H/L screw confusion

[Megatron]

When setting a carb, always set the low speed screw (and idle) first, before the high speed. This is because unless the idle system is dependent on the main jet system alterations in the low speed will affect the mixture at high speed.

 

Idle system indepentent of main jet system;

In these carbs the main jet and idle systems are arranged in parallel. Changes in the idle volume mean a change in full load volume. If the idle fuel volume is altered with the low speed screw, it is necessary to readjust the high speed screw.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.

 

 

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. Adjusting the low speed screw to change the idle fuel volume does not result in a change to the overall fuel volume.

 

In other words always tune L before H, just to be on the safe side.

 

Unfortunately, tachs are not much help in tuning non-stock saws. Especially ported ones, as one of the aims is to move peak power and torque to a higher rpm. So setting with a tach to factory spec is not appropriate.

Tachs are no good on rev-limited saws either, as soon as the saw starts bouncing off the rev limiter the tach goes nuts.

Best ways of tuning are by ear (if experienced) or "in the wood".

[s.varty]

This thread is mainly for future reference, the screws on the carbs on stihl saws. Am I right that the h is for top end revs and the l is for rich/leanness? If I open up an exhaust on a saw and it needs richening, I turn the L screw up right?

 

I'm a bit foggy with the details. Cheers!

 

Only alter those screws if you really need to, 9 times out of 10 your air filter needs cleaning or replacing,making the engine run rich as its not getting enough air.

[Cut4fun]

I'm not good at explaining. This Madsen's link explains H and L to a t.

 

Saw Carb Tuning

[RobRainford]

I've got a feeling a trip to the local saw boffins would probably save time and money if it went belly up!

 

All this info is good for learning. My ear is not set so I would probably make a mistake!