Weber stumble when racing

[1QuickS]

The anti-percolation holes developed by Weber in the early 1970s and popularized by PMO have saved many engines from destruction by fuel spilling onto exhaust pipes. However, there is an issue with these holes suffered by racers and posted in previous threads.

The anti-percolation holes are located in the top cover of the throttle body to redirect boiling fuel to escape the fuel well before it percolates up the vent pipes located between the intake air horns. The redirected path drains fuel into the intakes; better there than onto the exhaust headers. This path takes advantage of an existing notch in the top cover providing atmospheric air to the idle air bleed jet. Redirection of fuel in this fashion isn't an issue for a parked car since the fuel that spills into the idle/progression circuit gallery is naturally drained out after the engine is shut down.

However, if high-G cornering is experienced then fuel will flood the carb which is located on the inside of the turn (outer carb is OK.) Since the throttle is feathered during this activity the enrichment of the progression on the inner carb isn't noticed but when the throttles are opened just after the apex the inner carb has a flooded progression mixture. Of course the natural response is to open the throttles to get the power which squirts more gas with mixed results.

The picture shows the path of the classical anti-percolation drilling (copper wire) and where the idle air bleed is located (black arrow.)

Customers suffering this issue have plugged the anti-percolation hole(s) resulting in the elimination of the issue described.

Also shown is a recommended alternative to the classical hole location; the drill shows where to locate the alternate hole and the blue tape is the location of the bleed notch which allows the fuel to enter the cylinders.