ceecrb1

the very simple explanation is its doing the fuel to air mixture for the lower half of the throttle positions..

Altering this will effect the idle stability and the smoothness of transition from low to high rpms.

A basic way to tune an engine is just simply, set the high rpms first with a tacho... get highest rpm then richen to add just to remove a few rpm so it does not lean out and overheat.

Next just start lowering the throttle, if you find it becomes unstable, stop lowering then open or close the low speed needle till its better... then continue lowering the gas.... repeat..

now you should be able to give it sudden stabs at the gas and it should neither hesitate or stop..
If it hesitates, you can lean it a little.. but I mean just a TOUCH of the screw at a time.
If it stops, it is too lean so open.. but again, a touch at a time.
It is ok for it to hesitate just a TINY bit.. again this will keep it rich and help prevent overheating.

Once you have a good transition, re-check the high end.. just incase.

This method will give you the best reliability in the air... other methods will give you better strength etc etc but this has always worked for me and it keeps things simple..... plus the "theory" works in all motor types, from my os55ax to my 3d gasser.
(before people mis-quote me I KNOW it is not the same..... but i'm trying to simplify things).

Every change you make must be as small as you can make them and once the engine has warmed up... also dont try to rush it.
make a change... wait a few seconds, try things out a little THEN make another change... Just give it a few seconds longer than you think it needs.. just to be sure you are testing the change... and not the moment of transition between was was there and what you last changed... ie give it time for the changes to take effect.
dont go wild with the screwdriver.. you will just chase yourself.


I think it would be very hard for any manufacturer to be able to tell you vie email or phone how many turns the needle will need.. it depends on everything, prop type and size, fuel (mix, age and brand), weather, cone or no cone, diameter of fuel tubing, if in a closed cowl or not etc etc etc.