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 130 years engineers have contrived ingenious ways to harness its power to the engine’s induction system. Aided by barometric pressure, ignition and compression, the carburetor creates the air-fuel mixture that promotes combustion. 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, the carburetor has empowered naturally aspirated full-bodied 500cu in drag racing cars to speeds in excess of 213mph in a distance no greater than 1,320 feet!

It works by pressure differential; high pressure flows toward an area of low pressure. On starting and at idle, the area of lowest pressure on a carburetor is under the almost closed throttle plates (see second illustration below) and as the engine gains speed, the area of lowest pressure switches to the boost venturii (see first illustration).

On 4150-style carburetors, the boost venturii reside within the main venturii and low pressure is created by the constricting shape of their bores, which causes air speed to increase and, as a result, a siphoning effect is introduced at a boost venturi’s tiny air-fuel discharge orifice and through the air-fuel metering circuits back to the fuel bowls. (The faster the air speed passes through the venturii the more fuel is withdrawn from the fuel bowls and thus unleashing optimum engine power.) As the fuel bowls are vented to barometric pressure—an area of greater positive pressure than that of the discharge orifice in the venturii—the air-fuel mixture is compelled to travel from the bowls to the boost venturii.

This pressure difference at the venturii takes effect during the engine’s intake strokes; that is, as the pistons travel downwards on their induction strokes (and with their respective intake valves open), vacuum is created that draws the air-fuel mixture from the carburetor through the intake tracts and into the cylinders.

1. The boost venturii are carefully positioned within the main venturii. The constricted area (that is, the smallest diameter within their bores) serves to increase air speed, which, in turn, lowers air pressure, presenting the perfect placement for the booster’s tiny discharge orifice. Here, the air-fuel mixture is drawn and atomized in the fast-moving air stream to the cylinders.

 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.

2. The air bleeds draw air into the idle fuel wells and into the main fuel wells where it emulsifies (mixes) before being discharged through the idle ports and transfer slots in the base plate as well as the boost venturii up in the carburettor’s main body.

3. The most common problem that arises in carburetor tuning involves the over exposure of the transfer slots during the idle phase. When increasing the idle speed of a four-barrel modular carburetor, adjust both the primary and the secondary throttle plates. 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 plates are closed, the transfer slots should give the appearance of a small square when viewed from underneath.

4. 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. In tuning, always start by checking initial ignition timing, as it is frequently insufficient. Use 18 degrees BTDC as a starting point. If 18 degrees results in excessive total timing, retard it until the engine runs happily. Excessive total timing can be recognized by spark-knock under acceleration. Spark knock can also be caused by excessive compression ratio.

Lastly, in this area, in the Atlanta suburb of Doraville, a well-established company known as Lamar Walden Automotive has been remedying hot rod troubles since the early 1960s. For those residing in the southeast, you can reach them at (770) 449-0315.

5. 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

6. After setting the idle mixture check the float levels. With the engine 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.

7. 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 engine 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 engine 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.

To read trouble-shooting comments about Blow-through carburetors, read the story below:

Blow-through carburetors: 650, 750, & 850cfm Mighty Demons


  1. In my opinion one of the world’s best carburetors. Well done.

  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 / bogging 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 (ingress of debris). It can also implicate maladjustment of float level, failed float mechanism, or too high fuel pressure. Yet, if it does not leak through the vent as well, then the vent could be blocked. If blocked, this will likely prove to be the source of the problem.

  6. Regarding my 650 Speed Demon with mechanical secondaries: It will not hold a steady idle, instead it fluctuates. Yet, when the idle speed is set at 1,000rpm in the garage it remains there, but when taken out and driven, it might idle at 1,200rpm. Then at the next stop light it might fall to 700rpm. Stop for gasoline and when restarted it might increase to 1,000rpm. Its erratic behavior reminds me of the characteristics of a faulty throttle position sensor of a fuel injection system. It’s very inconsistent. Also, how are the butterflies adjusted. Thanks Paul

