the screw opens up the secondary throttle early allowing fuel from the secondary carb to enter the engine its probably just enough to get rid of the flat spot on your bike the ydis carb set up is designed to keep those flat spots from happening its a bit like variable valve timing the engine needs different volumes of fuel/air mixture through the rev range but its not a linear curve so the two part carb is a way around that problem.
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XT 630 SUPERMOTO
GSXR750L  KMX200
RF900 & GSX400X Impulse Projects
3.9 V8 Rangerover
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