You should not have to touch the lowend needle at this early stage,

First up you should check your fuel lines for leaks, are there any air bubbles in the fuel lines?

if the fuel lines are alright next thing you need to check is that your glowplug it not needing replaced,

there should be a good glow in the center of the plug with the igniter attached, if this is fine

try resetting the carb back to factory settings which i think may be 3 turns out on both needles,

thats what it was on one of my sh28 engines i use to have,

you also say that it will not run for very long when you manage to get it started,

try opening up the idle if you remove the filter and look into the carb you should see a 1mm gap in the carb,

if you can get it to start at these settings and it will not stay running try leaning out the high speed needle slightly,

just so as it keeps running,

after you get it to start, drive the truck slowly not revving past half throttle for few tanks before trying to lean it out anymore,

after about five or six tanks you can start to lean the high end out a little more,

let us know how you get on