    • Paul, Erratic Idle: Usually, this condition suggests the idle transfer slots are over-exposed (See note 5 of the story above) or the throttle plates are binding. The throttle plates (butterflies) must return to the stops at idle. If they stop part-way and by using a little force you can push them to the stop, then the throttle plates are binding.
      Also, did you read the other tuning stories listed in our sidebar, including “Six helpful tips on four-barrel carburetors” and “Handy tuning tips for modular carburetors”, which includes valuable advice on setting the idle mixture? In those stories as well as “More Handy Tuning Tips” you’ll discover the common errors that blight the carburetor’s performance. Here are some:
      1) Always start by checking initial ignition timing, as it is frequently insufficient. Use 18 degrees BTDC as a starting point. If 18 degrees results in excessive total timing, retard it until the engine runs happily. Excessive total timing can be recognized by spark-knock under acceleration. Spark knock can also be caused by excessive compression ratio.
      2) Adjust the throttle plates (the butterflies) to their correct position as described in those stories. The rules on transfer-slot exposure must be obeyed, otherwise the carburetor cannot perform properly. Usually, transfer slots are over-exposed because of insufficient ignition timing and/or the use of an inappropriate camshaft, both of which require excessive idle adjustment, which in turn over-exposes the transfer slots.
      3) Select the correct carburetor size. Frequently the chosen carburetor is too large.
      4) Similarly, select the correct camshaft as it, too, suffers from the same ill-advised illusion – bigness. Questionable combinations of parts result in mediocre-to-dreadful drivability. Lastly, always make adjustments with engine at normal operating temperature.

      Good luck Paul.

      PS: In this area, in the Atlanta suburb of Doraville, a well-established company known as Lamar Walden Automotive has been remedying hot rod troubles since the early 1960s. For those residing in the southeast, I should probably make this comment at the bottom of those stories mentioned above. Though Lamar sadly passed away in 2015, his capable son, Rob, assumed the helm. Reach them at (770) 449-0315.

      PPS: One further thought on your erratic idle. The problem could also be caused by weak springs on the counterweights of your distributor. MSD provides an assortment of springs for their distributors.

      • I’ve just rebuilt a 625 Road Demon Jr. for my 1969 Mustang 289. It starts and idles fine but dies when the throttle is moved. I’ve tried slow throttle and fast. Sometimes while starting it spits fire out the secondary. This car hasn’t run in 12 years.

        • Jonathan,

          Road Demon carburetors, particularly early production units manufactured in Dahlonega, Georgia, offered poor drivability because the transfer circuits were too lean. At the time of their introduction, Westech’s John Baechtel alerted the factory, suggesting they required larger idle-feed restrictors or smaller air bleeds on the primary side. For units already in circulation, the tuning solution was to slightly enrich the primaries and lean the secondaries, and drivability was restored. This said, don’t forget to check your initial ignition timing, your transfer-slot exposure at idle, and the condition of your distributor’s counterweight springs?

    • My 850 is fine at full throttle, but when it’s under load, after the clutch is out, it bucks in every gear.

      • Phil, You’ve provided little information for an accurate diagnosis. We know nothing of your initial ignition timing, your transfer slot openings, your engine size–all the usual suspects mentioned in the article. It’s probably not drawing sufficient fuel because the 850 carburetor is too big for the engine or insufficient timing or too lean.

  7. What are the factory settings on the 650. I need to start over using factory settings. Thank you

    • Donald: We’re not the best resource for original carburetor settings; Holley’s website may provide them. Our objective is to help those with intractable tuning troubles via the stories published on our website. We provide at least four articles on this subject, perhaps more. Good luck.

    • I have a 650 (1282010VE) that I inspected out-of-the box using a set of pin gauges (-0.0005″), micrometer verified. Jets: pri=70 (0.063″), sec=78 (0.085″). Idle air bleeds: 0.0695″- 0.072″ – I believe their target value was 0.070″. High-speed air bleeds: 0.042″. PVCR holes (x2): 0.059″. Cross channel/booster feed: 0.150″. IFR: 0.029″/0.092″, which are located below emulsion holes on metering block. The first accesses idle well, the second main well – the alternative IFR location at top of idle/transfer slot channel is effectively open at 0.115″. Emulsion holes (x3 per venturi) from top to bottom: 0.030″/0.029″/0.030″. Siphon break: 0.028″. Transfer slot feed: 0.123″. Idle feed: 0.120″. Power valve = 6.5 as shipped. Squirters = size 28 (I did not verify nozzles).

