by Had Robinson
updated January 3, 2020
As carburetors are used, they will eventually fail to deliver the proper air-fuel mixture to the engine. When this happens, the engine will run roughly or fail to run at all. The symptoms can be noticed at idle, in the mid-range, or at full throttle. A failed carburetor can burn up an engine if it is not fixed immediately.
It is assumed that the pilot has always used fresh, clean, high octane fuel and high quality 2 stroke oil. Note that use of ethanol fuels will greatly increase carburetor problems because of the tendency of ethanol/gasoline mixes to attract water and for ethanol to dissolve everything it comes in contact with, especially plastics and rubber. Ethanol also conducts electricity. This causes electrolysis of metal parts and is why pilots should never leave gasoline/ethanol mixes in the fuel system for long, especially in hot weather. Please read our fuel section to learn about the correct fuels, oils, and how to prevent problems caused by ethanol.
DO NOT STORE FUEL MIXED WITH OIL FOR MORE THAN A FEW WEEKS. Old gasoline and old fuel/oil mixes cause gum, varnish, and sludge buildup in carburetors. If the fuel contains ethanol, these problems will be much more common. Avoid ethanol blends if at all possible.
1. Fuel inlet needle
A common point of failure in diaphragm type carburetors is the wearing away of the tip of the fuel inlet needle. The wear is microscopic and not visible to the naked eye. When this happens, the pop-off and rest pressures will be lower causing the engine to run richer (and rougher). Engine idle is also badly affected because of fuel leakage into the carburetor. Leakage of the inlet needle can be noticed when attempting to adjust the carburetor's low speed system: the low speed needle can be completely screwed in (closed) with little or no effect.
If you have a pop-off gauge, you can check for this problem in less than a minute without disassembling the carburetor.
The inlet needle will eventually wear out/fail no matter what kind of fuel is used. The needle goes up and down up to 150 times per second.
Most pilots should rebuild their carburetors every year, at the least or after (25) hours of use, whichever is sooner. Pilots who fly extensively will notice problems with engine performance after about (50) hours. If ethanol fuels are used it will be much sooner. Routine carburetor maintenance and spark plug replacement are quick and easy fixes to the most common types of engine performance issues.
Just as common or more so is the hardening and deterioration of the diaphragms inside the carburetor. Because of the presence of ethanol in gasoline, this problem has become acute in recent years. When the fuel metering and/or fuel pump diaphragm become stiff, the engine will tend to run lean because the fuel pump cannot keep up with the demand for fuel. The valves in the pump will also fail to close properly further decreasing the pump's capacity. Sometimes, the fuel pump diaphragm will become so stiff that the engine will not run at all. Pilots who never purge their fuel systems before storage will have many more problems with the carburetor, especially if they use gasoline that contains ethanol.
3. Metering lever spring
This spring is critical for keeping the fuel/air ratio correct and weakens with age whether the engine is run or not. Ethanol fuels can also corrode it (and harden it) because of the presence of water dissolved in the ethanol. A hardened spring will lean out the fuel/air mix which can burn up an engine. This is an acute problem in humid environments. The pop-off pressure must be regularly checked. If the rest pressure is below specification, the engine will run rich throughout the throttle range. If the pop-off pressure is high, water has probably gotten into the entire fuel system.
4. Corrosion and gum inside the carburetor
The diaphragm material also is used for the valves in the pump chamber. However, even if the diaphragm has been replaced the valves can still leak due to corrosion or gum/goo on the surfaces inside the pump chamber.
Rebuilding the carburetor is the simplest way to solve most problems. Be sure to get the complete kit as it contains the inlet needle valve which is the most common part to fail. At the same time, be sure to replace the metering lever spring (must be purchased separately). This is important if you do not have a pop-off pressure gauge. If you do not have a pop-off gauge you will have to assume there is no water or debris anywhere in the fuel system that may be blocking fuel from getting to the engine. When rebuilding the carburetor, always check/change the spark plug. These items are available from Miniplane-USA.