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.

Engines produce vacuum, and over the past 130 years engineers have contrived ingenious ways to advance the carburetor’s powers to match engine developments.

Aided by barometric pressure, ignition and compression, the carburetor creates the air-fuel mixture that promotes combustion.  What’s more, it mixes gasoline with air in the correct ratio for combusting at varying engine loads, engine and air temperatures and altitudes.  Carburetors work by pressure differential; high pressure flows toward areas of low pressure.  Through a labyrinth of small-bore drillings in the 4150-style four-barrel carburetor, the vacuum draws a potent mixture of air and fuel.  So formidable is the mixture, the carburetor has empowered naturally aspirated full-bodied 500 cubic inch drag racing cars to speeds in excess of 213mph in a distance no greater than 1,320 feet!

On starting and at idle, the air speed is too slow to draw fuel from the carburetor’s main jets and through its boost venturii.  So idle fuel is drawn from a low pressure area under the carburetor throttle plates, which at idle are almost closed (see illustration No. 3 below).  As the engine gains speed, larger throttle openings provide sufficient air flow and the area of lowest pressure switches from the idle discharge ports to the boost venturii (see first illustration), which activates fuel flow through the carburetor’s main jets.

On 4150-style carburetors, as displayed in these illustrations, the boost venturii reside within the main venturii and low pressure (partial vacuum) is created by the constricting shape of their bores—the bores’ narrowest part—which causes air speed to increase and, as a result, a siphoning effect is introduced at the boost venturii’ tiny air-fuel discharge orifices and through the air-fuel metering circuits back to the fuel bowls.  The faster the air speed in the venturii, the more fuel is withdrawn from the bowls, thus unleashing optimum engine power.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 of the running engine.

The pressure difference at the venturii is activated by engine’s intake strokes; that is, as the engine’s pistons travel downwards on their induction strokes (and with their respective intake valves opening and closing), vacuum is created in a progression of gulps, drawing 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 (partial vacuum), 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.

Power Valve

On powerful engines at high speed or under high load, main jets cannot provide sufficient fuel without adversely affecting the carburetor’s performance at lower engine speeds, and power valves were introduced to resolve this difficulty.  Vacuum operated and located on the carburetor’s metering block and residing between and above the main jets, they have direct access to the fuel bowl, see illustration No. 8 below.

To make the phase changes from liquid to vapor, fuel 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 on the hot surfaces from combustion.  Complete combustion and best fuel economy requires an air-fuel mixture of almost 15:1.  Best power is achieved around 12.5:1.

When a cold engine fires and runs, combustion heat warms its parts quickly, enabling evaporation of the emulsified and atomized fuel from the carburetor.  But when starting a cold engine, where a moving film of fuel clings to the walls of the induction tracts (leaning the mixture), there are no hot surfaces to vaporize the fuel and choking is required to enrich the mixture.  In addition to heat, vaporization is also triggered by air speed, pressure, and surface area.

The carburetor’s choke mechanism (a mechanical valve) restricts the flow of air, and the ratio of this highly enriched starting mixture is reported to be as concentrated as one part of fuel to one part of air (1:1).  Fortunately, gasoline produces dense vapor at a temperature lower than most liquids—140 degrees Fahrenheit—and the small proportion of fuel that does evaporate induces a mixture sufficiently rich for a spark to ignite in a cold engine.  Once warm, opening the choke valve restores the carburetor to normal operation.


Tuning information with illustrations and captions are as follows:

Here we identify most of the chief components of the 4150-type four-barrel carburetor, illustrate their functions, acquaint you with common problems that adversely affect them, and advise you how to resolve them.

But first, 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. 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.

5. The Idle-Eze was created by Demon Carburetion to ease the complications associated with idle-speed adjustment. This device can introduce an extra source of idle air to the engine without disrupting the critical relationship between the idle-speed screws, the throttle blades, and the transfer slots.

6. Float levels are often too high; they should be placed in the middle of the fuel bowl sight glass. The initial float setting when the carburetor is being assembled at the Demon factory (the dry setting) is accomplished by removing the fuel bowl and, with it turned upside-down, the dimension between the inside top of the bowl and the top of the float is set at approximately 0.375in to 0.0400in

7. Installing larger accelerator pump discharge nozzles often eradicates a hesitation at off-idle, but frequently the fault lies not with the pump discharge nozzles 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 discharge nozzles. These nozzles 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 discharge nozzles 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 discharge nozzles.

8. A vacuum-operated fuel enriching device, the power valve (illustrated above and below) is located in the carburetor’s metering block between and above the main jets. It’s immersed in the fuel bowl on one side and occupies a source of vacuum on the other. Power valves are rated at half the intake manifold vacuum when observed at idle. For example, an intake manifold vacuum reading of 13in calls for a power valve rated at 6.5in.  The 6.5 power valve is identified with the numerals 65 stamped upon it (barely visible in the lower left image).  With an idling engine at normal operating temperature, the vacuum gauge can acquire a reading from vehicles with manual transmissions while in neutral. Automatic transmissions require the engine idling in gear with an assistant’s foot on the brake pedal.

