Depends on which carb you have. If it’s a Carter model N, the initial setting for both needles is approx 3/4 turn open. For the Tillotson MD the settings are 3/4 on the idle mix, and 1 1/2 turns on the main. On the MD, counter-clock rotation on the idle mix screw LEANS the micture..

Ya got me there ! I just know that’s a (some) Tillotson’s quirk. I suppose it’s a design ‘feature’.

I have a ’55 Scott-Atwater 16hp. It has the MD43B carb. Look on the oppisote side where it says Tillotson and the number should be there.
Ran for the second time just this past weekend. Mine idled very nicely with the top knob turned out 5/8 of a turn from gently closed. I did not get an opportunity to adjust the high speed as I was running on the Tippecanoe River in Montecello Indiana. 5mph speed limit. But just once I opened it up, not all the way, just enough to get her moving… and slid to the middle seat….. GPS said I hit 18.5 and it was still gaining speed before I shut it down…It definitely needs more adjusting at high speed. Hoping for about 25 or so, not sure if I will get it but hey one can hope… I am lucky both impellers are at the lower unit and it pumped water VERY well. Hope this helps.

Richard