Engines produce vacuum, and over the past 120 years, engineers have contrived ingenious ways to harness its power to the engine’s induction system. Through a labyrinth of small-bore drillings in the carburetor, the vacuum draws a potent mixture of air and fuel. So formidable is the mixture, it empowers naturally aspirated full-bodied 500-cubic-inch race cars to speeds in excess of 213 miles-per-hour in a distance no greater than 1,320 feet!
Although its fumes ignite, gasoline won’t burn and produce energy in its liquid form. Instead it needs to be emulsified (mixed with air), atomized, (separated into fine particles) vaporized (transformed to a gaseous state) and compressed in order for it to produce energy.
That’s where the carburetor comes into play.
In Carb Tuning Tips, Part 1, we provided a brief overview of the carburetor, including some basic carburetor anatomy. In Part 2, Demon Carburetion helps us identify a few more chief components of the modular, square-bore carburetor, acquaint you with common problems that can arise, and tell you how to prevent them.
Basic Carb Function
The carburetor takes responsibility for the emulsification and atomization of fuel, while the vaporization occurs in the induction tracts. In addition, the carburetor must meet the air-fuel ratio requirements of the engine.
To emulsify the air and fuel, small-bore drillings are placed in the air entries on the top of the carburetor. They are known as air bleeds. The emulsified mixture is then drawn rapidly through a multitude of small passageways and distributed into the fast moving air stream in the carburetor’s throttle bores for atomization. It is discharged through a dozen small ports, three in each throttle bore. They are known as idle, transfer slot, and boost venturi.
The idle discharge ports and transfer slots are located in the throttle bores of the base plate and draw their source of fuel from the fuel bowls via the carburetor’s idle wells. The boost venturis and their discharge ports are positioned near the top of the carburetor and discharge into the narrowest part of the upper throttle bores. Boost venturis draw their source of fuel from the fuel bowls via the main wells.
The #1 Tuning Problem: Sloppy Idle-Speed Adjustment
The top cause of carburetor troubles is the overexposure of the transfer slots at the idle condition. When increasing the idle speed of a four-barrel modular carburetor, adjust both the primary and secondary throttle blades. If you adjust the idle speed with the primary throttle blades only, you could upset their position relative to the transfer slots. Some of the adverse effects of an overexposed transfer slot at idle are hesitation, excessive richness, or poor running; it can even negate the idle circuit completely. In the idle condition, when the throttle blades are closed, the transfer slots should give the appearance of a small square when viewed from underneath the base plate.
Demon Carburetion even created a special component to ease the complications associated with idle-speed adjustment. This device, known as Idle-Eze, can introduce an extra source of idle air to the engine without disrupting the critical relationship between the idle-speed adjustment screws, the throttle blades, and the transfer slots. The Idle-Eze has access to air above the throttle blades and can transfer it beneath them, if necessary. This introduces an extra source of air to the engine that can alter the idle speed by as much as 500 rpm without touching the idle speed adjusting screws.
Setting the Idle Mixture: Patience Pays Off
Set the idle mixture to the highest vacuum by using a vacuum gauge connected to the constant-vacuum port of the carburetor base plate. Slowly adjust the first idle-mixture screw. Make one adjustment only to the first screw. The adjustment should be no more than an eighth or quarter turn. Then leave sufficient time for the carburetor to respond and move to the next adjuster screw. Gradually work your way around the carburetor, making just one, small, slow adjustment to each screw.
Float Levels and How to Set Them
After setting the idle mixture, it’s time to check the float levels. With the motor running the fuel level should be in the middle of the glass window of the fuel bowl. If it isn’t, loosen the lock screw on top of the bowl and adjust the nut clockwise to lower the float level and counter-clockwise to raise the float level. Make the adjustment by one flat of the nut at a time and wait until the carburetor has had a chance to respond.
The ‘dry setting’ is accomplished by removing the fuel bowl and, with it turned upside down, set the dimension between the inside top of the bowl and the top of the float at approximately 0.375-0.0400 inches.
Pump Shooters (Squirters): Bigger Can Be Better
Installing larger pump shooters often eradicates a hesitation at off-idle, but frequently the fault lies not with the pump shooters at all but with incorrect ignition timing.
There are at least two ways of reaching the off-idle position, either gently or suddenly. If the throttle is eased into the off-idle position and the motor stumbles, the idle circuits and the transfer slots are probably too lean. To cure this condition, either slightly undo all four idle-mixture adjusting screws to enrich the system or enlarge the idle-feed restrictors in the metering blocks. On the other hand, if the hesitation occurs under rapid acceleration, increase the size of the pump shooters. The pump shooters only serve to provide the initial shot of fuel, and together with the idle circuits and transfer slots, they provide the predominant fuel supply to the motor until the main circuits are operating through the boost venturi.
The pump shooters are also particularly useful during cold starts. One or two depressions of the throttle pedal provide sufficient fuel for starting. A variety of orifices are available, ranging from 0.025-inches to 0.052 inches. Usually, larger engines use larger orifice pump shooters.
Air Bleeds: Big Risk, Big Reward
Replaceable high-speed air bleeds aid in tuning the fuel curve. These tiny bleeds work in conjunction with the main jets, emulsifying the air and fuel. The smaller the air bleeds, the richer the mixture, which is the opposite reaction when compared with main jets. As air bleeds are reduced in size, the carburetor draws more aggressively and adds additional fuel. But installing incorrect orifices in the air bleeds will bring you bigger trouble than their jet sizes. It pays not to touch them unless you have access to a dynamometer and can analyze the test results.