4412 Carb Magic by Sean Murphy Induction

Last year we bought a Street Stock Dirt Track race car (now named Madd Maxx) to take a shot at running it in the dirt. The old bomber was a former Track Champion car at Route 66 Raceway, now known as Victorville Auto Raceway. With a rookie circle track driver and a young inexperienced crew, the car debuted at the next to last points race in 2008 and did surprisingly well.

The next two outings would not go as well and the driver began to complain about the throttle response. The crew assumed that it was just a rookie driver making excuses to throw the blame off of himself – that is, until someone asked if we had an SMI carb. After some investigation, we discovered one of the best kept secrets in dirt track racing — Sean Murphy’s carb magic.

About Sean Murphy Induction

Sean Murphy Induction (SMI), located in Huntington Beach, California, is home to carburetor expert Sean Murphy. Sean has been working on carbs professionally for the better part of twenty years.  Holding a Bachelor’s degree in Automotive Technology from Southern Illinois University, Sean knew he wanted to get into racing.  “Twenty years ago there wasn’t a School of Automotive Machinists or UTI like there is now,” Murphy told “Chaffey College in Rancho Cucamonga had a Race Car Technology program, so I went there. While I was going to school, I heard that this carb place in Ontario had an opening, so I went down there and bugged them and didn’t take ‘no’ for an answer.” At that point, he starting working at Brad Urban’s legendary “Carburetor Shop” in Ontario, California where he quickly became the shop manager.

His experience at Urban’s shop led him to the higher performance areas of carburetor technology, such as NASCAR’s Winston Cup and NHRA Pro Stock, where he did rebuilding, track testing, dyno testing, and tuning for most of the teams in those series. Sean was lured away from Urban’s shop to another historic carb shop, Mike Jones Carb in Huntington Beach. The shop was under new ownership as JET Performance and was struggling, so they hired Sean to help turn the business around.  After eight years at the shop, JET Performance Fuel Systems had become one of the largest suppliers of performance carburetors to performance warehouses in the country.

SMI was Born

In 2001, Sean had a vision that tighter quality control and precision blueprinting of carbs would provide a new level of personalized service and quality for those looking for the highest caliber of carb performance. Sean Murphy Induction was born out of that vision and has become one of the racers’ best kept secrets. Sean doesn’t mind being a well kept secret. “I’m three weeks behind on my orders right now, so don’t tell me that carburetors are a thing of the past,” he told us. We knew that getting Sean to custom build a Holley 4412 for our street stock car was going to make a huge difference.

We arrived at Sean’s shop an hour before opening because we didn’t want to miss an opportunity to see how he reworked a carb from start to finish. After waiting forty-five minutes, Sean pulled into the parking lot and opened the doors. We were amazed at the organization and detail in the shop. Laid out on the build table were all the components that he would need to build our carb.

The information sheet that we had previously filled out and faxed over to SMI was at the head of the build table. Sean makes it a requirement to get all of the information about your engine and your type of racing before committing to customizing a carb. It’s one of the most detailed forms we have seen and asks for component dimensions and brands. Sean told us that “the better the information, the better we can build your carburetor,” and it’s clear that he checks the details thoroughly.

Sean Murphy at the build bench starting work on our custom Holley 4412 Stage I Carb.

After a brief introduction, Sean got right down to work. Talking with us the entire time his hands were moving around the carb parts assembling the intricate pieces, Sean proved to be a wealth of information about engines and induction. We asked him what made SMI built carbs unique and in such mind-blowing demand by racers. “I think that because of my education and background, sometimes I have a better understanding of what’s happening inside the engine rather than just inside the carburetor, and how to make the carburetor work better with what’s happening inside the engine.”

Sean selects specific carb bodies for custom work. Here he checks the venturi for proper size with a venturi gauge.

Sean is also willing to back his work in a way that other carb builders won’t.“There are some good people out there,” he explained. “I’m not the only one building carbs. But I believe in my work so much that I am the only one with a buy back guarantee. You buy my carb and find that someone else’s works better for you, I’ll buy it back”.

Sean starts with a select carb body and mills the metering block and base plate mounting surfaces perfectly flat.

Prepping the Carb Body and Selecting the Right Components

Using a carb jig of his own design, Sean starts the build by milling the mounting surfaces of a hand picked carb body that has been cleaned and blasted with media in his blast cabinet. Even though the carb body has a fresh surface, Sean cleans the holes and passageways of the body to remove any burrs left by the machining process.

Removing any metal burrs left by the machining process.

Once the carb body has been prepped for assembly, Sean turns his attention to the metering block. He explained that Holley uses several different types of metering blocks. To reinforce that point, Sean pulled out three metering blocks to show us. He went on to explain that most are easily identifiable by the number of holes in the emulsion circuit of the metering block. According to Sean, “the emulsion circuit is a means of bleeding air and oxygen into the fuel before it leaves the carburetor.”

Three different metering blocks with three different configurations in the emulsion circuit. The block on the far left has two holes, the center block has three holes and the far right block has no holes in the emulsion circuit.

The emulsion circuit is designed to atomize the fuel so that it can be used more efficiently in the engine. More emulsion holes in the circuit “means that the fuel can be atomized finer, which is great for an engine designed for that,” according to Murphy. “Specifically, it has to be a motor which a very good combustion shape and higher compression motors.” We asked Sean what we would need for our 64cc stock chamber as far as meter blocks and emulsion circuits. He told us that we would do best with “the meter block with two holes in the emulsion circuit and some additional SMI modifications to the anti-siphon and hi-speed air bleed systems.”

The system circuits from the metering block mate up to these orifices on the carb body. Sean tells us that system crossover is a problem, especially with the vacuum circuits.