      I didn’t measure initial transfer slot exposure as that is tweaked during tuning anyway and is always an easy fix. The thing I noted, however, and you should inspect, is that the cross drilling that joined the idle well to the transfer slot/idle port/idle set screw had material wiped across the face inside the idle well causing about 40% occlusion – the well, apparently, is drilled last during production. I broke this free using my pin gauges to restore full communication. In this instance, removing idle-well plugs for proper cleaning is best, which means you need a replacement set if you find a similar condition. I used a 10x set of Jeweler’s glasses with LED’s for close inspection.

      Hope this helps and enjoy your ride.

      • I have an 850 Barry Grant Speed Demon on a 502 Chevrolet I rebuilt with a kit from Holley but it falls on its face when I punch it. I’m about ready to pitch the carburetor. I have a roller cam and a new MSD Street Fire distributor and had same problem before I rebuilt the carb and bought the distributor.

        • Eric,
          Don’t be too despondent. Carburetors are complex devices. Still, the symptoms you’ve expressed are vague; you haven’t divulged any of the fundamentals: your initial ignition timing, your transfer slot opening, or your float levels. Is the 502 you mention the GM 450hp crate engine? If so, is it fitted with down-leg boost venturii? These are unlikely to generate sufficient air speed to propel this engine with gusto. To improve its velocity, replace the down-leg boost venturii with the annular kind. Also, nothing is said about your transmission. Is it manual or automatic? The stall speed of a torque converter could also impede your vehicle’s acceleration. Moreover, what kind of intake manifold is being used? Is it of single- or dual-plane design? Dual plane would probably operate better on a 502, and an 850 with down-legs is, as they say, too much carburetor! You mention that when you punch it, it falls on its face. Again, this doesn’t clearly convey much: do you mean that it bogs or stumbles or is just powerless?

  8. I’m helping a friend who recently had a Demon carburetor installed. The car runs for a while then acts like it’s running out of fuel. When it sits for a brief period it will restart. The only way to ensure it runs is to keep the RPM up. Do you think it’s a fuel delivery issue or a carburetor incorrectly adjusted?
    Thanks for your time.

    • Jerry,
      From the information provided, your dilemma could be caused by failing fuel delivery. If the engine fades after reaching normal operating temperature and miles of driving, it could be affected by heat soak–fuel line becomes hot, perhaps too close to the exhaust, boiling fuel. Some install an insulated plate under the carburetor and/or re-route the fuel hose. Also, the fuel pump could be overheating and fading.

  9. Speed Demon 4-bl: Fills float bowls through air vents primary bowl leaking from underside of carburetor base plate from hole next to the Phillips screw that holds the base plate to the body. Thank you very much for any ideas what could cause this.

    • Mark,
      Your problem is likely caused by a faulty needle-and-seat valve or one that’s impeded by ingress of debris (or foreign matter as they used to say). Excessive fuel pressure could also threaten the proper operation of this valve. Lastly, ensure your floats are correctly adjusted. For details read accompanying article: “Handy Tuning Tips for Modular Carburetors”. You should find it at the top of the right sidebar.

  10. Hi Vic Moore, I want to thank you very much for taking the time to help me with my carburetor problem. The needle-and-seat did have debris. Fired up no leaks. Thank you!

  11. I have the Screamin’ Demon SDA 650 and have had my engine rebuilt, which ran fine when on the engine stand. But when installed in my truck, it shook pretty badly. I checked the fuel pressure, noting 3psi and switched to an electric fuel pump, which now records a steady 6psi. I was curious as to where my idle-fuel mixture screws were normally set; the front is 3/4 turned out from bottoming and the rear is a 1/2 turn out. I then tried the default of 1-1/2 turns out and the engine refused to idle. Any help would be greatly appreciated.

  12. Matt,

    This question is mostly answered in the picture and caption Number 4. Idle-mixture screws are adjusted to where the idle performs at its smoothest or where the engine carries its highest vacuum – check with your gauge. The setting of 1-1/2 turns out from bottoming is a baseline only suggestion. If the air-fuel mixture is rich, turn in, if lean turn out. Make small adjustments of 1/8 – 1/4 turn per screw, and allow the engine to run for a few seconds to ascertain any change. But always ensure the basics are in place beforehand, i.e. check ignition timing first.


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