Post by MCS on Mar 21, 2009 21:39:52 GMT -5
It has been mentioned a couple of time on the forum that some theory would be nice. Well here goes..
Both the Tillotson and Walbro web sites have technical pages on how their carburetors work so I will just point out some things that may be of interest. They are really a simple design but they are quite different from a float design in that they run in "all positions." The Tillotson HS and the Walbro SDC were popular in the good Homelites and they have similar operating designs.
A diagram of a HS carburetor
Fuel Pump
A flat flexible material with two flapper valves. Crankcase pressure and vacuum are applied to the one side of the diaphragm and as the piston goes up and down the diaphragm goes up and down. As it goes up, one valve seals and the other opens and fuel is drawn into the fuel chamber and as it goes down, the valves change state and the pump builds pressure against the closed inlet valve. The inlet valve is controlled by the metering circuit and when it is open, fuel will pass through the screen to the metering chamber.
Metering Circuit
There are four main parts: the diaphragm, the inlet valve, the lever, and the spring. The diaphragm has atmospheric pressure on one side and throat/venturi vacuum on the other side. The diaphragm and spring affect the operation of the inlet valve. This is where things get a little strange - especially if you compare it to a float carburetor.
When the saw is off, not running, there is no vacuum in the carburetor throat so there is equal pressure on both sides of the diaphragm. One side is through the hole in the cover and the other side is through the main and idle ports. The inlet valve is held closed by the spring.
Starting the saw
With the choke closed and the throttle locked open, cranking the engine creates a low pressure in the throat which is transmitted to the metering chamber through the idle and main ports. Atmospheric pressure will then force the diaphragm up and open the inlet valve. Continuing to crank the saw will fill the metering chamber with fuel, and when full, fuel will be drawn into the carburetor throat through the main and idle ports.
Idling
At idle the butterfly is closed enough to block the secondary idle port. Note the two idle ports in the picture. The idle mixture screw will control the fuel to the idle circuits. Since there is a pressure difference between the two idle ports, some air will be drawn into the idle circuit through the secondary idle port.
As the throttle is opened, fuel will be drawn out the secondary idle port too.
High Speed
As the engine speeds up and air flow increases in the carburetor, the venturi creates a low pressure area which causes fuel to flow out of the main fuel port. The Main fuel needle controls how much fuel can flow into the high speed circuit.
Back to the Metering Circuit
There is a constant balancing act going on in the metering circuit while the saw is running . First, the fuel pump has to be able to replenish the fuel as fast as it is being removed through the main and idle circuits. Second, the fuel side of the diaphragm - the metering chamber - has close to atmospheric pressure because of the flexible diaphragm. The pressure differential occurs between the metering chamber and the carburetor throat/venturi because of the restrictions of the mixture screws and the main and idle ports. This is what make the fuel flow.
Welsh Plugs and the Main Port - Welsh plugs are in the carburetor to plug up areas that were created during manufacturing but need to be closed during running. Most of the time these never have to be removed but they do give access to the main and idle ports. Also under the main welsh is a screen (HS) or a check valve (SDC) depending on who manufactured the carburetor. The screen is there to act in a similar fashion to a check valve as the SDC has. When the throttle is closed and there is no venturi vacuum, the wet screen or the check valve prevents drawing air into the metering circuit through the main port. The screen in not there in the HS to filter the fuel but it can get plugged up and block fuel to the main port. If you look at the picture, you can see where the screen is in the main port. It is the dark line just above the main mixture screw.
The diaphragm lever is adjusted in both of these carburetors so it is flush with the bottom of the metering chamber. Never stretch the spring or use any substitute spring. When assembling the carburetor the gasket goes next to the body, then the diaphragm, and then the cover. Incorrect assembly will prevent proper operation of the metering circuit.
If a saw runs with the mixture screws turned down, either fuel is getting past the welsh plugs or the mixture screws or seats are damaged.
If a saw runs lean and nothing is blocked in the mixture area or ports, then something is wrong in the metering circuit or maybe the fuel pump.
And, of course, if the saw start and runs but dies, you have to look at the fuel filter in the tank, the pump, or the inlet screen.
Also, in these two cycle engines the crankcase cannot leak. There are crankshaft seals and gaskets that prevent this and the reed valve if equipped. A leaking crankcase can screw up the fuel pump operation and prevent the required vacuum from developing in the carburetor throat. Bad rings also come into play!
