Want to get the most from a carb? Understanding and optimizing thevarious tuning points to really dial it in makes all the difference.Entry-level carbs have the fewest tuning options, while atop-of-the-line unit like this Race Demon RS features an incrediblenumber of tuning possibilities. With so many tuning permutationsavailable, you'd better know your stuff before digging in.
Tuning a carburetor can be done on many levels, depending on your ironand how you need to use it. A good ear and a slot-head screwdriver areall it takes for a curbside calibration on an old Carter BBD two-barrelsloshing gas down the pipes of a greasy 318. Not to shock you with thecontrast, but top Pro Stock teams spend more on carb calibration and therequired facilities and equipment than it would take to buy a room fullof Mopar Muscle staffers. Where do you fall on the scale?
Even a kidwith a screwdriver can benefit from a little knowledge of what thattwist of the screw is doing, while more serious Mopar motorheads cangain a fistful of power with a well-considered calibration. Carbs thesedays are available with more adjustments and tuning points than ever.Generally, entry-level carbs have fewer tuning points, often just theprimary main jets, idle speed, and mixture. Moving up the scale, thehigher-end carbs up the ante with replaceable secondary jets, whileracier carbs may incorporate four-corner idle adjustments, primary andsecondary idle speed screws, and perhaps replaceable air bleeds for theidle and main circuits. At the top end of the scale are the race carbs,which feature the greatest level of tuneability.
We selected the Race Demon RS to run through our tuning examples, simplybecause this carb is the most adjustable on the market. The carb isadjustable for venturi size and flow rate via replaceable venturisleeves. Other tuning points include idle speed adjustment at both theprimary and secondary throttle shafts, typical pump adjustments, fourcorner idle mixture, replaceable jets at both ends, replaceable powervalves, replaceable high-and-low speed air bleeds, removable boosters,air bypass jet provision, screw-in idle feed restriction, andreplaceable emulsion bleeds.
Demon's top-of-the-lineRace Demon RS includes all the tuning provisions mentioned, as well asreplaceable venturi sleeves, boosters, idle feed restrictions, emulsionbleeds, and auxiliary air metering. As an uncompromised race carb, thetuning flexibility is designed in, and the custom tuning possibilitiesare staggering. Tuning one correctly requires more than curbsideguesswork; you'd better have a well-instrumented dyno and more than alittle know-how before digging in too deep.
Fortunately, the factorycalibration on carbs, including the race units, put the more esotericsettings in a range that will seldom require custom tuning. Generally,jet changes alone will provide a carb that's very close. Even so, thereare more screwdriver slots in carbs than ever before, so a rundown onwhat all those little things do is worth a look. For your entertainment,we brought the mother of all tuner's carbs--the Race Demon RS--alongwith a burly 440 for a session on the Westech dyno, all to hunt downthat elusive "magic" tune.
Before any of the more serious tuning can begin, and before the engineis first fired with a new carb, get the base adjustments set first.These simple procedures are often bypassed, causing headaches later on.First on the list is the float level, made easy with Demon's large bowlsight glass. Brule recommends the fuel level adjustment be set one-halfto two-thirds of the way up the side glass.
WHAT IS IT: Idle speed screws.
WHO SHOULD CARE: This one is for everyone, from the kid with the BBD tothe heads-up racer running a pair of quads.
WHAT IT DOES: Sets the position of the throttle plates for idle.
WHAT CHANGES DOES IT MAKE: Screw in-faster idle; screw out-slower idle
TUNING TIPS YOU NEED TO KNOW: Idle speed is a function of the airgetting in, at the correct mixture. The idle should feed off the idlecircuit, which includes the idle port and partially the transition slot.Large idle openings position the throttle blades too far past the idleport for the mixture screws to be effective. Always use the least amountof idle opening to achieve the required idle rpm. If the mixture screwsdo not seem to have any effect on idle quality--as can happen whentuning with large cams with poor idle vacuum--chances are the blades areopen too far. Read on for other ways to get air in and close thethrottle idle setting. The rule of thumb here is to set the throttleblades to expose just a small portion of the transition slot below thebutterflies.