The Metering Block Circuits

Sean then went to work on the carb body, adding the SMI modifications that fine tune the carburetor to work with the engine. Starting with the air bleed circuit, he installs new air bleed screws and drills them to size. Size is dependent upon the components in your engine and the application. While Sean is very open about what he does, he will not explain how he comes up with just the right orifice sizes. That’s a proprietary secret based on twenty years of real world testing.

Modifying the bleed air circuit based on SMI proprietary specifications.

Continuing with the emulsion circuit in the metering block, Sean installs “blank” jets that only have a small pilot hole drilled in them. Based on the information supplied on the engine data sheet, Sean drills the “blanks” to the size that will maximize the efficiency of the circuits. The sequence of photos below show the orifice sizing process in the metering block.

Installing the "Blank" to custom size the orifice in the circuit.

Checking the hole sizing with a drill bit.

Custom sizing the circuit orifice with a drill.

Sean made sure to point out that our Stage I carb was not customized beyond the track rules but was more of a “blueprinted” carb. All holes and jetting were sized to the maximum allowable factory tolerances. Several SMI carburetors have been taken apart in tech sheds across the country and checked with pin gauges. The carbs have passed tech inspection in every case. Of course, if you want a carb built for maximum performance and rules are not an issue, SMI can perform a Stage II or Stage III carb build as well.

Installing the Boosters

After the metering block circuits were properly sized, Sean replaced the boosters in the carb body. “I install the boosters by fitting them to the carb,” Murphy explained. “Meaning that I take the mating flange and make it a precise fit.” We watched as Sean took a new booster and ground the mounting flange down a little bit, and installed them into the venturi making sure that they were mounted flush and centered. Once he was satisfied with the placement of the boosters, he swedged them into the carb body for a perfect fit.

Installing the boosters into the venturi.

Measuring the boosters for proper height.

Swedging the booster mating surfaces to the carb body for a precise fit.

Filing the excess flared flange down to ensure a flush mount for the metering block.

Finalizing the Build

With the majority of the “blueprinting” tasks finished, Sean installed a power valve in the metering block and a 10cc accelerator pump diaphragm into the float bowl. All that was left to do with these components was to assemble the metering block to the carb body, then assemble the float and float needle assembly into the float bowl, and attach the entire assembly to the carburetor body. Sean explained that SMI has always used O-rings instead of the fiber washers in the idle adjustment screws and in the float adjustment assembly. Now that the main components were installed, Sean began working on the throttle base plate to wrap up the build.

The float bowl with float and float needle installed.

The float was reworked for the circle track environment that the carb would be functioning in. Because the fuel would be pushed to the right side of the float bowl, the float was ground down to compensate. With the fuel pushing against the right side of the float bowl, the float would be lifted higher up in the bowl and shut off the incoming fuel (the float would sense an artificially high level).

With a diminished level of fuel in the float bowl during hard cornering, the carb would not have the available fuel on demand coming off the corner and into the straightaway (when you want it most). Sean has developed a grinding technique that removes just enough material from the float, in the right area, to eliminate that fuel starvation coming off of the corners.

The stock float on the top compared to the modified float on the bottom. Notice the material removed on the left side of the modified float.

The carburetor base plate required very little in the way of modification. Grinding down the throttle plate mounting screws to remove any unwanted airflow restrictions is “one of the few areas that you can grind material away and still be within most track rules,” according to Sean.

Grinding the excess material off of the throttle plate mounting screws.

Sean finished up the base plate by pulling off any vacuum ports and filling the ports with brass plugs. Double checking the base plate for proper dimensions and adding a smaller accelerator pump cam, Sean was then able to mount the base plate to the carb body. Now we were ready for the final touch, a modified discharge nozzle (super squirter) with Sean’s self-designed weight, spring, and discharge ball. He explained that this setup was more dependable than the stock weight and ball metering and was not prone to siphoning or bleeding off issues that the stock squirters encounter. The photo sequence below shows the new and improved “super squirter” being installed.

SMI's "Super Squirter" assembly.

Inserting the discharge ball.

Getting the discharge Screw, Nozzle and Spring ready for installation.

Installing the Discharge assembly into the carb body.

Finishing up the assembly by tightening the discharge nozzle screw.

Sean was finished putting our SMI Stage I 4412 Holley carb together, and we shook hands and shared some parting words. A true artist and magician, Sean didn’t give away all of his secrets, but he did tell us a couple of things that apply to all of SMI’s carbs:

  • Don’t over tighten the bolts on the float bowl when changing jets. There’s only one on each corner of the float and metering block that hold it to the carb body. Over tightening the bolts will cause the ends to lift over time and you’ll get circuit bleed over. If the vacuum circuits start to cross over, it will cause erratic operations and will give you a problem that you’ll never find.
  • Don’t touch the float level. If you call with a problem that you can’t diagnose, that is the first thing that I am going to ask you.

With that, we drove away from the SMI shop in wonder at the “Carb Magic” that Sean Murphy had performed on our Holley. We got back to the PowerTV garage and installed the carb. True to his word, the carb needed no adjustments, and we dared not adjust the float level. Our next trip out to the track verified the increase in engine performance, before we were sidelined with a faulty electrical system.


Sean Murphy Induction
Phone: 714.843.9169

Post A Comment

  • bret adams says:
    October 10, 2014 at 10:06 pm


  • tk says:
    March 18, 2016 at 8:11 pm

    put a bowtie on the car lol. Check with a good carb shop to match the carb to the build you have. If you are doing it on your own power valve choice and jetting will help air bleed adjustment are better done with a wat to monitor the air fuel ratio.


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