The guru of all gurus in carburetor function was without a doubt an obscure Dutch-Swiss dude named Danny. OK, his full name was Daniel Bernoulli and it’s possible you may have heard of him. Besides being an uber-smart mathematician/statistician/physicist/author, he put into words the theory of why carburetors work. Of course, unless you are also an über-smart physicist, you might not get all the little details and decipher the squiggly lines in his writing. That means that when it’s time to bolt a brand-new, shiny fuel mixer on your bitchin 383 with double hump heads and three-quarter race cam, you’re better off talking with the tech guys at the company you bought it from in order to get it running to the best of its ability. PHR recently spoke with the top techies at Holley, Quick Fuel Technology, and Edelbrock in order to glean their most helpful hints and help you avoid the most common mistakes. In no particular order are the most common problems and suggested solutions.
Number One: Incorrect Fuel Pressure Surprisingly, the most common problem with getting a carb working right is not having the correct fuel pressure. Quick Fuel Technology tech expert Zach Baker says: “These carbs are designed to run at 6.5 psi. You might be able to get away with 7, but we’ve had a rash of pumps that do 8 or 9 pounds, which is great from a volume standpoint of getting fuel to the front of the car, but the carburetor’s not going to be happy there so you have to run a fuel pressure regulator and a lot of people just don’t.” The head of Edelbrock’s tech department, Dave Stinson, recommends a regulated pressure of 5 to 5.5 psi on their carbs, with a max of 6.5 psi.
Number Two: Wrong Carb Size Choosing a 1,050-cfm Dominator for a 327-cube engine might impress the guys at the local car show, but it is a surefire recipe for a poor-running engine, unless it’s designed to turn 9,500 rpm. Kevin Baker: “If you’ve got a 500ci engine and all you’re going to do is fire it up, back it out of a trailer, and cruise it down to the Dairy Queen to light the tires off, you don’t need a 1,000-cfm carburetor to do that. A smaller carburetor will drive better.” Also, there is no truth to the myth that you can jet a carb from a 750 to an 850, or anything like that. The sizing is determined by the airflow through the venturi, boosters, and baseplate, not the fuel jets.
Number Three: Wrong Carb Design Typical four-barrel carburetors are set up as either square-bore or spread-bore design. The square-bore carbs have equally spaced and sized throttle bores while spread-bore carbs have two smaller primary bores and two larger secondary bores. As you may know, or would expect, there are intake manifolds designed for each of these. Adapters are available to allow cross breeding of square-bore carbs to spread-bore manifolds, and vice versa. Edelbrock’s Stinson says: “If the customer has a spread-bore like a GM intake that a Q-jet came off of, we’d recommend a number 2697 adapter kit.” It’s important to know that if you have a car with a low, flat hood, you may need to check hood clearance.
Number Four: Let Your Buddy Tune Your Carb He may be just as excited as you are with your new purchase, but before you let him loose with a box of jets and a flat-blade screwdriver, note how good his car drives and how long it took him to get it that way. He may indeed be an expert, but odds are slightly against it. A quick telltale sign of whether he knows his stuff is whether or not he has a full box of assorted jets, air bleeds, squirters, pump cams, vacuum gauge, and diaphragms and if he knows what the heck they are. If he’s only messed with Holley-style jets, he might not know what to do with Edelbrock-style metering rods.