Adjustment is made by loosening the lock screw, and turning theadjusting nut out to raise fuel level, and turning it in to lower it.Check the fuel level with the fuel pumps on (electric) or the enginerunning (mechanical).
WHAT IS IT: Idle mixture screws.
WHO SHOULD CARE: Anyone with a carb and consciousness.
WHAT IT DOES: Sets the quantity of mixture (idle emulsion) entering theengine through the idle circuit.
WHAT CHANGES DOES IT MAKE: Screw out for more fuel, richer; screw in forless fuel, leaner.
TUNING TIPS YOU NEED TO KNOW: Always adjust the screws the same amount,keeping the settings at the same number of turns out from a lightlyseated baseline. Demon carbs can be set at a baseline of one turn fromseated, and fine-tuned from there. If the mixture screws have no effect,look for an overly open idle speed screw.
WHAT IS IT: Auxiliary idle air bypass bleed.
WHO SHOULD CARE: Brawny big-cammed thugs like us.
WHAT IT DOES: Lets air into the plenum by a means other than past thethrottle butterflies.
WHAT CHANGES DOES IT MAKE: Idle air bypass will allow the throttlebutterflies to be closed down to make the idle circuit responsive wherelarge cams and low vacuum are dictating too much throttle blade openingfor idle speed adjustment. Demon carbs come with the "Idle Eze"adjustment in the center of the carb, while Race Demons use an orificein the same position. Tune in air here, and close the butterflies to thenormal position.
TUNING TIPS YOU NEED TO KNOW: Old-time carb tuners would drill thebutterflies to get this air bypass effect. Drilling was hit or miss,while the Demon carbs turn this into a simple adjustment.
WHAT IS IT: Idle feed restriction.
WHO SHOULD CARE: Experienced tuners solving an idle problem.
WHAT IT DOES: Similar to a main jet, but for the idle circuit.
WHAT CHANGES DOES IT MAKE: Larger orifice gives more fuel and a richeridle emulsion; smaller orifice makes for less fuel flow and leaner idleemulsion.
TUNING TIPS YOU NEED TO KNOW: The idle feed restriction is a drilling inthe metering block and is factory set in street carbs, but racier carbshave replaceable idle feed restrictions. More hairy cam tuning here,since lots of carb air bypass will require a fatter idle emulsion togenerate the needed idle mixture. Idle feed restriction tuning issensitive to small changes. Changes to the idle feed restriction willalter the required idle mixture screw setting, which may be necessary ifthe best idle quality setting is found to be in an out-of-whack screwposition (e.g., like nearly all the way in or way out). Hook up to asensitive Lambda and proceed with caution.
WHAT IS IT: Idle air bleed.
WHO SHOULD CARE: Carb savvy tuner trying to solve an idle problem.
WHAT IT DOES: Draws air in to emulsify the idle mixture.
WHAT CHANGES DOES IT MAKE: Larger bleed mixes in more air, making aleaner mixture. Smaller bleed has the opposite effect. Alters the timingor sensitivity of the idle circuit.
TUNING TIPS YOU NEED TO KNOW: Just a drilling in street-type carbs, butreplaceable tuning point in higher-end units. Not as sensitive as theidle feed restriction to altering idle mixture, but can change thesignal to the idle circuit and alter the smoothness of the transitionfrom the idle to main circuits.
Brule likes to set the primary and secondary idle speed screws to ahalf-turn from the "bottoming" position. Demons make the secondaryidle-speed adjustment easy with a conventional idle-speed screw.Fine-tuning will be done later with the engine running. | 
Brule will typically set a Demon carb's idle mixture screw at one-turnout from lightly bottomed at all four corners as a bench setting. |
It's shocking how much carb tuning is done to fix ignition problems.Before starting any carb tuning, make sure the ignition system isoperating to its full potential. We had a top-notch MSD Pro Billetdistributor, working through a Digital 7 ignition, plumbing the sparkthrough MSD wires. A bulletproof and reliable ignition system will helpavoid chasing your tail on carb tuning. | |