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Classic Inlines
603 W Pecos Ave
Mesa, AZ 85210
(602) 708-6650
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2V Carb Swaps & Log Modifications


Many inline enthusiast use aftermarket carb adaptors to install two barrel (2BBL) carbs on the integral log intake, replacing the anemic one barrel (1BBL). There are many different types, so you need to make sure you purchase the correct adaptor for your application and your choice of carburetor. Most of the adaptors offered are for Weber carbs, however there are those that allow Holley/Autolite 2BBL carbs to be adapted to the log intake. The most widely known adaptor from Offenhauser allows the use of triple one barrel carbs (3x1BBL).

While the adaptors seem straight forward, they are not. Not only do you need to make sure you purchase the correct adaptor, there are other issues you need to consider. Some adaptors need the original spacer to square the carbs orientation, so it is perpendicular to the crankshaft, however they are not absolutely required. In the photo below, the carb was installed without the spacer. While the end result is a misaligned carburetor, it probably doesn't effect the performance one way or the other. Personally I think it looks a little strange, but if your struggling to maintain hood clearance, it might be your only option.

Stacking up adaptors and spacers may result in inadequate hood clearance. However you may be able to compensate by using a drop base air cleaner, or an air cleaner with a thinner element. Another popular solution is to use a low profile carb hat with a remote filter element, as seen below. The other problem you'll need to deal with is throttle linkage. Some guys bend or modify the existing linkage, others rig up some sort of custom linkage, but most simply switch to a throttle cable. Unfortunately there aren't any kits available, so you'll still need to custom fabricate your own. The good news, fabricating a throttle cable is pretty easy, thanks to Lockar Performance Products. The other option is finding a donor car that came equipped from the factory with a throttle cable, such as late model Fairmont or Granada.

Upgrading to a two barrel carb over the one barrel definitely offers improved performance. However, when using a two into one (2-1) type adaptor, the air flow is funneled into the log intake through the existing one barrel opening. This limits the full potential of a two barrel carburetor, and if the carb is to large for the application, there's a good chance it will suffer from hesitation and flat spots coming off idle and under acceleration.

Cast Adaptor
Billet Adaptor with Insert

Most after-market adaptors are made of cast aluminum. They are designed with a flat bottom so they can be bolted to the intake from inside the adaptor. This design has the advantage of being shorter in height, which helps to maintain hood clearance. However the flat bottom, as well as the protruding bolt heads, create turbulence under the carb, which reduces airflow. While Classic Inlines offers the same style of adaptor, to maximize hood clearance, our adaptors are made from billet aluminum rather than cast. Our adaptors also include a billet insert that conceals the mounting bolts and funnels the airflow into the existing carb bore, thereby reducing turbulence under the carb, and increasing air flow.

Log Intake Manifold
Billet Adaptor with SS screws
Billet Adaptor with Insert

We also offer an optional Phenolic Spacer/Insulator which is designed to reduce heat soak (transfer of heat) from the intake manifold. This keeps your carburetor running cooler, which greatly reduces the likelihood of  vapor locks and/or other heat associated problems. Overall height of the adaptor and the insulator is 1.5 inches, which is still shorter than most other adaptors.

Classic Inlines Adaptor with Insert and Base Insulator
This set up is for a Holley or Autolite carb, however we also offer Weber adaptors.

While Classic Inlines makes and sells adaptors for Holley and Autolite carbs, we highly recommend using a 32/36 Weber DGV for stock or mild performance applications. The 32/36 Weber DGV is a progressive two barrel, which operates primarily as a one barrel carb, with the second barrel only opening when it's needed. The progressive design helps to eliminate troublesome flat spots and hesitation, which are common place with the Holley and Autolite synchronous carbs. However, this is not to say that Holley or Autolite carbs won't work with an adaptor. If the motor is built properly, the carb is set-up and tuned properly, and the carb is the correct size (CFM rating), Holley and Autolite carbs work nicely. Whichever carb you chose, be prepared to do some research and a little carb tuning to get it running right. While the carb may work right out of the box, nine times out of ten you'll need to rejet and make other adjustments to get the motor running at peak performance.

For more info, see the results we got when we dyno tested three 2V carbs, mounted with a 2-1 adaptor and when using a conversion adaptor on a modified cylinder head. We also wrote a tech article on Selecting the Right Carb, which covers the various carbs that can be mounted to the small six head. Finally we flow tested a couple cast iron log heads, an Australian 250-2V cylinder head, and our own aluminum head, to compare air flow (CFM). While the results were basically what we expected, it is interesting to see how the air flow increases with each cylinder head.

2V Conversion

Another method of upgrading to a 2V carburetor, which has become popular amongst the FordSix Performance forum members, is commonly call a 2V Conversion. This method incorporates mounting a custom carb adaptor to the log manifold, then hogging out the carb opening so it is the same size and shape as the 2V carb. Depending on the design, conversion adaptors allow the use of Holley, Autolite, and/or Weber carbs. The conversion method also offers improved hood clearance, depending on the adaptor, which carb is used, and whether or not a spacer is used. The conversion method offers substantial increases in power and torque, over the two to one (2-1) type carb adaptors, which was quite evident when we did our dyno testing last summer. I'm sure you'll agree that the results speak for themselves.

