Demon wants to tell you how to tune carburetors—in minutes!

Demon wants to tell you how to tune carburetors—in minutes!

Text by Sam Logan.
Pictures by Moore Good Ink.

Download text and hi-res images here.

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 500cu in drag race cars to speeds in excess of 213mph in a distance no greater than 1,320 feet!

01 DemonOverheadRLr

Engine vacuum draws fuel from the float bowls through the main jets into the passages of the metering blocks. The air required to emulsify the fuel in the metering block passages is drawn through these tiny air bleeds

02 Idle-airDischarge0185Lr

Empowered by engine vacuum, the idle air-fuel emulsion is drawn rapidly through a multitude of small passageways in the carburetor’s metering blocks and discharged into the induction system through eight small ports in the carburetor base plate. These are known as the idle discharge ports and transfer slots.

03 (0091)Lr

Emulsion bleeds convey air to the main fuel wells. It is here where the mixing process of the carburetor’s main circuits begins. The high-speed air bleeds above introduce the precise amount of air to the fuel in the adjacent vertical wells.

 It is fumes that ignite, as 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. The carburetor takes responsibility for the emulsification and the atomization processes while the vaporization occurs in the induction tracts. In addition, the carburetor must meet the air-fuel ratio requirements of the engine.

04 (0197) R2Lr

Through the discharge port in each boost venturi, emulsified main-well fuel is dispersed into the fast moving air stream and downward into the engine’s cylinder bores.

05 TransferSlotsSetRLr

The most common problem that arises in carburetor tuning involves the over exposure of the transfer slot at idle condition.
When increasing the idle speed of a four-barrel modular carburetor, adjust both the primary and the 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 over exposed transfer slot at idle are hesitation, excessive richness, or poor running. 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.

06 (0202) SettgIdleMixLr

Set the idle mixture to the highest vacuum reading by using a vacuum gauge connected to the constant-vacuum port of the carburetor’s 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 of the four screws.

 In this sequence of pictures and captions, we identify some of the chief components of the modular carburetor, illustrate their functions, acquaint you with common problems that adversely affect them, and tell you how to resolve them.

07 Idle-Eze-cutaway

The Idle-Eze was created by Demon Carburetion chiefly to ease the complications associated with idle-speed adjustment. This device introduces 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.









Demon Carburetion
(270) 901-3346

08 (0176)Lr

After setting the idle mixture 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 or counter-clockwise to raise it. Make the adjustment by one flat of the nut at a time and wait until the carburetor has had a chance to respond.
To check the original dry setting, remove the fuel bowl, turn it upside-down and set the float to this measurement.

09 Pump Shooter(0170)RLr

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 venturii.
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 0.025in to 0.052in. Usually larger engines use larger orifice pump shooters.



  2. Hello, I recently purchased a Mighty Demon 850 blow-thru but can’t seem to get it right. It won’t function at full throttle, but I am not a carb guy. Out-of-the box, it bogs and would appreciate some trouble-shooting help if possible

    • Neil, it could be the carburetor or the fuel system, as your trouble occurs at full throttle. The information we’ve provided in the story above addresses idle/ bog issues. Tune your top-end first, then address fine-tuning if necessary. It may require larger jets – but also check your fuel system to ensure it provides sufficient volume and pressure. To work properly, an 850 blow-through carburetor needs a large-displacement engine – and it’s even more difficult to tune if it uses down-leg boosters.

  3. My Speed Demon 650VS jerks when driving steady at 30/40 mph! What should I do to fix?

    • Tim, increase your jet sizes by one or two steps. Also, check your float levels and verify your ignition timing.

  4. I have Demon 750 that won’t stop pouring fuel into the rear bowls from the air bleeds

  5. From carburetor specialist Michael Knowles:
    James: Out of the air bleeds means the bowl is full of fuel, which suggests the fuel will also exit the vent, which leads to damaged needle-and-seat (trash ingress). It can also implicate maladjustment of float level, bad float, or too high fuel pressure. Yet, if it does not leak through the vent as well, then the vent could be plugged. If plugged, this will likely prove to be the source of the problem.


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