Maybe you should try tuning it first?
one screw is for the air/fuel mix (turn clockwise for more fuel and anticlockwise for more air) and the other screw is for tick over speed, it opens the butterfly inside, normally by putting pressure on the choke or the accelerator, this is pretty standard for all carbs.
before you start take the air cleaner off and get a clean rag like an old T-shirt or something, then (with the engine running) use the accelerator cable open the carb butterfly until the engine is runnign pretty fast (about 3000rpm (ish) then stuff the rag over the inlet to the carb, until the engine almost dies, and then take it off, never move the accelerator cable, just keep it where it is and the engine will stutter back into life, do this whole process twice, it basically runs with no air and only petrol which totally cleans out the galleries and stuff in the carb so you know its good before you start to tune it. It can solve a myriad of running probs too !
then your ready to start twiddling, in an ideal world you'll have a co2 meter but in the drive way you can use your ears....
let the engine get to operating temp then with the engine off turn the air/fuel mixture screw all the way anti clockwise and count the number of turns or half turns until its tight, but dont do it FT just until it stops turning with light pressure then unwind it (clockwise) two and a half turns.... this is a good starting point.
start the engine and listen to it, if its a little uneven sounding or slow turn the screw one half a turn in either direction and see which way makes the engine sound better, then try turning it half a turn in the same direction again, keep going until it starts to sound worse, then backit up half a turn to the good spot again.
then you need to set the idle speed, after setting the air/fuel mix you need to turn the idle speed in or out untilt he engine is ticking over at about 800-950rpm, you'll know when it sounds right, clockwise will speed the engine up and anti clockwise will slow it down.
thats a basic tune but for best results get it done with meters etc etc.
