[Tom Vaught]

Jet Extensions
Uncovering the secondary jets can cause a stumbling problem, and no amount of accelerator pump tuning will cure it. This occurs when a car leaves the line so hard that the fuel in the secondary bowl is pushed up against the rear wall, uncovering the jets. Quick Fuel, Holley, and others offer Jet Extensions that correct this situation. Whenever Jet Extensions are installed in a carburetor equipped with a Secondary Power Valve, the Power Valve must be removed and the PV hole plugged. Jet size must then be increased accordingly

This is because the Power Valve fuel inlet passage is .250" HIGHER than the Jet Opening in the jet when mounted on the rear metering block.

When you add the Jet Extensions you also need the Notched Black Float installed in the rear bowl.
Jet Extensions have progressed from a small brass tube soldered into a Holley jet (usually about 3/4" long and 3/16" outer tube size to Large Diameter 'Slip on" extensions, to "Holley sold" machined extensions that go into the metering block and then the Holley jet screws into the end of the extension.

The Slip-On Jet Extensions made from a fuel resistant plastic (when new) seemed to seal well but after several jet swaps would sometimes leak air and not pull fuel from the rear of the bowl properly. The metal (usually brass) Slip-On Jet Extensions never did seal well and always leaked air. They basically were 'Feel Good" extensions but did nothing except get your money.
The Holley Machined Extensions seem to work best and last a long time without air leaks. Many people buy a hand full of the things and install them on the most used jets. Then use a wrench to swap them out vs a screwdriver.

Notched Floats

Notched Floats have been around for a long time. They work well with the different Jet Extension parts out there.
Depending on if the "Notched Float" was created by the racer or purchased by the racer can be the difference between a leaking and poor functioning Rear Float vs a properly working float. Some racer modded ones work well based on the sealer used on the float.

So now you have the "Trick of the Week" Jet Extensions and "Notched Float" in your "Race Holley Rear Bowl" and you still have a hesitation when you launch the vehicle on the line.

Many Racers miss the fact that the Power Valve Stem that seals the Power Valve from the Fuel Bowl is .250" higher vs the Jet Opening therefore the Main Wells will see air past that Power Valve when the car accelerates and the fuel rushes to the rear of the fuel bowl. So now your Jet Extensions and Notched Float really did nothing to remove the hesitation in the acceleration curve.

So for Drag Racing with a "Hard Launching" vehicle, the Power Valve circuit needs to be blocked with a Power Valve Plug and the Jetting raised from 6 to 10 jet numbers. Holley actually offers carbs with no Power Valve Drilling in the Rear Metering Block for that reason. Plugs not required, PV gaskets not required (at that Position) and just basic Rear Jet Changes required to tune for max MPH.
So Holley Engineers Did that "delete" Mod for a reason.

Bowl Vent Baffles.

Holley basically offered two styles of Bowl Vent hardware: A Brass Deflector that installed on two bowl gasket pins and a press-in Plastic Vent "Whistle"

The Brass Deflector was for "Non-Raced" street vehicles that were never accelerated hard or braked hard.
The Plastic Vent "Whistle" was for vehicles that could be driven aggressively. Braked harder or accelerated harder.
When the vehicle was "Braked Hard", the Whistle "Inlet" being 3/4 of the way into the back of the fuel bowl kept fuel from banging off the metering block, moving thru the vent tube, and entering the air stream and passing thru the engine as a very rich mixture.
So we solved that problem.

But then you have the RACER who accelerated hard. When the vehicle was "Accelerated Hard", the Whistle "Inlet" being 3/4 of the way into the back of the fuel bowl allowed fuel that banging off the rear of the bowl wall, to pass thru the vent tube, and enter the air stream and pass thru the engine as a very rich mixture, causing a hesitation part way down the track.

So then a Compromise Solution was required.
The Bowl Vent Whistle was trimmed to about 1.5" (from the steel retention pin to the end of the Whistle). (About .400" removed from the whistle and the .250" square opening at the top of the whistle recreated in the new shorter length whistle.

So now fuel banging off the rear wall of the Float Bowl would collapse like a surfing water wave before entering the fuel bowl whistle.

So now you know a bit more about what Racers do certain mods and why Holley added given parts to a carburetor.

Tom V.