Since the 21BB engine has a 3 needle carb, you have to be a little careful that the low and mid speed needles don't hit each other when adjusting the carb. Here's how to do it, follow the steps in this order:
Idle Adjustment: Rotate the Idle Adjustment Screw (setting "C" on page E-5) so that the carb opening is 2mm. This setting is like a doorstop, it prevents the carb from closing so far that the engine shuts off when you apply brakes.
Mid Speed: Open carb all the way, screw in gently clockwise until the screw stops, don't force it! Then unscrew counterclockwise 3 1/4 turns, at that point it should be flush with the side of the carb.
Low Speed: Close the carb all the way (except for the 2mm gap for Idle Adjustment), screw in Low Speed screw gently clockwise while watching the Mid Speed screw, stop when the Mid Speed screw starts rotating. Then open the carb all the way and unscrew the Low Speed screw counterclockwise 3 1/2 turns.
High Speed: Carb can be open or closed, screw in gently clockwise until the screw stops, don't force it! Then unscrew counterclockwise 3 turns.
See page E-5 of your 21BB instruction manual if you're not sure which needles are which.
The actual settings that work best for your engine will vary depending on the type of fuel, altitude, humidity, age of the engine etc., those settings are starting points that should get the engine running fairly well.
Here's a good article on carb tuning from www.rcnitro.com:
Super Engine Tuning
Answers to all of your Frequently Asked Questions can be found here.
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