Carburetor Flat Spots

By-pass Orifices

The drillings (orifices) in the throttle chamber close to the throttle plate are known as “by-passes”. The purpose of these is to ensure a smooth transfer from idle to normal speeds. (i.e. Transfer from the fuel provided by the idle port to the fuel provided by the main jet discharge arm). As an example, the 34 Pict 3 has three by-pass orifices that operate by a step effect as the throttle plate passes each one. By virtue of the distance of the throttle plate from each drilling, increasingly less fuel is discharged from each of the by-pass orifices as the throttle plate opens. This allows more fuel to be discharged from the by-passes until the main jet discharge arm begins to supply more fuel, and less fuel when the discharge arm is giving a full supply of fuel.

Throttle Body Image

Flat Spot Conditions (hesitation in acceleration)

A “flat spot” usually occurs when accelerating for the following two reasons:

  1. The throttle plate has passed the by-pass orifices and fuel is not yet being drawn from the main jet discharge arm. In this situation the throttle plate is opening wider and less fuel is being drawn from the by-pass orifices.
  2. Oppositely, the throttle plate has not yet reached the by-pass orifices.

Remedies for the Flat Spot Conditions

In the two “flat spot” conditions above the following may be done to remove the condition:

  1. In condition No. 1 above, a hole drilled in the edge of the throttle plate will enable the orifices to operate with a wider opening of the throttle plate, and it also will allow the throttle plate to be closed slightly more when idling.
  2. In condition No. 2 above, Filing a small amount off of the lower edge of the throttle plate (beveling the edge) on the side nearest to the orifices will bring the edge of the throttle plate nearer to the first of the by-pass orifices.