AIR-FUEL MIXTURE

IN THIS SECTION, YOU WILL FIND THE FOLLOWING SUBSECTIONS:

PREPARING TO ADJUST THE CARBURETOR
ADJUSTING THE CARBURETOR AIR AND FUEL MIXTURE
SET THE HIGH SPEED SYSTEM MIXTURE
SET THE IDLING SYSTEM MIXTURE
IDENTIFYING PROBLEMS WITH THE FUEL-AIR MIXTURE
CORRECTING PROBLEMS WITH THE FUEL-AIR MIXTURE

PREPARING TO ADJUST THE CARBURETOR

The following procedures apply to almost all types carburetors. Before beginning maintenance on engine, should read service manual. Special considerations or procedures mentioned in manual should be noted and followed. Carburetor and service kits available for most engine models. Good idea to use kit if available.

Must adjust carburetor if engine getting too little or too much fuel or getting no fuel at all. Don’t automatically assume carburetor at fault if engine not running well. Should always make following series of checks before attempting to adjust carburetor. This will help ensure carburetor to blame:

–Start the engine. If starts and runs for few seconds then stops, not enough fuel getting through carburetor. Trouble most likely in carburetor, will need to be adjusted.
–Run engine at slow speed until warms up.
–Open throttle suddenly. Engine should accelerate smoothly and evenly. If rough acceleration, air and fuel mixture too lean.
–Check exhaust. If engine gives off black smoke, air and fuel mixture too rich. Remember that two stroke engines will produce some smoke. So look for rough idling as additional indication rich mixture.

Before carburetor adjustments made, following steps should be carried out. Unless you perform these steps, settings you make may not be accurate:

1)Fill fuel tank with clean fresh fuel. For two stroke engines, use proper oil and gas mixture; for four stroke engines, use gasoline. Exceptions to this rule involves engines with suction type carburetors that have neither pumps nor reservoirs for fuel. With such carburetor, proper adjustments for fuel air mixture vary with fuel level in tank. Therefore tank should be filled only halfway. Half full tank will provide good average when finding proper setting for mixture.
2)Check fuel tank’s vent, located in tank cap. Blocked, restricted, or closed vent will cause flow fuel to carburetor to get cut off. Engine can be started with malfunctioning fuel tank vent, but won’t run long.
3)Check fuel filter and strainers. If engine has fuel line, check line for kinks that inhibit proper flow.
4)Inspect air cleaner. Clogged or blocked air cleaner will choke engine, resulting in rich mixture.
5)Check condition throttle, governor linkages. Make sure they move freely without binding.
6)Inspect intake manifold for air leaks.
7)With two stroke engine, should also check crankcase for air leaks. Leak can be pinpointed by putting drop of oil on suspected spot and running engine. Air leak will cause oil to disappear. Air leak can be corrected by tightening flanges. Check spot again. If leak still present, gaskets have to be replaced. May also need to remove carburetor to check for broken or missing reeds in reed valve assembly.
8)Check ignition system, especially spark plug.

ADJUSTING THE CARBURETOR AIR AND FUEL MIXTURE

Most new carburetors can’t be adjusted. Older model may have two screws used for adjusting air to fuel ratio.

–The idle mixture screw for setting idling system’s mixture.
–The high speed load screw for setting high speed system’s mixture.

Some simple carburetor’s don’t have idle mixture screws. In such carburetor, ratio of fuel within idle mixture determined by size carburetor’s fuel discharge port. Can use following steps when setting ratios for air and fuel mixture:

1)Locate adjustment screws on carburetor. In case new carburetor, may be no way to make adjustment. Refer to service manual to learn where adjustment screws located. If manufacturer’s service information unavailable, look for two spring loaded screws on carburetor. Idle mixture screw usually be closest to engine. Other screw usually be high speed load screw. If not possible to tell one screw from other, start engine and operate at idling speed. Tighten one screws (usually by turning clockwise). If engine slows down or stops, you’ve found idle mixture screw; if no change, must have turned high speed load screw and closed high speed jet.
2)Close both carburetor’s jets. Tighten each of screws using small screwdriver (usually by turning clockwise). Adjust screws carefully. Tightening too much will cause needles to jam into seats, which can damage needle valves.
3)Loosen idle mixture screw about 1 turn. This can vary between ¾ and 1 ½ turns, depending on carburetor. Consult manufacturer’s service manual if available.
4)Loosen high speed load screw about 1 or 2 turns; exact number will depend on carburetor.

