The key to successful fuel flow is setting and maintaining the appropriate float level. In our scenario we are describing what is all too common and the multitude of typical corrective actions can create what's commonly referred to as a "temperamental carburetor."

Q I have a 600-HP motor with fuel pressure set at 9 PSI. I can't seem to get my carburetor adjusted so that I can easily handle the car in the pits (and the staging lanes) without sacrificing either top end MPH or low speed driveability. I've tried readjusting needle and seats, plus adjusting my fuel pressure, based on weather and track conditions. Can this be resolved for the best of both worlds?

A The commonality of most carburetor float bowls, Holley, Demon and Quick Fuel Technology along with other brands, suggest sthe universal problem and condition is potentially pervasive. The root of the problem is operator error from the lack of understanding of the float's function. The most common "fixes" are more fuel pressure or larger jets, and/or adjusting the needle and seat. Trying to "fix" the situation with fuel pressure without knowing the real basis of the problem will set up a vicious cycle; the carburetor is way too rich at low speed and way too lean at top end. Not wanting to burn up your motor; you increase the jet size; now the bottom end is even worse than it was before. Yes, the problem can be solved by making sure the float is positioned approximately 3/8-inch from the top of the float to the inside top plane of the fuel bowl and lowering the fuel pressure to between 6 and 7 PSI.

Q Why is the position of the float in the bowl and its pivot point on the needle and seat so critical to fuel flow?

A The reality of carburetor function is, to cut this down to the simplest analogy, to provide fairly even fuel flow, based on demand. There is a distinct difference between fuel flow and fuel pressure. Simplistically, more fuel pressure does not mean more fuel flow. The increase in fuel pressure must be coordinated with an increased capacity to handle greater fuel flow. However, the described concern here has little to do with that condition. In reality, and this is true with any carburetor setup (Sportsman single four-barrel to Pro Stock mammoth multi-carburetors), the angle of leverage set by the float will determine the amount of fuel flow through the needle and seat.

The float bowl image shown here demonstrates how the float should be set to obtain optimum leverage, allowing flow based on most demands (high RPM and low RPM). Setting the needle and seat using the sight port can be misleading and puts significant limits on the float's function. The typical position from this effort is shown on the left. Setting the float level (setting the static tension on the needle and seat) before the carburetor is assembled will provide a much better result. The Pro teams tear down their carburetors to measure the float drop after each run so they know the needle and seat will open fully on their next run down the track.

We tested two carburetors, both using the same pressure regulator with a .120-inch needle and seat. The one with the float set in its proper position will flow more fuel at approximately 5 PSI than the same needle and seat combination flowing at 9 PSI, causing the unavoidable lower float position. At 9 PSI, the float is positioned .075-inch below the correct dry level setting, thereby limiting its movement and reducing the opening of the needle and seat. Yes, in theory there was more fuel entering the bowl, but because the needle and seat could not move to its optimum opening, 9 PSI actually results in less flow. Unfortunately, the needle and seat will react negatively to the higher pressure without the proper float drop. Coordinating all of the components and functions of the fuel system is the best plan to achieve fuel demand success. The QFT service technicians do an excellent job of analyzing fuel system needs.  DR