Both the Tillotson and Walbro web sites have technical pages on how their carburetors work so I will just point out some things that may be of interest. They are really a simple design but they are quite different from a float design in that they run in "all positions." The Tillotson HS and the Walbro SDC were popular in the good Homelites and they have similar operating designs.
A diagram of a HS carburetor
Fuel Pump
A flat flexible material with two flapper valves. Crankcase pressure and vacuum are applied to the one side of the diaphragm and as the piston goes up and down the diaphragm goes up and down. As it goes up, one valve seals and the other opens and fuel is drawn into the fuel chamber and as it goes down, the valves change state and the pump builds pressure against the closed inlet valve. The inlet valve is controlled by the metering circuit and when it is open, fuel will pass through the screen to the metering chamber.
Metering Circuit
There are four main parts: the diaphragm, the inlet valve, the lever, and the spring. The diaphragm has atmospheric pressure on one side and throat/venturi vacuum on the other side. The diaphragm and spring affect the operation of the inlet valve. This is where things get a little strange - especially if you compare it to a float carburetor.
When the saw is off, not running, there is no vacuum in the carburetor throat so there is equal pressure on both sides of the diaphragm. One side is through the hole in the cover and the other side is through the main and idle ports. The inlet valve is held closed by the spring.
Starting the saw
With the choke closed and the throttle locked open, cranking the engine creates a low pressure in the throat which is transmitted to the metering chamber through the idle and main ports. Atmospheric pressure will then force the diaphragm up and open the inlet valve. Continuing to crank the saw will fill the metering chamber with fuel, and when full, fuel will be drawn into the carburetor throat through the main and idle ports.
Idling
At idle the butterfly is closed enough to block the secondary idle port. Note the two idle ports in the picture. The idle mixture screw will control the fuel to the idle circuits. Since there is a pressure difference between the two idle ports, some air will be drawn into the idle circuit through the secondary idle port.
As the throttle is opened, fuel will be drawn out the secondary idle port too.
High Speed
As the engine speeds up and air flow increases in the carburetor, the venturi creates a low pressure area which causes fuel to flow out of the main fuel port. The Main fuel needle controls how much fuel can flow into the high speed circuit.
Back to the Metering Circuit
There is a constant balancing act going on in the metering circuit while the saw is running . First, the fuel pump has to be able to replenish the fuel as fast as it is being removed through the main and idle circuits. Second, the fuel side of the diaphragm - the metering chamber - has close to atmospheric pressure because of the flexible diaphragm. The pressure differential occurs between the metering chamber and the carburetor throat/venturi because of the restrictions of the mixture screws and the main and idle ports. This is what make the fuel flow.
Welsh Plugs and the Main Port - Welsh plugs are in the carburetor to plug up areas that were created during manufacturing but need to be closed during running. Most of the time these never have to be removed but they do give access to the main and idle ports. Also under the main welsh is a screen (HS) or a check valve (SDC) depending on who manufactured the carburetor. The screen is there to act in a similar fashion to a check valve as the SDC has. When the throttle is closed and there is no venturi vacuum, the wet screen or the check valve prevents drawing air into the metering circuit through the main port. The screen in not there in the HS to filter the fuel but it can get plugged up and block fuel to the main port. If you look at the picture, you can see where the screen is in the main port. It is the dark line just above the main mixture screw.
The diaphragm lever is adjusted in both of these carburetors so it is flush with the bottom of the metering chamber. Never stretch the spring or use any substitute spring. When assembling the carburetor the gasket goes next to the body, then the diaphragm, and then the cover. Incorrect assembly will prevent proper operation of the metering circuit.
If a saw runs with the mixture screws turned down, either fuel is getting past the welsh plugs or the mixture screws or seats are damaged.
If a saw runs lean and nothing is blocked in the mixture area or ports, then something is wrong in the metering circuit or maybe the fuel pump.
And, of course, if the saw start and runs but dies, you have to look at the fuel filter in the tank, the pump, or the inlet screen.
Also, in these two cycle engines the crankcase cannot leak. There are crankshaft seals and gaskets that prevent this and the reed valve if equipped. A leaking crankcase can screw up the fuel pump operation and prevent the required vacuum from developing in the carburetor throat. Bad rings also come into play!
