Carburetion Systems

1. Carburetion Systems

2. Overview • Purpose of Carburetor • Types of Carburetors • Gas, Oil, & Air

3. The “BIG” Picture • One of the “Big 3” systems • Ignition • Compression • Carburetion

4. What is a Carburetor? • A carburetor’s primary purpose is to produce a mixture of fuel and air to operate the engine.

5. Purpose of the Carburetor • Secondary Purposes & Requirements - Change air fuel ratio and amount of fuel that is correct for different circumstances such as: • Cold or hot starting • Idling • Part throttle • Acceleration • High speed operation

6. Carburetor Theory • Atmospheric Pressure • a constant downward force of air on the Earth • usually varies between 13 and 15 lbs per sq. in. • areas of low pressure must be created in the carburetor to create air flow

7. Carburetion • Gasoline engines cannot run on liquid gasoline. It must be vaporized and mixed with air in the proper proportions for varying conditions.

9. How does it work? • Air enters the top of the carburetor and is mixed with liquid fuel.

10. How does it work? • The air fuel mixture is forced into the intake manifold by atmospheric pressure and burned in the combustion chamber of the engine.

11. Air-Fuel Mixture • The mixture will vary depending on the conditions. The proportion is given as the number of pounds of air compared to the number of pounds of gasoline.

12. Air-Fuel Mixture • At normal operating speed a small engine will use an air-fuel mixture of about 15 pounds air to 1 pound of gasoline

13. Pressure Differences • Carburetors work on the principle of air pressure differences. When discussing pressure differences we will talk about • Vacuum • Atmospheric Pressure • Venturi

14. Vacuum • An absolute vacuum is an area completely free of air or atmospheric pressure. • Although an absolute vacuum is not reached in a small engine, any pressure less that atmospheric pressure is generally referred to as a vacuum

15. Atmospheric Pressure • Atmospheric pressure is the pressure produced by the weight of air molecules above the earth. • A partial vacuum is produced by the piston on the intake stroke. When the intake valve opens, atmospheric pressure forces air through the carburetor to fill it.

16. Venturi Principle • A venturi is a restriction in an air passage that increases air speed or velocity.

17. Venturi • This increase in velocity reduces pressure causing fuel to be drawn into the air stream. • Particles of fuel are vaporized by air rushing through the venturi.

18. Carburetor Theory (con’t) • Airfoil low pressure airfoil high pressure

19. Because the main discharge nozzle is extended into the airstream, an airfoil is also created, further lowering the pressure “above” the nozzle. • If an engine ran at only one speed under ideal conditions, the story of carburetion would end here.

20. The carburetor must be able to adjust the air/fuel mixture to the conditions presented.It must run smoothly and economically at widely varying speeds. • Richer mixture--more gas added to mixture. • Leaner mixture--less gas added to mixture. • To make these adjustments, a threaded needle valve is added at the bottom of the main discharge nozzle. • This regulates the amount of fuel that gets to the venturi

21. To further regulate the mixture, two “air regulators” or butterfly valves are also added: • These restrict the amount of air flow through the carburetor--either manually or automatically. • This action decreases the power and speed and the richness of the mixture within the engine. • Throttle valves restrict air movement at all speeds and are generally manually controlled.

22. Throttle Open Throttle Closed

23. Choke valves restrict air movement at start-up to allow for a richer mixture and can be manually or automatically engaged.

25. To further regulate the air/fuel mixture at lower speeds, another threaded needle valve is added. • This allows the engine to run smoothly and economically at very low speeds by allowing a slight bit of fuel to slip past the throttle valve. • Note: At low speeds, the throttle valve is fully closed. • Idle speed should be 1750 RPM on all B&S engines.

26. Vaporization • Although the venturi breaks the fuel into fine particles, it is further vaporized by the heat of the engine in the intake manifold and by the swirling action of the air in the combustion chamber.

27. Combustion • Cold fuel is difficult to vaporize, this is why we choke or prime a cold engine to help get it started. • Over choking or priming can cause raw fuel to be pulled into the combustion chamber resulting in bypass or a condition known as flooding.

28. Summary • Since a gasoline engine does not operate on liquid fuel, it is the responsibility of the carburetor to provide it with an air-fuel mixture. The carburetor operates on the principles of the following: • Vacuum • Atmospheric Pressure • Venturi

29. Types of Carburetors • Gravity Feed • called a “Flo-Jet” by Briggs & Stratton • Vacuum Feed • called “Vacu-Jet” • Pulse Feed • called “Pulsa-Jet”

30. Flo-Jet- Gravity feeds or float type. • Vacu-Jet-Suction feed-Tank is usually below the carburetor and must have atmospheric pressure to operate. • Pulsa-Jet- Utilizes a fuel pump to distribute the fuel.

