Comparing the Motorcraft 4350 Spreadbore Carburetor

From 1976 or so until 1978, the Jeep 360 and 401 4bbl engines used a unique spreadbore carburetor known as the Motorcraft 4350. This carburetor was also used on some late-70's Ford 460's, but nothing else. This means that rebuild kits and replacement carburetors are relatively hard to come by. A popular question concerns what carburetor can be used to replace the 4350, and how it can be adapted to the original AMC 4bbl intake for the 4350.

One common mistake is to confuse the spreadbore pattern of the 4350 with the pattern of another popular spreadbore carburetor, the Rochester Quadrajet (or Q-Jet for short). As the images on this page will show, the two spreadbore patterns are VERY different, and do not easily interchange. A squarebore-pattern carburetor is more easily adapted, but is still not simple or cheap.

First, our two cuplrits:
 
 

Rochester Quadrajet Spreadbore Carburetor

Motorcraft 4350 Spreadbore Carburetor

Neither is very pretty, huh? The big difference lies underneath, however:


The Quadrajet is on the left, and the 4350 is on the right. Immediately evident is what is meant by "spreadbore"; the two primary barrels, or venturis, that the engine usually draws air and fuel through are very small, while the secondary throttle bores are significantly larger, and provide the extra fuel needed for full-throttle acceleration. This design enables this four-barrel carburetor to provide good fuel-economy, provided the driver refrains from opening the secondaries.


 

What you might notice, however, is that the spacing of the throttle bores is very different on the two spreadbore carburetors. The Q-Jet on the left places the center of the smaller primary bores in-line with the centers of the secondary bores, while the 4350 places the primary bores much closer together. You may also notice that the secondary bores are different sizes, and that the four corner mounting holes for the carburetors are in different locations. These differences are very evident when the gaskets for the carburetors are switched (4350 gasket on the Q-jet carb on the left, and Q-jet gasket on the 4350 carb on the right):

The difference in mounting holes, as well as the size and spacing of the throttle-bores, means that the carburetors will not interchange.

The 4350 does share the mounting bolt pattern and primary bore position and spacing with the standard squarebore pattern used by most Holley, Carter and Edelbrock 4bbl intakes and manifolds, as you can see from the 4350 gasket on a Holley 750cfm 4bbl:


 

The use of an ordinary open or 4-hole aluminum spacer to adapt a squarebore carburetor to the 4350 manifold is complicated by the design of the manifold mounting pad for the carburetor. Unlike most manifolds, which have a flat milled mounting surface for the carburetor that extends to surround the mounting bolt holes, the AMC 4-bbl for the 4350 used a smaller scalloped pad, on which a resin spacer plate rested, sandwiched between the carb and the manifold.

This prevents the use of a standard square-bore open-spacer; there will be massive air-leak from around the scallop between the primary bores:

A four-hole spacer would avoid that leak, but there would still be a step obstructing a portion of the secondary bores. When I lay the stock plastic spacer over a squarebore carb, you can see the overlap of the bores:

One possibility would involve enlarging the secondary bores with a die-grinder in order to remove that step. The manifold would need to be removed and thoroughly cleaned, in order to ensure that no metal filings made their way into the cylinders. The following image indicates approximately how much material would need to be removed:

If you want a simple bolt-on solution that does not require grinding or removing the intake, there is one adapter that does the trick: the four-hole TD-Performance aluminum spacer/adapter (part # TRD-2199) that has the squarebore pattern on one side and the four hole pattern for the 4350 on the other:

It is not cheap ($46.99 at Summit when I bought mine), but it  includes the longer mounting studs and the gaskets, and more importantly, it provides the right pattern for bolting on any squarebore carburetor.

[Return to the Tech Pages]../index.html