As I understand it, both affect the ratio of fuel vs air. Is there a conversion function between them? e.g. each needle position is equivalent to 0.010 float height?
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Pretty well sums up carb tuning! Voodoo and black magic!
There are no tables, no charts, no go-bys. Every carb and engine behaves a little different. Sooo….testing and data gathering is all you have to work with. The thing is, no two days are the same conditions and no two tracks are the same elevation, so the air isn’t ever exactly the same.
I set my float and never touched it again for tuning. I would adjust the needle only based on weather at the track. With our series we would see air density altitudes from as low at 500 ft below see level (New Orleans or Houston on a cool dry high pressure day) to 6500 above sea level (Amarillo on a very hot, and unusually damp low pressure day). It’s a hopped up lawn mower engine!!! That 0.1 HP isnt worth the chase.
I’ve never understood the carb height affecting AFR. The bowl holds X amount of fuel. As the fuel is used it’s replenished constantly until the fuel level reaches a point where the float closes the needle and seat. Usually when the throttle is closed and fuel isn’t being greatly used.
The needle and seat are sized to be able to pass more fuel that the engine can suck at full throttle.
If you can get the float to open / close right at the main jet intake hole then maybe, just maybe, you can get the engine to run lean at a certain throttle position.
But I can’t see it as a constant change in AFR.
Can someone please explain how it works.
Thanks
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I don’t know how much the float height affects AFR, but I understand how it does. The higher you lift a fluid the more powerful pump you need. As you lower the float height the fuel has to be drawn a greater height up the jet. Since the air flow has a limited amount of energy to pull the fuel up the jet it pulls it up more slowly the lower the float height. As a result the AFR is leaner. How much, I don’t know. The affect on HP for LO206, I also don’t know
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Hi Todd, thanks for responding. I think I now get it.
the fuel is pushed up the jet by atmospheric pressure to the float level. Let’s say to 2mm below the top of the needle tube. At this point the low pressure caused by the venturi sucks the fuel out of the needle jet.
But if you lowered the float level by 2mm, then the fuel level in the needle jet would be the same level and now 4mm below the top of the tube. That would mean it would have a harder time drawing fuel out of the needle jet and hence run leaner.
Really high or really low float heights would of course either create a flooding or starvation situation.
I think we both would like to know the full explanation but my “Internet degree” in Fluid Dynamics says that makes sense. 
Plausible?
Absolutely plausible. You are correct it is the atmospherice pressure pushes it. The venturi only creates a lack of pressure. The difference in pressure allows the atmosphere to push the fuel to the jet.
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