T P F C: Transient power fuel control

Car carburetors are usually equipped with a accelerator pump system which injects or pumps a small amount of fuel, which can be adjusted by varying jet sizes and or pump stroke, into the air stream. The system is linked to the throttle and comes into play particularly when the throttle is snapped open. Basically the system consists of a plunger, which operates a diaphragm to pump fuel through a fixed orifice or an interchangeable jet. A non-return ball valve prevents fuel returning to the float bowl when the plunger activates the system via the diaphragm. This overcomes the momentary hesitation or stumble that engines experience when sudden throttle applications produce what is commonly termed a flat, spot'. The accelerator pump bridges the tiny gap that usually exists when the pilot circuit hands over charge to the slide cutaway and then on the needle jet and needle to provide smooth transition from low throttle opening to over halfway up the range. Usually a certain amount of overlap exists between the various circuits in most carburetors and fine-tuning them can bring about satisfactory results. Sporting applications would generally require the added enrichening provided by the accelerator pump. It remains debatable whether such a system or a street hike like the CBZ is necessary. Anyone blipping the throttle without actually needing to would activate the system and inadvertently be wasting fuel. CBZ owners please take note). The CBZ is special in many ways and to determine how effective these additions are in actuality we configured a few tests of our own. To start with it was decided to deactivate the T P F C system to determine its effectiveness. By disconnecting the cable, which brings the system into action, we rendered the system inoperative. Normally an accelerator pump injects the fuel to compensate for its leanness, which accompanies sudden application of the throttle. We ran the CBZ to simulate conditions where, since the pump was inactive, a stumble or hesitation would result from the quick throttle opening. We repeated the runs over again varying speeds and rpm levels to evoke a flat spot. But finally in the absence of any discernible difference in the running came to the conclusion that the pump may not be having as large an influence as to be perceptible in a real world situation. At least not enough to disrupt the power application in a disturbing way. This points to the pilot circuit in conjunction with the needle and needle jet combination enjoying a rich enough setting so as not to reed the extra enriching from an accelerator pump.