Supersix - your last comment could be quite true.
The best part about the internet is that anyone may post
The worst part about the internet is that anyone may post
Having said that, consider the following:
Carburetors are rated in CFM (cubic feet per minute flow). The major contributing factor is the diameter of the carburetor venturi. The same can be said for the diameter of the bore of the intake manifold. Carburetors are rated at a SPECIFIC vacuum. Again, the same can be said for the intake manifold. If you have a normally aspirated engine; there is a maximum amount of air/fuel mixture that may be drawn through a specific size hole in the manifold; as the engine vacuum and the size of the hole are the controlling factors. Assuming the factory one-barrel carburetor can provide this maximum, no more will be gained REGARDLESS of the size of the carburetor installed. This concept is not true if you install a blower; as you can FORCE more air/fuel through the hole than you can PULL through the hole.
Now consider the adapter:
This is a highly un-technical analogy, but true. Think of two different situations on an interstate highway that cause a tie-up. The first is normal highway construction where signs with arrows educate motorists over a 2 or 3 mile period to form into a single lane. The traffic will merge and by the time the traffic reaches the bottleneck there will be one lane. This will be the maximum amount of traffic that can pass in one lane (at a given speed).
Now consider an accident; and the first trooper on the scene attempts to establish traffic control (at the scene). The traffic will back up in ALL lanes, and then jerkily merge right at the bottleneck. Less trafic will flow through the bottleneck under these conditions.
Your manifold is the bottleneck. With a two barrel carb you have two "lanes" that must merge. Compounding the difficulty is that the velocity of the air/fuel mixture at the throttle of the carburetor is less than at the manifold. As the adapter squeezes the two holes down to one, the velocity must increase, and disrupts the flow of air/fuel. This is the reason for my original comment concerning the length (height) of the adapter, and why it should be "shaped".
Now consider a four barrel:
But not just any four barrel! Put an 1150 Holley dominator on this engine and your earlier comment was right on! Please note that I suggested a specific four barrel - the Carter 9400s or 9410s. Both of these are 400 CFM carburetors that are progressive "demand" secondary carburetors. Many talk about vacuum or manual secondaries erroneously, so I try to avoid these terms. The "demand" secondary will open ONLY if the engine provides sufficent "demand" for the additional CFM. Otherwise, it is a two-barrel. But utilizing a four barrel manifold, you eliminate the bottleneck, and allow the engine to breathe as necessary. The same can be done by utilizing a multicarb manifold but the four barrel is easier. Another carb that may be used (if you like Holleys, I don't) is the Holley 390 CFM, but it will require modifications by someone who likes (or at least understands) Holleys.
As far as doing your own thing:
The "school of hard knocks" is probably the finest teacher available. By all means try your adapter and two barrel. If your original carb was shot (as the Rochester B series often is) you will probably see improvement; as the Rochester 2G series two barrel is MOO one of the finest two barrels ever made. I still find it difficult to believe the same company made both the B and the 2GC.
You may see a noticable improvement. However, the concensus of those of us who have attended "the school of hard knocks" is that the improvement will be less than other methods discussed. These forums are a wonderful place to learn.
And there is no single "perfect" solution. If there were, all carburetors would have been produced by Carter
Jon.
