Yes this screw. So this screw missing wouldnt cause a air leak for the intake? Is this where the jet is located? Is this just a screw that I could stick a 6mm bolt into or is it a special design? Is it meant to be an adjustment or just a access screw for the jet? I was adjusting it maybe why it fell off.This screw? That is the power jet access, it normally wouldn't stop the engine from starting but being lean all ready this would for sure make it more difficult, Treatland would have the part, or any small 6mm cap screw would work.
Carb's are carb's they all basically work the same and require being jetted/tuned to the application, this carb one of my favorites has 5 points to be tuned the pilot jet, air screw, needle, main jet and the power jet. Tuning starts from the bottom up pilot jet first and the air screw, with a proper sized pilot jet the air screw should be around 2 turns out and idle properly then the needle is for tuning the transition from idle/pilot jet and upto 3/4 throttle then it's the main jet upto full throttle and the power jet adds fuel at WOT when the velocity is high enough to pull the extra fuel, and this is to help when on the pipe so as not to go lean at peak rpm.
You could try a pumper/diaphragm type carb as it needs no jet changes but you still need to tune it adjusting both the slow and high speed needles (much easier) but then they need a pulse line fitted to the case or base of cylinder and if the engine flows enough a secondary fuel pump may be required.View attachment 160100