Back to Basics: Holley Jet Extensions

Click Here to Begin Slideshow

Click the transbrake button, pin the throttle on the two-step, release the button and hang on. The car hooks and rockets forward. Then, just as you’re watching the tach as it races toward redline, the car noses over. After what seems like an eternity, it picks up again.

What’s up with that? Simple. The car stumbled as it accelerated hard because it experienced a lean condition. It simply ran out of gas.

Rewind for just a second: It seems not long ago that racers turned carburetors sideways on tunnel ram intake manifolds. The reasoning was at least twofold: This oriented the carbs so that they’d actually fit better over the respective ports and, equally important, it meant you actually had better control of the fuel within the carb bowls. It more or less cured the potential for fuel starvation.

The quandary is this (tunnel ram or otherwise): When a Holley 4150 or 4500-series carb is mounted normally (inline – one bowl facing the front and one bowl facing the back of the engine), fuel rushes toward the primary (front) jets as the car accelerates. It also means fuel runs away from the secondary (rear) jets. Not good.

In the old days, there were all sorts of quick (and not-so-quick) fixes. Some worked; some didn’t. Today, the cure is simple. You can simply buy all of the parts you need from Holley (see the last slide for a list). The fix takes an hour or so if you’re leisurely with your time. Here’s how it’s done:

With the fuel bowls empty, pry off the secondary bowl along with the secondary metering block. Be very careful when removing the bowl, because the bowl gaskets can stick. You'll find the jets in the metering block. FYI, today Holley fits their carbs with non-stick gaskets. You'll appreciate them if you have to dig into the carb (for example, to perform quick jet changes). Some non-stick gaskets are softer than the old gaskets and require a re-torque. Along with the stock bowl and metering block, you can see the jet extensions and the notched float at the lower left.

This is a closer look at a Holley notched float. Notice how it is physically milled out to clear the jet extensions? Note, too, that the Holley jet extensions are a bit different than some aftermarket jobs. The Holley models physically move the jet outward. Some of the aftermarket pieces are simply plastic extensions that slip over the jet. The Holley system is obviously much more secure.

This is how it works: The stock jet is removed. The jet extension is screwed into place. Next, the jet(s) is screwed directly into the extension. Simple. Here, the carb is equipped with a power valve.

In some cases, you’ll have to swap the power valve for a power valve plug. The reason is that fuel moves away from the power valve too, as it is situated in the same proximity as the jets within the metering block. Keep in mind that the power valve typically adds 6-10 or so jet numbers of fuel to the mix when it opens. Obviously, if the power valve is removed, the jetting will have to be increased. We can’t give you exact numbers for the jet size increase, as each application will vary.

The stock float in some Holley carburetors is a hollow brass job, just like this. As you can well imagine, once the jet extensions are installed, they hit the float and in turn, the float can’t function.

Obviously, the float has to be removed and replaced with a notched job. In this photo, you can see all the components found inside the float bowl. To get it out, the needle and seat assembly (lower left) is removed first. You simply loosen the lock nut and screw the works out. Two screws (lower right) hold the float in place.

You’ll need to transfer the float spring from the brass float to the new nitrophyl job.

Note the orientation of the spring in this photo.

Reassemble the float back into the bowl. This is how it should look.

After the needle and seat is installed, we eyeballed the float level (float upside down). Compare this to the earlier photo of the brass float in the bowl.

Holley offers a matching nitrophyl float for the front bowl. It’s not notched (obviously, it’s not necessary to have jet extensions on the leading bowl). Installation is the same as the back bowl.

This is a list of the parts used in the article:

Description, Part Number

Holley Jet Extension Kit (for one bowl), 122-5000
Holley Notched Secondary Float kit (for one bowl), 116-14
Note: Holley also offers a combination notched secondary float/jet extension kit under part number 116-15
Holley Nitrophyl Float Kit (un-notched) for one bowl, 116-3
Holley Power Valve Plug & Gasket, 26-36