ORIGINAL: Flyboy Dave
Both of you have to realize that both needles influence the part throttle mixture.
Yes they certainly do "influence" partial throttle settings.
But consider this....
"If" you have the carb adjusted properly, top and bottom, the engine
will run crisp and clean from idle to full blast. This is because the carefully manufactured tapered
needle see to that chore. Sure, if the top end is set too rich, it will throw off the whole range.
If the low speed is set too rich, the engine will "load up" and will be running too rich as you
advance the throttle to a point where it will clear out....providing the high speed is set correctly.
If the high speed is set too rich as well, the engine will be sloppy everywhere.
The key is the taper on the needle takes care or everything between idle and full speed, and will
do a really good job of it....as long as the low and high speed are set correctly, the needle takes
care of the rest. Trust me, that's how it works.
FBD.
Sure
"If" things are set correctly we wouldn't be having this discussion and everyone buying an engine wouldn't have trouble. The point was to clarify how the carb works.
Please correct the following statements if you believe them to be wrong.
1. The first carb adjustment is wide open throttle mixture. Run the engine wide open, peak it with the main needle and back off as much/little as you like. For most engines with a given prop and atmospheric conditions, this setting will not change(essentially fixed as Dar says).
2. Second, throttle back and idle for a while, open throttle and check transition. If transition is poor, adjust low speed needle to correct.
3. Some carbs will required you repeat steps 1 and 2 a few times to dial in the carb because the idle needle influences the high speed needle, Saito comes to mind.
4. If you are really picky about the running mid range mixture (not transition) this can be adjusted via the idle needle, but you must check transition afterwards to make sure it is acceptable.
All of this changes depending on fuel supply pressure, nitro content, even oil content in the fuel. It's all one huge compromise and relies mainly on proper carb design to achieve a good running midrange mixture. Tuning it wil standard fuel mixes is simple as long as you follow the above steps.
Dar, maybe you haven't ever tried as it's not normal adjusting procedure, but if you open the main needle at part throttle it does have some influence on the mixture. It is still a restriction(not as much as the low speed circuit at part throttle) and there is a small pressure drop in the fuel flow past the main needle. It is in series as you say, so it is not left out of the equation, but the influence is smaller. I experience it starting my four stroke diesel conversions. To start, I open the main needle 1/2-1 turn to richen the mixture. I start at 1/4 throttle. If I close the main needle to the running setting (still at 1/4 throttle) before it warms up, it quits. I have seen this with air bleed and OS and Magnum two needle carbs.