Barry Grant's new Old School Triple-D Six Shooter system combines the vintage look with contemporary performance. Due to the progressive linkage, the engine runs off the center carburetor most of the time; all three reach wide open simultaneously.
Of all the mechanical icons there are in the performance world, few things are more impressive than multiple carburetors. Adding another carburetor to an engine was one of the earliest tricks hot rodders came up with to enhance performance and virtually every combination from two to eight has been tried, but when it comes to V-8 engines, the coolest combination is undoubtedly three twos.
While adding carburetors seems simple enough, there's actually more to it than meets the eye. One of the most common misconceptions is that when carburetors are added the jet sizes have to be reduced to keep the engine from running too rich, but that's not the case. Although the jets determine the air/fuel ratio, the amount of air passing through the venturi determines the amount of fuel that is discharged, not how many carburetors there are. Granted this is an oversimplification, but if one carburetor delivers a 14:1 air/ fuel ratio and another carburetor just like it is added to the engine, the air/fuel ratio is still 14:1, there's just a larger volume of that mixture. For maximum performance rejetting may be required with multiple carburetors, but the same holds true for single carburetor applications.
Obviously the purpose of multiple carburetors is to supply the engine with a larger quantity of air and fuel, but just because a little mixture is good, a lot isn't necessarily better in some instances. At idle and low engine speeds, one twobarrel carburetor can satisfy the needs of most V-8 engines and three are two too many. At higher a rpm, most small carburetors can't supply the volume of air and fuel required, but three of them can. So, in most cases you'll find three twobarrel combos equipped with progressive throttle linkage. At idle and low speeds (until half or two-thirds throttle depending on the linkage design/adjustment) only the center carburetor is operating. As the throttle pedal is pushed further towards the floor, the end carburetors begin to open and all three reach wideopen simultaneously.
Because carburetors only deliver fuel when air passes through the venturi(s), if the throttle blades are completely closed the carburetor is effectively shut off. And that brings us to the differences between the carburetors used in three two applications. In most cases, the end carburetors are different from the one used in the center, that's why you just can't bolt three "regular" carburetors on a manifold and expect good results. Think of three twos operating in a manner similar to a four-barrel. The center carburetor is the primary supplier of fuel at idle and low speeds, and the end carburetors are the secondaries that open when you stick your foot in it all the way.
Like most carburetors, the center in a trio has all the normal systems including idle, transfer, main, enrichment, and choke. And in most three two arrangements, all the carburetors have the same CFM rating (some factory installations have center carburetors that were bigger than those on the ends, others used center carburetors that were smaller than those on either end). But while all the carburetors may look the same, there are normally significant differences in those used on the ends. Generally they lack idle circuits and the throttle valves are thicker to seal more effectively and ompletely close off airflow when running off the center carburetor only, choke butterflies are also eliminated.
One of the systems found in most carburetors is an accelerator pump. It provides that squirt of fuel when the throttle is opened and it helps with starting and preventing flat-spots in acceleration. When the throttle is opened suddenly, the airflow through the carburetor increases faster than the heavier fuel droplets can, and the engine will go lean momentarily causing a stumble. The accelerator pump provides an extra volume of fuel until the main system in the carburetor can "catch up" with the demand. With three twos, when all three carburetors are suddenly opened, the function of the accelerator pumps is critical.
Another important feature found in carburetors is the enrichment, or power system. When the manifold vacuum drops as a result of the throttles being opened, the power system supplies a richer fuel mixture. In some three two installations, the power system is eliminated and the carburetors are simply jetted appropriately.
Variations On The Theme
* Triple-D Six Shooter
It you're looking to add three deuces to you classic truck, there are a number of options. Two of the best come from Barry Grant. Known for race winning carburetors, Grant has also been paying attention to the needs of street driven vehicles, and addressing the traditionalist in that crowd, he recently introduced his Old School version of the popular Triple-D Six Shooter induction system.
Based on Grant's new Triple-D intake manifold, the carburetors used on the Old School system have a gold chromate finish, a chrome fuel log with red fuel lines, and traditionally styled Rush air cleaners. But don't let the old timey look fool you; this system incorporates the latest carburetor technology. Rated at 250-cfm each, thanks to the progressive linkage, the Old School system provides excellent low speed throttle response (along with surprisingly good mileage) from the center carburetor and plenty of performance when all three are wide open.
As you might expect, the center carburetorin this system is different from the other two; it includes an electric choke, an idle system with two external mixture adjustment screws in the metering block, and a power valve that opens when the manifold vacuum drops below 6.5 inches. Two vacuum ports are provided: one ported for the distributor's vacuum advance and one full time for a transmission modulator (if they are not used, one or both can be capped).
To keep things simple, the only external adjustments on the outer carburetors are for float levels. And although the end carburetors do actually have idle circuits, they are linked to the transfer slots and the mixture and idle-speed adjusting screws are preset (no synchronization needed). The throttle plates are almost .040-inch thicker than those of the center carb to reduce mixture leakage into the venturi bores at idle.
* The Demon 98
Over the years there have been who knows how many multi-carb manifolds produced for three-bolt carburetors like the venerable Stromberg 97 and Holley 94. And while they were reasonably good carburetors in their day, those days were more than 70 years ago. Lots of things have changed since then and carburetor technology is certainly one of them. What the world needed was a modern carburetor with a vintage three-bolt mounting flange, and that's just what you get with Barry Grant's Demon 98.
