A 2V carb swap is one of the most popular modifications performed to increase the performance of the small inline six, even though most have no idea what to expect. Many suppliers sell the ever popular 2 into 1 carb adaptor, claiming gains of 20 horsepower or more, while others condemn them. They believe the adaptors are just a waste of money, offering little or no benefit. And that the use of such adaptors will only result in hesitation and flat spots under acceleration.

While we too have our doubts, our thoughts are on a slightly different path, in that we question the value of the modification rather than the validity. In other words, we believe that with the proper carb selection, additional power can be gained using a 2V adaptor, However we question the amount of money spent, verses the amount of power gained? Example: If one were to make a modification that resulted in 5 horse power at a cost of $500.00, the ratio would be 1HP per $100 dollars spent, or a 10:1 ratio.

While Classic Inlines sells a few 2V adaptors, due to customer demand, in most cases we recommend using an alternative method which requires modifying the log cylinder head. In our opinion, this method offers a much better power to dollar ratio. However it requires removing and re-installing the cylinder head, which may be a major drawback for those seeking a quick fix. For more information on this method please read our tech article, which details the modifications required, and gives step by step instructions.

In any case, neither method has been dyno tested or documented to our knowledge. Therefore, Classic Inlines decided to put them to the test. In our summary, we will list the advantages and disadvantages, the cost involved, the power gained, and the power per dollar (P/D) ratio for each method.

Note: While we were waiting to do the dyno test, after the head swap, the Maverick was taken out to the track to see how it would do. Previously, the best ET was a 17.35 @ 75 mph, with an average ET of 17.50 @ 74 mph. With the modified head the best ET was 16.35 @ 82 mph, with an average of 16.47 @ 81 mph, over six runs. That's a full second faster with the modified log, where the carb is mounted directly to the intake, rather than using a 2 into 1 adaptor. The ET translates to approximately 102 RWHP, which would be a gain of 25 HP over the YF-1V, and 20 HP over the Holley 350 mounted with the 2V adaptor. If the dyno test proves this out, I'd say a modified log is definitely the way to go, and worth the extra effort of pulling the cylinder head to have the intake machined so that it will accept the 2V carb without using an adaptor.

In summary
When we compare the numbers from dyno #3A, verse dyno #3B, we see some very interesting results. First and foremost the advantage of mounting the 2V carb directly to the intake, verses using a 2 to 1 adaptor, are quite evident. We gained a maximum of 5RWHP when we used an adaptor, and 24RWHP when the carb was mounted directly to the log intake, or a gain of 6% verses 24%, respectively.

All three carbs produced nearly the same maximum HP when using the 2V adaptor, with the Holley producing the highest average HP, besting the Autolite by 4HP and the Weber by 2HP. Both the Weber and Autolite carbs accelerated smoothly, while the Holley had a definite stumble around 2100 rpm. The Holley 350cfm may be just a tad bit to big for a stock 250ci, while the Autolite 240cfm was probably a little small, which would explain the lower HP gains. The Autolite also peaked sooner than the other two, which are also signs that it was a bit to small.

Prices on the Weber carbs can vary greatly from one supplier to another, so check around. We have found that Langdon's Stovebolt offers the best price on the Weber Carbs, and the adaptor needed for this swap. Holley and Autolite carbs can be purchased from several online suppliers, including Classic Inlines, or you can buy a used carb at just about any swap meet for $50-$100 bucks.

All things considered, we feel the Autolite carbs are the best solution for those using an adaptor, or directly mounted, providing they are sized properly. Not too small, but not to big either. The Autolite's, unlike Weber and Holley carbs, come in seven different CFM ratings, so it should be pretty easy to match the proper sized carb to your specific engine build. Plus they are the only carbs that were designed with Annular Discharge, which we feel is far superior to other designs. However, that's just our opinion. They also have a one piece float bowl and main body casting, which eliminates leaks and maintenance.

Annular fuel discharge, by definition, is where the fuel is discharged into the bore of the carburetor from a series of small holes around the perimeter of the booster venturi, rather than a single hole. This method of discharging the fuel breaks up and partially atomizes the fuel for better throttle response, improved fuel economy, and more power. It's also as close as you can get to fuel injection, using a carb.

Dyno: #1 - #2 - #3 - #4 - #5 - #6 - #7 - #8 - #9 - #10 Track: #1 - #2 - #3 - #4 - #5 - #6 - #7 - #8 - #9 - #10