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So you've just finished bolting up that brand new engine. A freshly built shortblock, shiny aluminum heads, a hot cam, full-length headers, and a new intake. What are you going to put on top of it? Probably the same, tired and old Holley 650 you've been running for the past five years? If this scenario sounds all too familiar, don't sweat it you're not alone. We all like the latest, bright and shiny go-fast parts, but when it comes to shelling out several hundred dollars on a new carb, most of us can't justify the expense, especially when the significant other knows how much you've already spent on your project.

Fortunately there is an option that is better than new, but doesn't cost as much. A professional rebuild. We're not talking about picking up a $20 a gasket kit, but rather a complete teardown, blueprinting, and recalibrating to achieve maximum performance, or fuel economy, depending on your exact requirements.

What you end up with is a carburetor that performs better than a new one, because assembly line irregularities are fixed and the carb is tuned to your engine, rather than a "out of the box" calibration. We recently had an opportunity to renew our Holley 650cfm double pumper that had served us well for nearly six years, and in fact had served someone else for a few years before that.

After building up a new motor we noticed our 650 simply wasn't performing the way it should. The idle screws seemed to be less responsive and the throttle shafts leaked a little fuel. Throttle response off idle and cruise was also not what we'd expect from a fresh engine. Knowing we were way over budget as it was, a brand new carb was out of the question.

We tried rebuilding the carb using a Holley rebuild kit, but the results were mediocre; we fixed some of the leaks, but the carb still felt sluggish. A Holley rebuild kit is probably a good idea every ten-fifteen thousand

miles, or once a year, but on a neglected carb like ours, it really demanded more work. A fellow racer suggested we contact BIGS Performance, a carburetor shop in Wisconsin. BIGS has over 25 years experience rebuilding and customizing Holley's, from daily drivers to circle track engines. We called up BIGS and were surprised to talk directly with owner Lynn Bignell. Lynn spent a good half-hour with us discussing our carb, its problems, and what we wanted to achieve in terms of performance and fuel economy. Lynn and his son Jesse explained to us their procedure of complete disassembly, checking and milling flat each mating surface, correcting and modifying the fuel delivery passages, and other "secret" modifications and calibrations specific to our engine combination (in this case a naturally aspirated, 302 which spends the majority of it's time at the drag strip.)

The total cost for a BIGS rebuild starts at about $150, depending on the level of modifications, but nevertheless a considerable savings over a comparable Holley or Barry Grant performance carburetor.  By the way, we didn't tell them until after the conversation that this was a magazine project, so the personal attention we got is definitely standard practice at BIGS.

Here is a general idea of the work BIGS performs during a typical carburetor rebuild and tuning. We'll cover some of these in detail.

  • Complete disassembly
  • Thorough cleaning to remove varnish, dirt and corrosion
  • Check and modify circuits for correct size
  • Check and clear any casting obstructions
  • Check and correct surfaces for flatness
  • Includes float bowl, metering plate, air bleed, throttle
    shaft, and boosters to meet or exceed factory
    specifications
  • Remove all metering block brass plugs and clear fuel
    well of factory casting
  • Check air bleeds and adjust if necessary
  • Change fuel emulsion signal for even fuel flow and
    distribution

Rebuilding and Tuning
Over the course of a several days BIGS performed their "magic", meticulously going through every piece of our carburetor, just as they would do with a carburetor for a restoration project, or for a high-rpm oval track motor. We were stunned at just how different the carb looked when BIGS got through with it. Our grimy and sooty carb was now powder coated in brilliant gray. The choke horn that we had crudely hacked off had been milled completely flat, as had the base plate, body, and jet blocks.

At first glance we could tell BIGS had modified the fuel and air bleeds. Upon removing the fuel bowls, we could also tell the numerous passages had been cleaned out, deburred and resized where necessary.

Although BIGS wouldn't reveal all their secrets, we could spot a few modifications without much disassembly (see side bar.)

 
(Dyno Testing)
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Although BIGS wouldn't reveal all their secrets, we could spot a few modifications without much disassembly. In the higher-flowing downleg venturis (compared to the venturis in the photo of our original carb.) BIGS also smoothed and contoured the throttle bores, shaved the throttle shaft and installed button head screws, all in the aim to increase airflow through the carb while reducing turbulence and promoting better air-fuel atomization.
 

We told BIGS that our 9.5:1 302 typically ran a stout solid-roller camshaft and single plane intake, both of which made the idle quality tough to manage. BIGS made some internal modifications to the carburetor, such as the different idle air bleed orifice shown here. These types of modifications not only require extreme precision, but they require years of knowledge and experience. Unless you know carburetor function inside and out, we recommend you leave it to the pros to make such adjustments.
 

To eliminate a lean bog upon launching, BIGS installed jet extensions secondary metering blocks. This prevents the jet from being uncovered  as fuel is forced back in the bowl due to the momentum of the vehicle off the line. The screw-in extensions shown are preffered over the "slip on" style that Holley sells, which can fall off the jet.
 

Solid nitrophyl floats were also installed to maintain a consistent fuel level in the bowls,m and to clear the extensions. Stock brass floats can develop small leaks which cause them to fill with fuel and change the fuel level.

 

 

 

 

 

 

 

 

 

 

 

 


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