Carburetor guys are a funny lot. We get our first performance-oriented carbureted car and generally go crazy. We learn about carburetors. We debate the benefits of one type over another. We collect them. We trade them. We put them on the shelf in the workshop after we run them for a while because we have to go out and try the next big thing.
But every carburetor enthusiast at one time or another stumbles across The Perfect One. The carburetor that runs and performs flawlessly no matter what we ask it to do. bolt it on a mild 305 and take it to a cruise night? No problem. Your friend's big-block tow rig needs a carb for a couple of days? No sweat. Want to take it bracket racing or out to the autocross? Again, not an issue. Ford, Chevy, or Mopar, all it'll need is a little tweak or two here and there, and you're good to go.
The Perfect One in our shop happens to be a Holley 4150-an 800-cfm double-pumper that carried the List No. of 4780-2 when it left the factory years ago. But the operative word here is "years." Scores of rebuilds and kits and pump cams and jet changes and pump cams and linkage changes had started to take a toll. Our go-to carburetor was starting to get a little long in the tooth, so we turned to Willy Krup and the experts at Willy's Carb & Dyno Shop in St. Charles, Illinois, to whup some mojo on it.
This wasn't a decision to be taken lightly. But Willy's is the carb shop responsible for the Holleys on Greg Anderson's NHRA pro stock engines, and on Joe Gibb's cars in the Winston Cup series, and more than 35 Busch teams. Willy's, established in 1985, builds carburetors for asphalt and dirt circle tracks, drag racing, marine, truck pulling, and pro street applications.
The first step upon arrival at Willy's is a session on top of one of the shop's three 400ci dyno engines hooked up to a Superflow SF-902 x-dynamometer to see how it's metered right from the start. This is an important part of the diagnosis-why go to greater expense than you need to if a carb only needs some minor adjustment? "If it was calibrated well and then it doesn't run properly, we know what we need to do," Krup says. "Or it could just be that the jetting or the air bleeds aren't right. A lot of times you can just go in and correct that kind of stuff and help the carb out quite a bit without spending a lot of money on it. It also gives us a guideline so when somebody sends us something we know how much power they're picking up when they get it back. You can almost grade their happiness off that before they even get it."
After a pull or two on the dyno, the carburetor is then stripped completely down to its component parts so the work can begin in earnest. First up is a session on the bench with hand deburring and chamfering done before the carb body, bowls, and throttle plate are sent to spend about 45 minutes in a vibrating tank filled with glass beads. Glass beading the carb will also help debur some of the stuff and will take the finish off the carb to give it a whole new cosmetic look-it comes out of the tank looking like freshly-cast alloy.
After the application of a new dichromate finish to inhibit corrosion, the next step is to mill all the gasket mating surfaces on the main body so they're true and flat to ensure good internal sealing as well as to curb any external leakage. The choke horn can also be removed at this point. "With a new carb, you're relying on Holley's mold to provide gasket mating surfaces that are flat and true," Krup says. "I promise you that they're not straight and true right out of the Holley box. We usually take off between 0.025- to 0.030-inch to clean them up."
Getting all those gasket surfaces flat and true so there's no surface leakage at all is the key to getting a carburetor to perform correctly. Krup says, "there is all sorts of circuit bleeding on a factory carburetor. You always hear the story, 'I got this new carburetor and it ran good for a while, but a month later I had to start moving the idle mixtures around, and they won't stay adjusted.' The gaskets are temporarily sealing the circuits, but then they start bleeding because you're trying to seal something up that's not flat and true. So some of the fuel from the idle circuit goes over into the transfer or power circuit-it'll just constantly be bleeding from one circuit to another, and the thing will never run right."
Meanwhile, the throttle plate is machined for and fitted with a new Teflon throttle shaft for proper throttle shaft clearance. Krup says, "this is just about 0.001-inch. The Teflon bushings don't require any kind of lubricant. Holley has a steel shaft in a dry aluminum throttle plate. That's just asking for problems. Teflon holds the shaft in place, and it can run dry. We put in lube holes so that if you ever get dirt or debris in it, you can use one of those little red tubes that come on WD40 cans that will slip right in there and blow any kind of dirt or debris out of that area."
At this point, the carb gets glass beaded again before having a final alodine finish applied and reassembled with brand-new parts. We elected to have some modifications done to our carb from a host that Willy's is capable of.
"As far as the calibration is concerned, we replace all the air bleeds in the body," Krup says. "We didn't change the boosters on this one-that's a modification we do, but this one came out close enough on size to where we didn't have to."
Krup is looking for a certain air/fuel ratio when the carbs are dyno-tested. He says, "That's usually right around 12.7:1 to 13.2:1. The air bleeds help us tailor that air/fuel ratio on the top end. There are also idle air bleeds that help us tailor the idle up to about 2,500 rpm. If the air/fuel ratio starts getting a little rich on us once we get past 6,000 rpm or so, we'll go in there and open up the air bleeds a couple of thousandths. That will give us the same mixture through the midrange, but the top end will be a little leaner."
One trick addition to our Holley was the addition of Willy's own externally adjustable metering blocks so there is no longer any concern about fuel spillage or torn gaskets should a jet adjustment be necessary. Krup says, "The factory Holley metering block is about as straight as your leg is. You're using four corner points to pull the thing snug and seal the thing all the way across. So the body isn't straight; the metering block isn't straight. The metering blocks we put in there are 100-percent flat except for the accelerator pump which is 0.004-inch higher because it's a pressurized port. The raised surface makes it crush the gasket even tighter in there so all the surfaces stay sealed up 100 percent."
Willy's metering blocks also feature replaceable metering valves. Each "click" on the adjusting screws provides a two-jet number change richer or leaner. There are six different valves to choose from, providing primary or secondary jetting equivalents ranging from 60 to 68 all the way up to 90 to 98. Our carb was fitted with a pair of blocks providing jetting adjustments from 70 to 78, as well as a pair of jet extensions and a slotted float on the secondary side to ensure the engine gets plenty of fuel during hard launches.
Our carb's throttle plates were additionally drilled with extra air holes so the unit will now idle on an engine with a big, lumpy cam without burning your eyes. Krup says, "It ought to sit there and just thump along right at 900 rpm."
Since Willy's dyno-tests each carb that comes into the shop when it arrives, it's no surprise that each completed carb goes back onto the dyno for tuning and evaluation after receiving a makeover.
Our results? How about 20 lb-ft of torque and about 24 hp. "We feel like we get the carburetor as close as we can without having to be right there with the customer," Krup says. "Very few carburetor shops go through the added time to run them after they've been worked on. Running them on the dyno afterwards and tuning them ensures it will idle and there will be throttle response. it's not going to hesitate and it's going to do everything we possibly have control over. I've seen terrible cases where somebody gets a carburetor back from some place, and its got fuel shooting out the top or the thing flat doesn't idle because some of the circuitry is messed up."