This is something I wrote for another forum. Someone had asked me about 3 needle carbs. Hope this helps.
I have never understood why so many magazines and engine tuners have chosen to surround the three needle carb with so much mystery. I believe it is a great adjustment that when understood correctly allows the racer to adjust to a finer degree where the engine makes it’s power in the lower RPM range, but to understand it fully I think we should use the correct terminology. Fuel goes through the high end “needle,” which is actually a valve which controls the total amount of fuel that enters the carb., then passes through the mid range “needle,” which is also a valve, then goes around the low end needle. The “mid range” or fuel metering valve, is the female counterpart to the male low end needle. All carbs have this “mid range” valve. Only some are adjustable. Remove your air filter and look into the carb venturi. When the carb is opened from idle you will see the low end needle pulling out of the fuel metering valve. There are two things happening here. The air supply and the fuel supply are increasing at the same time. You can also see that the fuel is being introduced into the air stream at a specific point. Suppose you unscrew your fuel metering valve 3 full turns while screwing in on the low end needle 3 full turns, the ratio of fuel to air stays the same, but the air slide must open further before it exposes the point at which the fuel is introduced into the air stream. Now the carb has to open further before the engine starts to make it’s power, and this gives it the effect of moving the mid range around. The same is true in reverse.
Remove your entire high end valve assembly, and look down into the hole where it screws into the carb body. You should be looking directly into the orifice in the fuel metering valve that the fuel must pass through to enter the carb, if not then adjust the fuel metering valve until you see it. At the most this should only be ¼ of a turn. Once the orifice is inline with the high end valve so that fuel can enter it directly after passing through the high end , score a mark on the outside of the carb body inline with the fuel metering valve adjustment screw to indicate externally that they are inline. Turn the F.M.V. adjustment screw another 90 degrees and you will see another orifice in the F.M.V. Mark it’s location the same way. You now should have two marks on the carb body that will indicate that the orifice is properly aligned with every 90 degrees/¼ turn of the F.M.V.
Now when you go to the track, and you get your engine tuned correctly and you want to adjust where the engine starts to pull you can make adjustments to the low end needle and the F.M.V. in ¼ turn increments. Keep in mind that the F.M.V. must always line up with the marks you made, and when changes are made to the F.M.V. the low end must always adjust the same amount and in the same direction as the F.M.V. or it will change your fuel mixture on the bottom I.e. if the F.M.V screws in two turns the low end needle screws out 2 turns.
Just for kicks here’s my routine for tuning an engine:
1. Get engine to temp (190+)
2. adjust top end from rich to lean (engine screams with faint smoke trail)
3. Adjust bottom end from rich to lean (bring car to stop, engine idles for 10 full seconds, from stop pull full throttle engine should take off with puff of smoke and no hesitation. Bring car to stop again and pinch fuel line close to carb. Engine dies after 5 seconds = too rich. Engine dies before 3 seconds = too lean. Engine dies between 3-5 seconds = just right Goldilocks!
4. Set idle
5. Go whoop butt!!
Thanks for the questions and good luck in racing. -Joey