So you've made up your mind to buy a custom built carburetor for your racing engine. Your goals should be to have a carburetor that allows your racing engine to perform as expected with more horsepower that's easy for the driver to control as well as a metering package that allows the engine to not burn excessive fuel during part throttle operation, warmup caution laps driving back down the return road etc. You want and expect a "super carburetor" one that performs above and beyond an out-of-the-box Holley, Demon, Quick Fuel etc. The carburetor package should be well thought out with sensible tuning and modifications where required to accomplish these goals and maybe it should look "cool" too. Shown here is an example of a custom carburetor that does look "cool" but as you will soon see has many serious shortcomings that any racer should avoid at all costs.
As you can see, this Dominator has the cool look down cold with its billet booster inserts, colored throttle plates and nice HP style body. It looks nice and it would look great in any race car's engine bay. But the good looks are deceiving because this custom built carburetor leaves a lot to be desired in both performance, drivability and even safety.
Upon closer examination when inspecting the carburetor a loose booster insert was found. In the Dominator style carburetor with an annular booster "banjo" the insert is a separate piece that in this case is machined from a piece of billet aluminum and then anodized an attractive purple color. Sometimes when the inserts are manufactured there can be problems with the anodizing process and the parts must be stripped and then re-anodized for proper appearance. When this is done sometimes the parts can be a little "off" on size and end up just a little too small for a nice snug fit into the banjo. When this happens the only choice is to discard the bad insert and install a new one that's up to spec size-wise not to go ahead and install the faulty piece and hope nobody notices the poor fit or worse yet has it fall down into the running engine and jam the throttle wide open. If you see this problem it's not something that just "got that way" over time, it was like that from the beginning.
These two pictures while not very exciting show another very serious problem with this carburetor build and yet another oversight that could have as a consequence, disastrous results... Shown in the picture on the left is the accelerator pump arm in the closed position. It's on the cam just a bit less than 50% of it width. When the throttle is opened fully as in the picture on the right the arm is just barely still on the cam and trying to slip off to the left or body side of the plastic cam. If this would happen the throttle could end up stuck in the wide open position!(note photo of Allan Potter after a stuck throttle) This is especially true after the carburetor has been used for a while and the cam has had a chance to wear which it will do very quickly due to the poor alignment to start with when the carburetor was brand new.
These photos show the front of the carburetor where the metering block is attached. It's obvious from the photo on the left that the carburetor body has been set up in a mill and machined flat on this surface. This operation is not a bad thing, probably not necessary if starting with a new body but still quite harmless and in this example nicely done. What isn't so obvious is how the transition circuit of the carburetor has been unnecessarily and irreversibly modified, thus ruining the body for use in any sensible carburetor build! Notice in the middle photo how the transition slot has been removed by drilling through the body, plugging the hole and then drilling a new hole and inserting a transition "tube" instead. Compare to the unmodified Dominator shown in the photo on the right. The purpose of the transition slot is to ease the transition as the throttle is opened from a closed throttle idle position to wide open throttle. The reason a slot is used is so that as the throttle blade is opened it gradually exposes more and more of the transition slot and thus meters more and more fuel(to mix with the increasing amount of air) from the idle circuit(also feeds the transition slot) making the transision smooth and perdictable. In the modified carburetor instead of a slot we now have a hole and a tube mounted above the closed throttle and completely ineffective in making any transition. This made this carburetor nearly useless when driving in a just-off-idle throttle position, stalling, chugging getting the plugs black. Why you would take a perfectly good carburetor and perform this modification remains a mystery. Even a call to the carburetor builder who built this example failed to shed any light as to why this modification was performed and if it ever served any useful function.
These two photos show a bottom view looking up into one of the throttle bores on the modified Dominator. In the photo on the left the transition circuit mod can be clearly seen(it's the little tube sticking out below the intermediate circuit discharge tube). In the next photo the very long booster insert can be seen. Why this was needed along with the 100 jets again remains a mystery.
On this carburetor the metering blocks were nothing special but for some reason had a 4 hole emulsion package with the top two holes at .026 and the bottom 2 at .028 with a .036 main high speed air bleed. Holley blocks with screw in bleeds were used with no power valve provision.
The purpose of this page is not to put down anyone's work or to fault someone for trying to make a living. What it is trying to do is to give the reader a bit more insight into what goes into a well built custom carburetor package. It's the little things that count. The engineers who did the design work for major carburetor companies (in this case Holley) were very smart people and that fact should always be in the mind of any carburetor builder, modifier or user(racer). With that said, if the carburetor package begins to stray very far from what was originally built by in this case Holley; it's probably seriously flawed in some way and really needs to be reevaluated as to it's purpose and usefulness. Attention to detail in all areas of the carburetor build is the only way to be truly successful and to build a superior product. Bobby at Competition Carburetion takes his work very seriously and is not only willing to spend the time needed to develop a superior carburetor package but to build it and deliver it to you as well.