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

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

To read trouble-shooting comments about Blow-through carburetors, read the story above.


  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.

  13. I have a new 650 Street Demon carburetor with vacuum secondary and electronic choke. It was supplied on a Chevrolet 350 crate engine. When the car sits for a few days (4 or 5 in 90F-plus) it will not start. However, a shot of starter fluid into an open carb and the engine fires and runs well. Idle is just below 1000rpm, although it needs tuning, as it fires through the exhaust until fully warm.

    The carburetor seems to have no fuel in the bowls for starting, but once it starts (with a shot of starting fluid) the mechanical fuel pump delivers fuel to the carb. I believe an electric fuel pump would resolve the complaint, but this should not be necessary on new equipment. Are the bowls losing fuel by leakage or is the fuel evaporating?

    The specifications of the engine are as follows: RPS alum race heads, dual-plane Edelbrock Performer intake, Comp 292 camshaft, mechanical fuel pump (on front of engine), and an MSD ignition with mechanical advance. The engine transmits via a 6spd Tremec T56 manual gearbox.

    This is my weekend hot rod–a 1955 Chevy BelAir–and I feel discouraged that I have to shoot starter fluid to get it to run. Are Street Demon 650 bowls an issue or is it the week of sitting? I am a fuel injection guy so I’m not familiar with this experience.

  14. Robert,
    As the car sits in the summer heat of 90F-plus the fuel is likely vanishing by evaporation. Unlike modern fuel injection to which we’ve become accustomed, it’s not unusual with a carbureted hot rod in these conditions to pump the throttle pedal a few times and crank it a bit till the engine fires.

  15. I have the same problem as Robert: it takes forever to start–no gas. But I know where it is going. It is going into the engine oil. What is causing this?

    • Mr. Bond,
      I respectfully submit that your engine is experiencing two different conditions. There is insufficient volume of fuel in the float bowls to pollute the engine oil with gasoline.

      I will first address the hard starting when the engine sits for a few days.

      Modern gasoline is formulated for a closed fuel system that operates at a higher pressure than the approximate 7 psi that a carburetor does. Thus, it is prone to evaporate at a higher rate than gasoline from the past. Since every carburetor has by design a vent to atmosphere, the fuel has the opportunity to evaporate. You may try opening the hood after you park the vehicle so under hood temperature is reduced during heat soak. Often this helps but it may not. It is worth a try.

      Regarding the gasoline in the oil, and working under the assumption that it is indeed there, it is the result of the following:

      Engine running too rich.
      Choke misadjusted and engaged too long.
      Many cold starts and short run cycles.
      Mechanical fuel pump leaking fuel into crankcase.

      Regardless of how the fuel is getting into the oil, you need to get it out. Gasoline destroys the lubricity of even the best oil and will cause excessive wear, especially to the piston rings.

  16. I recently installed a 750 Demon and Performer intake on a 1979 Trans Am with a 6.6 403. The issue I am having is that under heavy acceleration (more than 1/2 pedal) it bogs and the car sputters until you release the pedal. Idle screws are currently 2-1/2 turns out and idle is smooth and steady at 750 rpm.

    • From Michael Knowles, former Demon technical advisor: Chris, you haven’t said if you checked the basics–ignition timing, transfer slots, float levels etc., as recommended in the article.

      Further, your Firebird engine generates fairly low compression and power (under 200hp) and your rear gear is probably high (in the 2.50 to 2.70 range), which slows engine revs, impeding the 750 double-pumper’s ability to open up from off-idle to full throttle.

      If the engine was redone, generating about 10:1 compression ratio and with the appropriate rear gear, say around 3.40 – 3.50, your carburetor and intake manifold could function with tuning. Also, a 750 vacuum-secondary carburetor with stronger secondary spring could work as well. The heavier spring in a vacuum-secondary carburetor delays the opening of the secondary throttle plates–until the engine generates sufficient air speed to open them. But this application is unlikely to generate sufficient air speed to open them fully, thereby downsizing the carburetor to the engine’s needs.

      The tuning would start with 14-20 degrees of initial ignition timing, depending upon the revised camshaft’s specifications, and about 36-40 degrees total advance and with lightest distributor springs. Light springs in the distributor accelerate the ignition timing’s advance curve, improving the engine’s efficiency and its ability to rev quicker, consuming more cfm from the carb. In addition, make sure the transfer slots aren’t overly exposed on the carburetor’s primary side (open secondary side as necessary) and, if it still bogs, try increasing the size of the accelerator pump discharge nozzles. However, given this Firebird’s existing state of tune, the real answer is that it would run better with a 600 or 650 max size carburetor.


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