Similar in basic design and with most of the same features as the Six Shooter carburetors, there are two versions of the Demon 98. One is designed to be a primary carburetor and comes with either an electric or mechanical choke, idle speed and mixture adjustments. The other is a secondary style without those features.
Certainly the appearance of the 98 is reminiscent of the vintage carburetors it's meant to replace, but from a functional standpoint there's no comparison. The 98 uses contemporary circuitry like adjustable air bleeds to tailor fuel curves for the best throttle response, fuel economy and power output as well as acceleratorpump circuits with interchangeable nozzles that can be changed to suit the engine's needs. But for those who have run those early carburetors the 98 is meant to replace, the most welcomed improvement is the Demon can handle six to seven pounds of fuel pressurewithout leaking like a sieve.
If your goal is to make the engine in your classic truck look traditional, nothing beats triple carburetors. And thanks to Barry Grant, now there are two ways you can have that traditional appearancewith contemporary performance.
Tuning Tips
You may have heard stories about the difficulties tuning multiple carburetors, but the truth is it isn't difficult with either the Six Shooter or Demon 98s. Barry Grant recommends the following procedure:
Set the float levels of all three carburetors just below the mid-way point on the sight-glass windows. Check the ignition timing, 14 to 16 degrees initial is suggested. Once the engine reaches normal operating temperature, set the idle speed, then find the best idle quality by adjusting the two idle-mixture screws (either inward or outward) a quarter turn at a time.
Trouble Shooting
Like anything mechanical, there may be carburetion issues to deal with from time to time. Here's some troubleshooting advice from Barry Grant.
* Blackens Spark Plugs While Driving:
Use smaller main jets, lower float levels, lower power valve number, and reset throttle plates (butterflies).
* Blackens Spark Plugs At Idle:
Reset throttle plates, tighten mixture screws, lower float levels, reduce fuel pressure, clean air bleeds, use smaller number power valve, and increase initial ignition timing.
* Backfires Through Carburetor:
Loosen mixture screws, increase size of the accelerator-pump nozzles, raise float levels, reset throttle plates, increase size of the main jets, and check ignition timing.
* Backfires Through Exhaust:
Tighten mixture screws, decrease squirter size, lower float levels, reset throttle plates, decrease size of main jets, and check ignition timing.
* Doesn't Pull Well At Wide-Open Throttle:
Increase size of main jets, raise float levels, and increase fuel pressure.
* Float Levels Won't Adjust:
Reduce fuel pressure, clean needle and seat assemblies, and ensure floats move freely.
* Float Bowls Drain Down After Shutoff:
Retighten bowl screws and power valve; replace bowl, metering block, and power valve gaskets, and clean the air bleeds.
* Fuel Drips From Boosters:
Lower float levels, reduce fuel pressure, and reset throttle plates.
* Fuel Drips From Accelerator-Pump Nozzle:
Loosen tension on the spring-loaded accelerator pump arm.
* Fuel Leaks From Throttle Shaft:
Lower float level, reduce fuel pressure, clean or replace needle and seat assembly, reset throttle plates, clean air bleeds.
* Fuel Shoots From Vent Tube:
Lower float level, reduce fuel pressure, clean or replace needle and seat assembly, and ensure float moves freely.
* No Adjustment On Mixtures Screws:
Reset throttle plates, increase initial ignition timing, lower float levels, and reduce fuel pressure.
* Rich While Driving:
Reduce size of main jets, lower float levels, use smaller number power valve, and reset throttle plates.
* Rich At Idle:
Reset throttle plates, tighten idle-mixture screws, lower float levels, reduce fuel pressure, clean air bleeds, use smaller number power valve, and increase initial ignition timing.
* Runs After Shutoff:
Reset throttle plates, lower idle speed, and check ignition timing.
* Sluggish At Fully Open Throttle:
Decrease size of main jets, lower float levels, and reduce fuel pressure.
* Smells Rich At Idle:Reset throttle plates, tighten idle-mixture screws, lower float levels, reduce fuel pressure, clean air bleeds, use smaller number power valve, increase initial ignition timing.
* Smokes At Idle:Reset throttle plates, tighten idle-mixture screws, lower float levels, reduce fuel pressure, clean air bleeds, use smaller number power valve, and increase initial ignition timing.
* Smokes Under Acceleration:Decrease size of accelerator-pump nozzles, lower float levels, reduce fuel pressure, and reset throttle plates.
* Hesitates Under Light Acceleration:Reset throttle plates, raise float levels, and reset idle-mixture screws.
* Hesitates Under Hard Acceleration:Increase size of accelerator-pump nozzles, reset throttle plates, and raise float levels.
* Surges At Idle:Reset throttle plates, loosen idle-mixture screws, and raise float levels.
* Surges At Cruise:Reset throttle plates, raise float levels, loosen idle-mixture screws, install larger main jets, and install higher number power valve.
* Won't Idle Down:Reset throttle plates, check idle-mixture screws, lower float levels, check initial timing, check throttle linkage, and check for vacuum leaks.
* Won't Stay Running Below 1500 Rpm:Reset throttle plates, check idle-mixture screws, lower float levels, and check initial timing.
* Won't Start:Prime carb with fuel, squirt accelerator pump, check float level, and check ignition system.
* Won't Stay Running:Reset throttle plates, check idle-mixture screws, lower float levels, and check initial ignition timing.
