Late model Merury intake manifolds explained.

[tubman]

Things seem to be a little slow around here lately, so to try to get things going, I'll submit a post I made on the H.A.M.B. a week ago that shows the differences between the various Mercury manifolds that can be used with the various Rochester 2G carburetors. One of the differences is that while American manifolds are cast iron, aluminum versions were used in Canada. In the first picture, a Canadian manifold is shown on the left, while an American manifold is on the right.

The Canadian manifold shown has a special plate I make to allow the use of 2G carburetors on these manifolds. 2G's have large voids in the cast iron base that almost overlap the edges of the carb base on the manifold making these plates necessary. They are installed with a Merc base gasket on the manifold and a GM gasket on the carb. Notice also that the throttle bores on the GM carb are larger than those on the manifold. They are also not concentric with those on the manifold, so a guide should be used to ensure they end up in the right place. The 2G throttle bore is 1 7/16", while the Merc manifold is 1 1/4'. That doesn't sound like much, but when you calculate the differences in area, it is over 30% larger. No wonder the 2G is a performance upgrade.

If you mount the 2G backwards, the throttle linkage and fuel line almost line up exactly. Adding a ball-stud to the 2G throttle arm will allow the Ford throttle rod to attach, and a slight bend in the fuel line will allow it to be connected. As a result, the choke ends up pulling the wrong direction, so I end up using the automatic choke. Both Mercury's and Chevrolets came with either conventional (choke mechanism on the carb) or "divorced" (choke mechanism on the manifold. The second is a picture of a conventional choke on my '51.

The third picture is of a Merc manifold with a divorced choke.

Luckily, Chevrolet used carbs with both types of chokes, so if you look hard enough to find a matching carburetor, hooking up the choke is simply a matter of shortening the heat tube (conventional) or making a slight bend in the actuating rod (divorced). These chokes are pretty simple and work well, so I have never used a manual choke on one, but I am sure it could be done.

The only shortcoming with this set up is that the linkage geometry does not allow the carburetor to open all the way. I changed my linkage so it would and it got even better. Unfortunately, the linkage geometry would not allow me to set the idle at less than 1500 rpm, so I changed it back. Increasing the throw without adding extra pedal effort is not as easy as it sounds, but I'll get it. This is the only real disadvantage of the 2G swap that I have encountered.