31. Flo-Jet Carburetors • Gravity feed system • fuel flows by gravity to the carburetor • gas tank must have a vent hole to provide atmospheric pressure to “push” fuel to carburetor • The float is found in the bowl. • As fuel flows into the bowl, it raises and lowers the float.

32. Flo-Jet Carburetors (con’t) • The float is attached to a needle valve. • The needle, along with its seat, work together to turn on and off fuel flow to the main discharge nozzle. • Float level • should be high enough to allow an ample supply of fuel at full throttle, yet remain low enough to prevent flooding and/or leaking. • should be set at 5/16 of an inch on our engines.

33. Flo Jet Float Type

34. Another type of Float Carburetor

35. Pulsa-Jet Carburetors • Only “true” fuel system • contains a diaphragm type fuel pump and a “constant level” fuel chamber • Newest design of carburetors for small engines • can obtain just as much (or more) horsepower as older, more complicated float-type carburetors • This is due to the fact that it provides a constant fuel level directly below the venturi • very little “lift” is needed to get the fuel to the carburetor

36. Pulsa-Jet Carburetors (con’t) • The venturi can be made larger, allowing more air/fuel mixture into the engine allowing an increase in horsepower within the same sized engine. • See diagram for operating principals

37. Pulsa Jet

39. Vacu-Jet Carburetors • Fuel tank is below the carburetor • Again, atmospheric pressure is employed to help get the fuel to the carburetor. • Air pushes down on the fuel in the tank, when the piston starts downward on the intake stroke, it creates an area of pressure that is lower than that of the atmospheric pressure. This causes the fuel to rise through the pickup tube and travel toward the main discharge nozzle.

40. Vacu-Jet Carburetors (con’t) • Vacu-Jet carburetors require a richer mixture setting because the fuel system “lags” behind the fuel requirements of the engine at the high speeds that most small engines run at. • As the throttle closes to idle, the leading edge takes a position between 2 different sized discharge holes that are found on the main discharge nozzle. • The larger of the holes now becomes an area of high air pressure and the flow of fuel ceases.

41. Vacu-Jet Carburetors (con’t) • The smaller hole now becomes an area of lower pressure and fuel continues to flow--just enough for the engine to idle. • Choke valve is also different: • There are many problems with this choke system including sticking open and closed, especially when remotely controlled.

42. Linkage • There are many set ups for the throttle linkage and the governor • Always note and record the linkage before dis-assembly

43. Carburetor Adjustments • The following applies to many Briggs and Stratton Carburetors, however, many carburetors have no adjustment

44. Carburetor Adjustments • Adjustment of the needle valve for maximum power • Causes overheating, early or late detonation, and short valve life • No accelerator pump in Flo-Jet and Vacu-Jet models so engine will “kill” if throttle is opened suddenly. • “Floods” the engine causing “raw” fuel into the combustion chamber (cylinder). • dilutes the crankcase oil washing away the film on the cylinder wall causing “scuffing”and lost compression.

45. Carburetor Adjustments (con’t) • Throttle Adjustment • To adjust for maximum power and efficiency: • turn the screw in (clockwise) until the engine speed decreases. • This indicates a lean mixture. • Turn the screw out (counter-clockwise) until the speed increases and again decreases • This indicates a rich mixture.

46. Carburetor Adjustments (con’t) • Slowly turn the main needle valve back clockwise to a point midway between the 2 readings • This method can also be used to set the maximum speed the engine will run by first opening the throttle all the way.

47. Carburetor Adjustments (con’t) • Idle Speed Adjustment • Very similar to Throttle Adjustment • IS NOT necessarily the slowest speed at which the engine will run. • Can be any speed you choose

48. Carburetor Adjustments (con’t) • Again, turn the screw in (clockwise) until the engine speed decreases. • Lean mixture • Turn the screw out (counter-clockwise) until the speed increases and again decreases • Rich mixture • Usually, the idle adjustment needs to be reset with each new application of the engine.

49. Gas, Oil, & Air • Gas-- • use fresh, clean, unleaded gasoline with a minimum of 77 octane • In Minnesota, the minimum octane allowed by law is 87. • Purchase an amount that can be used up within 30 days. • Ethanol an/or methanol is fine to use in today’s small engines.

50. Gas, Oil, & Air (con’t) • Oil-- • Detergent oils keep the engine free of gum and varnish deposits and generally keep the engine cleaner. • No other additives should be used in the oil.