Lean idle. Rich cruise.
A wonderful explanation from Tuner off of another site where the question was asked “how does bleed air effect the booster signal?”
“Gentlemen,
As always, circumstances alter cases.
For the purpose of a thought experiment look at the carburetor from the often referred to opposite point of view, the venturi doesn’t draw the fuel, the atmospheric pressure pushes it. Instead of thinking of the booster signal as drawing against a resistance or restriction of the fuel and air entering it, consider it in the sense that air pressure is pushing fuel and air through the circuits against the resistance forces of friction and gravity.
As an actual experiment to illustrate how increasing bleed air can increase the liquid fuel flow, put a straw in a longneck bottle with water in it and blow. Obviously, the air bubbles displace liquid and push the water up the neck of the bottle and out the top if you blow hard enough. The atmospheric pressure is pushing air into the bleed system and main well just like your breath is pushing air into the liquid in the bottle. Remember, atmospheric pressure is pushing both fuel and air into the main well and toward the nozzle in the venturi.
If you use a carbonated beverage the air entering the liquid causes a disturbance which causes the CO2 to flash into vapor bubbles in the liquid which further drives the liquid up the bottle neck and illustrates how the vapor pressure of the more volatile components in fuel can affect the liquid flow. Also obvious is how larger amounts of bleed air or vapor can reduce the liquid fuel delivery by occupying a corresponding larger portion of the flow path.
Obviously, the effect of increasing liquid flow is most significant in the range of very low venturi pressure difference (vacuum or signal, if you wish) from atmospheric, which is at and closely above the beginning of flow from the main in most common single 4bbl applications we deal with. A lot of venturi area, such as with multiple carbs or very large carbs on small engines, may operate in such a low pressure differential at maximum engine speed the phenomenon of bleed air increasing fuel flow can be in effect in all operating conditions. Most usually, in my experience with the common Holley-like single 4bbl applications, bleed air richens the low power range at and just above tip-in and leans the high power range closer to full throttle.
Confusion enters the conversation when the definition of operating range isn’t made clear. Sometimes, the bleed effect is considered in reference to WOT only, but from the bottom to the top of an RPM range. Other times, the bleed effect is discussed in the context of percentage of load or throttle position less than full throttle. The two different circumstances can appear to have conflicting requirements in the bleed configuration. In my view, that seems to be the result of working out the bleed arrangement using a dyno and only considering WOT in a narrow RPM range near maximum power. Obviously, the bleeds effects are significant from curb idle to WOT and maximum engine speed.
The bottom line is, you gotta dink with all the little holes until you like how it runs, no matter what size they end up. If the engine likes it, so be it.
As always, circumstances alter cases, but things are more like they are now than they ever have been.
“
A wonderful explanation from Tuner off of another site where the question was asked “how does bleed air effect the booster signal?”
“Gentlemen,
As always, circumstances alter cases.
For the purpose of a thought experiment look at the carburetor from the often referred to opposite point of view, the venturi doesn’t draw the fuel, the atmospheric pressure pushes it. Instead of thinking of the booster signal as drawing against a resistance or restriction of the fuel and air entering it, consider it in the sense that air pressure is pushing fuel and air through the circuits against the resistance forces of friction and gravity.
As an actual experiment to illustrate how increasing bleed air can increase the liquid fuel flow, put a straw in a longneck bottle with water in it and blow. Obviously, the air bubbles displace liquid and push the water up the neck of the bottle and out the top if you blow hard enough. The atmospheric pressure is pushing air into the bleed system and main well just like your breath is pushing air into the liquid in the bottle. Remember, atmospheric pressure is pushing both fuel and air into the main well and toward the nozzle in the venturi.
If you use a carbonated beverage the air entering the liquid causes a disturbance which causes the CO2 to flash into vapor bubbles in the liquid which further drives the liquid up the bottle neck and illustrates how the vapor pressure of the more volatile components in fuel can affect the liquid flow. Also obvious is how larger amounts of bleed air or vapor can reduce the liquid fuel delivery by occupying a corresponding larger portion of the flow path.
Obviously, the effect of increasing liquid flow is most significant in the range of very low venturi pressure difference (vacuum or signal, if you wish) from atmospheric, which is at and closely above the beginning of flow from the main in most common single 4bbl applications we deal with. A lot of venturi area, such as with multiple carbs or very large carbs on small engines, may operate in such a low pressure differential at maximum engine speed the phenomenon of bleed air increasing fuel flow can be in effect in all operating conditions. Most usually, in my experience with the common Holley-like single 4bbl applications, bleed air richens the low power range at and just above tip-in and leans the high power range closer to full throttle.
Confusion enters the conversation when the definition of operating range isn’t made clear. Sometimes, the bleed effect is considered in reference to WOT only, but from the bottom to the top of an RPM range. Other times, the bleed effect is discussed in the context of percentage of load or throttle position less than full throttle. The two different circumstances can appear to have conflicting requirements in the bleed configuration. In my view, that seems to be the result of working out the bleed arrangement using a dyno and only considering WOT in a narrow RPM range near maximum power. Obviously, the bleeds effects are significant from curb idle to WOT and maximum engine speed.
The bottom line is, you gotta dink with all the little holes until you like how it runs, no matter what size they end up. If the engine likes it, so be it.
As always, circumstances alter cases, but things are more like they are now than they ever have been.