SET THE HIGH SPEED SYSTEM MIXTURE

High speed load screw (also sometimes power adjusting needle or main adjusting needle) controls fuel flow at high speed or under full load. Should be adjusted only when engine at normal operating temperature. Otherwise setting will provide inaccurate fuel flow when engine at normal operating temperature. Follow these steps when setting high speed system’s mixture:

1)Run engine at full throttle and, if possible, under load. Load can be simulated using dynamometer connected to power shaft of engine. With tachometer, can get reading of engine’s speed while you adjust high speed load screw. One simple type tachometer is vibrotachometer. Vibrotachometer placed on cowling of engine, senses vibration produced by power strokes then displays rpm reading on scale. Scale should be read when vane arm that exits vibrotachometer becomes stationary; at this poin, exact rpm can be read in window at top of instrument.
2)Increase engine load until speed begins to decrease.
3)Tighten high speed load screw (usually by turning clockwise) until engine begins to lose power. Closing jet in this way leans out mixture to point that engine can’t attain full horsepower.
4)Loosen screw until engine rpm drops. When jet opened to this point, engine burning rich mixture. Ideal adjustment between two extremes producing high fuel efficiency but low power and low fuel efficiency but high power.
5)Slowly tighten screw down again until engine runs smoothly and efficiently. Best way to pause after every 1/8 turn, allowing engine few seconds to adjust to new setting.
6)Check setting by suddenly opening throttle while engine under full load. If screw properly adjusted, engine will accelerate without hesitation. If engine hesitates, screw should be loosened. If engine to be operated in cold weather or under heavy load, should set mixture slightly richer than normal. Can be achieved by loosening screw an additional 1/16 turn.

SET THE IDLING SYSTEM MIXTURE

Idle mixture screw controls fuel level when idling. Refer to these steps when setting idling system’s mixture:

1)Warm up engine.
2)Set throttle to recommended idling speed by using tachometer. In most cases, carburetor’s ideal speed can be set using adjustment screw that controls how far throttle plate will close. This procedure varies depending on carburetor so should consult engine’s service manual.
3)Slowly tighten idle mixture screw until engine begins to slow. Pause after each 1/8 turn to give engine time to adjust to new mixture.
4)Turn screw back slowly to enrich mixture with fuel until engine operates smoothly. If engine unaffected by one or two complete turns of idle mixture screw, may be some extensive trouble with carburetor. Could include leaking float valve or diaphragm, too high fuel level in bowl, or deposits in manifold near throttle plate.
5)Recheck setting for high speed mixture. By adjusting idle mixture screw, have made changes to air to fuel ratio. So previous setting made with high speed load screw may now be incorrect.
6)Recheck idle mixture adjustment.
7)Check engine for proper operation. Operate at high and low settings to see if it accelerates smoothly and efficiently from idling speed.

IDENTIFYING PROBLEMS WITH THE FUEL-AIR MIXTURE

Checking the spark plug—examining color of spark plug good way to check for problems with air and fuel mixture. Light tan to gray plug indicates normal engine conditions. Lack of fuel indicated by whitish gray hue on plug’s insulator.

Oil can seep into combustion chamber and foul plug. Faulty valve seats, rings, scored cylinder walls can be responsible. Oil fouling revealed by black deposits caked onto plug. Contaminants fouling fuel may also leave black tint on plug. Unlike oil fouling, these deposits will have smooth rather than caked on appearance.

In typical two stroke engine, oil added to fuel. Since engine burns oil, plug will naturally display oil fouling. Doesn’t indicate anything wrong. With two stroke engine’s plug, fuel system problems indicated by black deposits that have smooth wet appearance. When examining plug, also check its odor. Excessive fuel in mixture can leave behind a gasoline smell on plug.

Checking the exhaust—is good way to identify problems with fuel mixture. After warm up time three to five minutes, exhaust should be free of smoke. Blue gray smoke indicates engine burning too much oil. Black smoke indicates fuel mixture too rich.

CORRECTING PROBLEMS WITH THE FUEL-AIR MIXTURE

The air cleaner—if engine seems to be running rich, don’t begin repairs by removing carburetor. Often air cleaner to blame. Partial blockage will cause reduced air flow, creating rich mixture. To troubleshoot air cleaner, remove and clean out housing. If filter element dirty, clean or replace. Then, run engine for few minutes. If exhaust clears up, air cleaner was problem.

The choke—another common cause mixture too rich is broken or poorly adjusted choke linkage or cable. After you’ve removed air cleaner, make sure choke plate wide open when choke linkage in RUN position or when choke cable control fully depressed.

The carburetor—carburetor final item to check if engine isn’t operating properly and you suspect fuel system. Hard starting, stalling at idle, no power at high speed all can be caused by malfunctioning carburetor. Many modern carburetors aren’t adjustable. If carburetor can be adjusted, service manual for engine will provide fuel mixture starter settings for idle and main jets.

Sometimes fuel mixture may be so lean engine doesn’t run at all. Can test system to see if lack of fuel the problem. First remove plug. Next, insert bottom of drinking straw or thin tube into some fuel. With about one inch straw in fuel, place thumb over opening at top of straw. This will trap small amount fuel in bottom of straw. Fuel will stay there until you lift thumb. Take straw over to engine, hold end containing fuel into opening in cylinder left by plug. Lift thumb slightly, allow about ½ inch fuel to drip out of straw into cylinder.

Reinstall plug, reconnect plug wire and attempt to start engine. If engine runs or attempts to run, there’s problem with fuel getting to engine. Normally this requires carburetor rebuild. First check to make sure there’s enough fuel in tank, and that no fuel lines crimped or clogged. If fuel system has fuel shut off valve, make sure it’s open and functioning properly.