What are the Differences in Big Holley 4150 Carburetors? Part 3

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In the last couple of issues, we took a look at the differences in big CFM Holley 4150 carburetors. We covered boosters, bowls, accelerator pumps, cosmetics, overall materials used and so on. This time, we’ll examine throttle shafts, baseplates and a not-so-little thing called a “by-pass valve” found on the XP series carbs.

There are subtle differences in the throttle shafts found on all three carburetors. For example, the Classic Double Pumper makes use of a conventional “1/2 round” throttle shaft with regular screws. Meanwhile, the Ultra HP uses a similar throttle shaft; however, it incorporates smooth profile button head fasteners.

The XP is different. Here, the throttle shaft is fully slabbed and undercut to allow for improved airflow. Smooth profile button head screws are also used to fasten the throttle plates to the shafts. It’s a small thing, but the primary throttle shaft on the XP is capped, which positively prevents vacuum leaks. Holley also provides a machined accessory-mounting hole that allows for easy addition of an adjustable primary throttle stop.

When it comes to baseplates, the Classic Double Pumper and the HP series carbs use a conventional cast baseplate. The Double Pumper includes two provisions within the baseplate to tap in for vacuum. The HP does not have these provisions. On the other hand, the XP series carb includes a base plate machined from 6061 billet aluminum. This allows for a superior gasket-sealing surface on both ends – carb body to base plate and base plate to intake manifold. You can also see the elongated mounting holes. What’s with that? Simple. The slotted mount holes allow you to use the carb on a standard 4150-series intake or on an intake designed for a Dominator (no adapter required). In addition, the baseplate incorporates a “machinable” boss that can be opened up to provide for a vacuum source if required. One fact about the billet base plate that few recognize is that it’s robust. With older carburetors, you’d occasionally come across a cracked or broken baseplate (almost always caused by over-exuberant tightening – usually with tools way too big for the job). The billet baseplate is simply a stronger component designed to withstand the rigors of competition.

A very interesting addition to the XP series of carburetors is the integrated idle bypass valve. This valve is similar to what you might find in many fuel injection applications. In a fuel injection setup, the idle bypass keeps airflow up when the throttle is closed. With older carburetors, this was accomplished by either drilling holes in the throttle plates or by adjusting the curb idle screws. With a radical (big duration and/or big overlap) cam, you can get in trouble by opening the throttle too much, exposing the transfer slot. That doesn’t happen here. The idle bypass is pre-set by Holley and it can be easily fine-tuned by way of a simple screwdriver to compensate for a big lumpy camshaft. In truth, the folks from Holley recommend you set the idle bypass before you touch the curb idle screws.

Sounds like a good plan, but if you’ve been staring at the main image of the carburetors, where do you set it? Check this out: Holley installed the idle bypass valve adjustment screw right underneath the air cleaner stud. If you use the stud, simply remove it. Peer inside the stud boss and you’ll find a screw (with a flathead screwdriver slot) inside. Simply turn the screw to set the idle bypass valve. Chances are you won’t have to touch the curb idle (speed) screws at all. Not only that, Holley includes a nifty screwdriver with the carbs.

That’s a wrap for this segment. Next time around, we’ll remove the bowls and metering blocks and look inside. Meanwhile, check out the accompanying slideshow. There’s a lot here and it shows how Holley builds different carbs for different applications.

Here’s a look at the throttle shafts on the Classic Double Pumper. It’s a standard half-round shaft with conventional throttle plate screws.

The HP series car uses a similar throttle shaft; however, the throttle plate screws are more aerodynamic button head jobs.

Meanwhile, the XP carb makes use of a fully slabbed, profile milled throttle shaft arrangement; it too uses aero button head fasteners.

While it might seem like a small thing, the throttle shafts on the XP are positively sealed.

The baseplates fitted to the Classic Double Pumper and the HP carb are cast.

On the other hand, the XP baseplate is manufactured from billet aluminum. Obviously, this helps to prevent damage caused by overtightening carb studs.

The XP carburetor has mounting holes for the carb that are slotted. This means you can bolt the 4150 to an intake manifold designed for a 4500 Dominator without using an adapter.

Flip all three carbs over and you can see the Classic Double pumper is street-car friendly with a couple of vacuum source locations. The other two have none.

On the other hand, the XP baseplate has a boss that remains un-machined (its actually partially finished). You simply have to open it up (drill and tap) to provide a vacuum source if you need one.

Note the air cleaner stud location on the XP. When the stud is removed (not a big job) or not installed (as shown here), there’s an adjustment port for the idle bypass system.

A small screwdriver is used for the adjustment.

It’s a slick way to overcome big grumpy camshafts that don’t want to idle. The text offers more info.