The two-barrel Rochester 2G wasn't exactly designed with racing in mind, but with the righ
A few issues back we ran an article in Circle Track on modifying the Rochester four-barrel carburetor for maximum performance in circle track racing. We went to Sean Murphy, owner of Sean Murphy Induction (SMI), as a resource, and he provided a wealth of information on everything from choosing the right version of the venerable Rochester to tuning tips.
That article was so well received, we decided to go right back to the well. What we realized the last time around is that we left out all the Bomber class and IMCA racers who are required by the rules to run a two-barrel original-equipment carburetor-which almost always means the Rochester 2G. Unfortunately, because the designs are so different, most of the tips Murphy provided for the 4G simply won't work with the 2G. Instead, this carburetor requires an entirely new approach.
"It's different," Murphy says, "but it's also a much simpler carburetor. It's very versatile and it's quite durable. If you want to start working on and modifying your own carburetors, the 2G isn't a bad place to start."
According to Murphy, Rochester began producing its two-barrel carburetor in 1954. It went into mostly General Motors automobiles and trucks. Large-scale production lasted until 1979, meaning the Rochester 2G is still plentiful and readily available for racers looking for hardware. There isn't much in the way of rebuild kits available, but Murphy says it's usually easier and cheaper to buy a second carburetor and strip it for parts anyway.
Over the years, SMI has built a winning program developing Rochester 2G carbs for entry-level racing classes, making the company one of the leading builders of these carbs. Following are Murphy's tips in his own words for getting the most power possible from the venerable 2G.
Carb Sizes and CFM
Even though the 2G ended major production in 1979, it still lived on a little further in boats, heavy equipment, and even school buses. In the heavy-equipment applications, some of the carburetors had governors on them even so those can actually be more desirable because they are the biggest ones.
The cfm (cubic feet per minute-a measure of how much air the carburetor can flow) on the Rochester 2G ranges from about 225 cfm in the smallest applications up to what Rochester calls a 435 cfm. But the 435 is actually commonly referred to as a 500-cfm carb in circle track racing because it has the same diameter venturi bore and throttle bore sizes as the Holley 4412, which is a 500-cfm carb. Rochester just ran a different test on the flow bench to rate its carbs than Holley did, but generally, if you flow them both on the same bench, each will flow right around 500 cfm.
Throttle Plates and the Idle Mixture
One of the few things that basically works the same with these carburetors as the four-barrel Rochesters is the throttle blades. Photo 1 shows the throat holes in the throttle plates, which are relatively large. In these photos the holes are 0.180-inch. It doesn't matter where on the plates you put the holes but their size does matter.
The purpose of the holes is to allow enough air through the carburetor and into the engine so that it idles correctly without having to open the butterflies and expose the transition slots. And with a race engine, you are almost always running a larger camshaft which requires more air, so those holes will have to be enlarged. The transition slot is barely visible in this photo, which is what you want. You can see it on the lefthand side in the throttle bore at the 12 o'clock position.
As you are drilling out the holes, be careful to only make it larger in small steps. This is true with a lot of the changes you make on the carburetor-since a race motor will want more air and fuel than a street motor, just about everything you do will be to make the channels and openings larger. But it's always easier to make a hole bigger than it is smaller. In fact, in some cases, like with the throttle blades, it can be practically impossible to make the holes smaller. If you wind up making the holes too large your only option will be to go out and find new throttle blades to replace the ones you've ruined. So, try to sneak up on those throat holes until they are just the right size.