough guide.
Larry Van Deusen is a technical advisor at the State University of New York's College of Agriculture and Technology (Cobleskill, N.Y.).
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