The only drawback to this method is that the cylinder head must be removed for machining, then reinstalled. However this offers the perfect opportunity to install a mild performance cam, as well as installing larger valves, doing a three angle valve job with back cut valves, and the chance to do a little port work to get the most out of a converted cylinder head.

To mount the 2V conversion adaptor, the intake manifold needs to be milled flat, which means milling off the existing carb base. Prior to milling, it is generally preferable to build up the low spots on the intake manifold first, which can be accomplished by furnace brazing or by using some sort of permanent metal filler or putty. Once the low spots have been filled in and the manifold milled flat, the carb bore (or carb opening) is hogged out to the same size and shape as the 2V carb, thus creating a true two barrel manifold. The final step is mounting the conversion adaptor, which is done by drilling and tapping three to four holes in the manifold. The adaptor is sealed to the log intake using gasket material, high temp silicone, or in some cases, JB Weld.

HINT: We like to make a light pass over the log first, which helps to identify the low spots, then do a final machining after the metal filler (or brazing) has hardened. In the photo below you can clearly see where metal filler was use to fill in the low spots, as well as the hogged out carb hole, which was port matched to the custom adaptor (as seen below).

Once the log modifications have been completed, the face of the cylinder head needs to be milled to true the cylinder head surface, assuring a good seal to the block. Milling is also done to reduce the chamber volume, thereby increasing the compression ratio. In most cases the stock compression ratio is raised to 9.0-9.5 for increased performance and better throttle response. This is also a good time to back-cut the valves, do a three angle valve job, and if your budget allows, a little port work. Porting a log head is difficult (to say the least) and time consuming, as the only way to get to the the intake runners is up through the valve pockets. However a good shop with the proper tools, and a good technician, can work wonders on the log head.

Here's the same cylinder head (as shown in the photo above) with the conversion adaptor installed. Note the wide spacing of the mounting screws, which gives this particular adaptor plenty of stability.
(click image for larger view)

Regardless how you go about it, this is probably one of the best performance mods you can make, short of purchasing an OZ250-2V cast iron head, or the new aluminum head. It's worth the extra effort and cash, so if you have the time and money don't hesitate making the decision. You won't regret it. For a side by side comparison, check out our dyno results.

If you can't find a shop to do the work, Classic Inlines offers cylinder head rebuilds, 2V modifications, and port-n-polish work at very reasonable rates. For example, the labor to rebuild a log head runs around $600-650, plus parts. This includes the 2V modification, port work, all machine work, and final assembly. It also includes the installation of larger valves (175/150) and valve seats, new guides and seals, back cutting, and a three angle valve job. Every head is hot tank, magnaflux'd, and pressure tested, to assure our customers are getting a cylinder head that is free of defects. If your interested in a rebuilt and/or modified cylinder head, please see our Cylinder Head page for pricing, or contact us via e-mail.

Ignition Concerns

The other thing you need to think about when doing a carb swap, is the ignition system. If you have an Autolite 1100-1V carb, you may need to swap out the distributor as well. The early (63-67) 1100's came with a Spark Control Valve and were designed to work hand in hand with the stock Load-O-Matic distributor. If you swap to a carb that doesn't have the Spark Control Valve, you'll need to swap the distributor as well. And while the later (68-69) 1100's didn't have a Spark Control Valve, the stock distributors operated on mechanical advance only. Lacking vacuum advance, these distributors offered poor performance and should be swapped out with a distributor that has provisions for ported vacuum advance. If you swap over to a 2V carburetor, you'll definitely need to ditch the stock distributor and install a Duraspark, DUI, or other aftermarket distributor.

CAUTION: For more information, please read our tech article on the Load-O-Matic distributor and how it works hand in hand with the Autolite 1100-1V carb. Before you swap carburetors, it is essential that you understand the differences between the various distributors and how these differences relate to selecting the right carburetor and dizzy.

More Photos
Classic Inlines Weber DVG Conversion Adaptor
Classic Inlines Weber DVG Conversion Adaptor - mounted
Classic Inlines Weber DVG Conversion Adaptor with spacers
Classic Inlines Weber DVG Conversion Adaptor with 32/36 carb
Weber DVG carb with Classic Inlines alloy carb hat
This shot shows why the spacers are needed to clear the V/C
Classic Inlines - Autolite/Holley Conversion Adaptor - Black Anodized
Mounted conversion adaptor

Clean and simple, yet very effective - note throttle cable
Here's another shot of the throttle cable set-up - note carb hat.
milled intake - by others

homemade adaptor mounted

homemade adaptor - with three mounting screws

here, a spacer was used between the adaptor and the carb
carb mounted and plumbed

here's another shot of a throttle cable set-up

Click here to purchase Carburetors, Adaptors, and Related Products
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Future Article
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