10-18-2017, 03:47 AM
Doug,
I think you have over simplified the delicate ba1ance that is maintained by the spring pressure under the fulcrum arm.
It is true that the drop in pressure in the venturi is a significant factor but your remarks left out the other major factor which is the output pressure of the fuel pump. Pressure drop in the venturi will result in movement of the metering diaphragm due to the difference between atmospheric pressure on the dry side and the below atmospheric pressure condition at the venturi. As the delta between the venturi and atmosphere pressure levels grows the force of the diaphragm against the fulcrum arm will grow. Simultaneously, the residual pressure from the pump will be pressing on the tip of the needle that is exposed in the seat to the incoming fuel. The combination of the force on the fulcrum arm and the tip of the needle are what ultimately cause the needle to move off the seat and allow fuel to flow. I use the term "residual" when speaking of the pump output pressure as the pump is out of phase with the intake stroke of the engine. The typical built in pump as seen in the Tilly carbs produces pressure as the piston is moving down the bore and compressing the charge in the case. The pressure drop at the venturi is generated by air flow through the venturi as the piston is moving up the bore and expanding the volume of the case.
"Pop off pressure" is a simple reference point we use to quantify the spring pressure we have chosen to use at any given time. Depending on the carb design and the engine package it is used on the "correct" pop off pressure can range anywhere from as little as 5psi to as high as 30psi. Generally speaking, kart racers tend to gravitate toward setting the pop off pressure as low as possible while still maintaining enough seal to prevent drooling and flooding. However, doing so usually means the resulting mixture adjusting screw settings will be such that the openings for the low pressure signal coming from the pressure drop in the venturi will be muted. In many cases, the overall performance of the carb can be improved by increasing the spring pressure and opening the mixture screws to allow a stronger signal to influence the movement of the metering diaphragm. A very extreme example of the balancing act would be the way I ran my 125cc McCulloch enduro setup back in the late 70s. Due primarily to a very effective slippy pipe design, the BDC 16 double pumper carb pump output pressure on my engines was so high I used 30 psi of pop off pressure to keep the pump from blowing the needle off the seat at certain engine speeds. To compensate for the high spring pressure I would end up opening the mixture needles nearly twice as far as if I ran the pop off down around 15 psi. One interesting aspect of the different combinations of carb tuning setups was that I did not burn any more fuel in an hour race with the jets open twice as far.
To summarize... pop off pressure measured the way we do it in the karting world is simply a reference point to allow is to quantify the changes we make when using different spring pressure under the fulcrum arm. There is no absolute "correct" value for any particular carb. Venturi pressure is an important factor that influences the spring pressure that will work best but it must be considered along with pump output pressure and jetting to gain a better understanding of how and why changing spring pressure can help or hurt carb performance.
BTW... on a closing note.... just to complicate things a bit.... when we discuss the BDC line of Mac carbs there is another major design component that other carbs do not have which is the duck bill circuit that creates an artificial bump in pressure on the dry side of the metering diaphragm under certain conditions. Properly managed the duckbill circuit can overcome the weak signal at the venturi that occurs at low engine speeds when the carb is basically too big for the engine it is installed on. That is a subject for another day.
Steve O'Hara
I think you have over simplified the delicate ba1ance that is maintained by the spring pressure under the fulcrum arm.
It is true that the drop in pressure in the venturi is a significant factor but your remarks left out the other major factor which is the output pressure of the fuel pump. Pressure drop in the venturi will result in movement of the metering diaphragm due to the difference between atmospheric pressure on the dry side and the below atmospheric pressure condition at the venturi. As the delta between the venturi and atmosphere pressure levels grows the force of the diaphragm against the fulcrum arm will grow. Simultaneously, the residual pressure from the pump will be pressing on the tip of the needle that is exposed in the seat to the incoming fuel. The combination of the force on the fulcrum arm and the tip of the needle are what ultimately cause the needle to move off the seat and allow fuel to flow. I use the term "residual" when speaking of the pump output pressure as the pump is out of phase with the intake stroke of the engine. The typical built in pump as seen in the Tilly carbs produces pressure as the piston is moving down the bore and compressing the charge in the case. The pressure drop at the venturi is generated by air flow through the venturi as the piston is moving up the bore and expanding the volume of the case.
"Pop off pressure" is a simple reference point we use to quantify the spring pressure we have chosen to use at any given time. Depending on the carb design and the engine package it is used on the "correct" pop off pressure can range anywhere from as little as 5psi to as high as 30psi. Generally speaking, kart racers tend to gravitate toward setting the pop off pressure as low as possible while still maintaining enough seal to prevent drooling and flooding. However, doing so usually means the resulting mixture adjusting screw settings will be such that the openings for the low pressure signal coming from the pressure drop in the venturi will be muted. In many cases, the overall performance of the carb can be improved by increasing the spring pressure and opening the mixture screws to allow a stronger signal to influence the movement of the metering diaphragm. A very extreme example of the balancing act would be the way I ran my 125cc McCulloch enduro setup back in the late 70s. Due primarily to a very effective slippy pipe design, the BDC 16 double pumper carb pump output pressure on my engines was so high I used 30 psi of pop off pressure to keep the pump from blowing the needle off the seat at certain engine speeds. To compensate for the high spring pressure I would end up opening the mixture needles nearly twice as far as if I ran the pop off down around 15 psi. One interesting aspect of the different combinations of carb tuning setups was that I did not burn any more fuel in an hour race with the jets open twice as far.
To summarize... pop off pressure measured the way we do it in the karting world is simply a reference point to allow is to quantify the changes we make when using different spring pressure under the fulcrum arm. There is no absolute "correct" value for any particular carb. Venturi pressure is an important factor that influences the spring pressure that will work best but it must be considered along with pump output pressure and jetting to gain a better understanding of how and why changing spring pressure can help or hurt carb performance.
BTW... on a closing note.... just to complicate things a bit.... when we discuss the BDC line of Mac carbs there is another major design component that other carbs do not have which is the duck bill circuit that creates an artificial bump in pressure on the dry side of the metering diaphragm under certain conditions. Properly managed the duckbill circuit can overcome the weak signal at the venturi that occurs at low engine speeds when the carb is basically too big for the engine it is installed on. That is a subject for another day.
Steve O'Hara