Influence of L screw on full-load fuel voume.

[Megatron]

As I said in the MS201T thread, on some carbs the low speed screw will influence the full-load fuel volume.

 

Idle system independent of main jet system

 

In these carburetors the main jet and idle systems are arranged in parallel (separate fuel passages for H and L screws). Changes in the idle fuel volume mean a change in the full-load fuel volume. If the idle fuel volume is altered with the low speed screw (L), it is necessary to readjust the high speed screw (H).

 

When the idle setting is made leaner, the overall fuel-air mixture becomes leaner too. This increases engine speed as well as the risk of engine damage as a result of overheating.

 

Note: different model carbs for the same model saw can vary with respect to having dependent or independent idle systems. E.g. the Zama C3M carbs for the Stihl 044 have independent idle systems, however, the Walbro HD carbs for the 044 have dependent idle systems.

Edited September 30, 2011 by Megatron
Formatting

[Megatron]

Idle system dependent on main jet system

 

In these carburetors the idle system branches off the full-load system after the high speed screw. It is not directly connected to the metering diaphragm chamber. Full-load fuel flow is determined by the high speed screw (H). Adjusting the low speed screw (L) to change the idle fuel volume does not result in a change to the overall fuel volume.

[Logan]

on Stihl models, are the dependant idle carbs less common generally?

[Megatron]

on Stihl models, are the dependant idle carbs less common generally?

 

Actually it is the opposite. The carbs with idle systems dependent on the main jet system are more common nowadays. Generally the independent ones are on older models or belong to older saws.

 

I meant to say before the full-load semi-fixed jet is not an exclusive feature of the dependent carb, you can find it on independent ones also. I am not aware of an independent carb having the part-load jet though, maybe due to design or again due to age?

[cheshire cat]

ever done any pop testing and if so what do you think of it as a guide/starting point

[Megatron]

ever done any pop testing and if so what do you think of it as a guide/starting point

 

The pop-off and re-seat pressures will vary from carb to carb and are not that important. It only matters that that the inlet needle does open and reseat at a pressure of more than about 10PSI. This will ensure the fuel pump pressure does not force the needle to open under operating conditions where the pressure from the fuel pump is usually 5-7PSI. You must remove the metering diaphragm when performing this test otherwise the pressure required to un-seat the inlet needle can damage it. Also the carb should be flushed with fuel before any testing.

 

Usually I just perform a leakage test on the carb, intact. Again flush the carb with fuel to wet the fuel passages and moisten the diaphragms. Apply a pressure of about 11PSI via fuel hose. If this pressure remains constant for a few minutes you can assume the carb is airtight.

If the pressure drops;

The inlet needle is usually not sealing which is usually caused by foreign matter in the valve seat or sealing cone of the inlet needle is damaged or inlet control lever is sticking. Also check the metering diaphragm for distortion and replace if necessary.