From the manual (had it downloaded from when I had an SH28 quite a few years ago):
HIGH SPEED NEEDLE VALVE
The high speed needle valve is used to meter the air/fuel mixture at
full throttle. Using a flat blade screwdriver, turn the needle clockwise
to lean the mixture or turn the needle counter-clockwise to richen the mixture. When you start the engine for the very first time, the needle valve should be turned in completely, then backed out 3 full turns. This is the factorybreak-in setting.
LOW SPEED NEEDLE VALVE
The low speed needle valve meters the air/fuel mixture at idle and during transition from idle to full throttle. Turn the low speed needle clockwise to lean the mixture. Turn it counter-clockwise to richen the mixture. The low speed needle valve is preset from the factory, but minor adjustments may need to be made to suit your application: fuel used, glow plug and environment all contribute to the setting.
To reset the low speed needle valve back to the factory setting, follow these simple steps:
l Open the carburetor barrel completely.
l While holding the barrel open with your finger, use a flat blade
screwdriver to turn the low speed needle valve clockwise until it stops.
From this point, turn the low speed needle valve counter-clockwise
3 full turns. This is the factory break-in setting.
I just uploaded the manual to my FTP site so you can download it:
SH 28 Manual