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

1) 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

2) 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

3) 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

4) 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

5) 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

6) 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. 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.

07 Idle-Eze-cutaway

7) 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

8) 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

9) 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 / 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 (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.

  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 provide an assortment of springs for their distributors.

  7. I purchased a 750 Mighty Demon blow-thru carburetor about 2 years ago but it won’t act right. I adjusted its air-fuel ratio to 14.5:1 and went for the first drive, but it began bogging and popping, and I had to two-foot it to get it back. When I re-checked the air-fuel ratio it was dead lean, recording 17:1. I adjusted the mixture screws and reverted it to around 13.8 – 14.5:1 and drove it again. When I returned, it was crazy rich, idling at 11 and even 10:1. It has behaved this way since new. Once it went so lean during a dyno pull (also wastegate stuck) it recorded around 12lb boost and lifted a ring land off a piston and trashed the motor. Now with a new 355 all-forged built and installed, the carb still acts with the same deficiencies. The fuel system is top notch (Magnafuel everything), fuel cell, -10 feed line, and rock-solid fuel pressure but still can’t get this carburetor to act right.

    • Jess: Your 750cfm carburetor is too large. Blow-through kinds are estimated to almost double the size of the carburetor. Yours will probably exhibit a lean tip-in condition that needs enriching. Switching to annular boost venturii might help. They act to downsize the carburetor, and they atomize the fuel better than the down-leg kind. Also, it’s better to disregard the air-fuel meter initially and concentrate on getting the engine to run well first. A carburetor at idle usually won’t run at 14.5:1. And if it did, the consequences could be fatal — much too lean. You mention “bogging and popping” both of which suggest leanness. So, set your idle to its highest vacuum — that is, where the engine runs at its highest rpm or its smoothest rpm.

      But before all of this, ensure the engine has sufficient initial timing at idle and that the throttle plates are set correctly (see adjacent note to Paul Kuran) and ensure your regulator is boosted correctly to achieve proper fuel pressure. Also, read our story on setting the idle mixture properly “Handy tuning tips for modular carburetors” and attempt to achieve your best idle as mentioned above. If your idle mixture is too rich, it will smell rich, and the engine will lose rpm and die. Similarly, if it’s too lean it will die. An easy way to determine which condition exists on a rough-running engine is, when it’s idling and begins to die, introduce some fuel by activating the accelerator pump. If this rejuvenates it’s too lean.

      To introduce rapid response at higher engine speeds and to avoid the lean condition (caused most likely by the large carburetor) will probably require a visit to a tuning specialist or, alternatively, the purchase of a 650 blow-through carburetor or better still a 600 if such exists. When a forced induction engine accelerates from no boost to 10psi, instantaneous velocity in the carburetor’s metering circuits is vital, and sometimes only a tuning guru, as mentioned in my previous response, playing with the sensitive air-bleed circuits, can accomplish it.

      Lastly, this suggestion may not be welcome but a more fruitful option would be to switch to a Holley Sniper efi system. Tuning a blow-through carburetor to its fullest potential is elusive. Hoping this helps.

  8. 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.


Submit a Comment

Your email address will not be published. Required fields are marked *