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

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

 By Fergus Ogilvy

Demon Bow-thru carburetor - overhead viewBowling Green, KY: For turbocharged and centrifugal supercharged applications using a bonnet, Demon Carburetion offers three Mighty Demon Blow-through carburetors. Operating with up to 18psi of boost they are available in 650, 750, and 850cfm.

To increase the vacuum signal to the carburetor under boost and to enrich the calibration, they are equipped with annular boost venturii, large screw-in power-valve channel restrictors in the metering blocks, and 0.130in needle-and-seat valves in the fuel bowls. The bowls also contain non-collapsible solid nitrophyl floats, unlike their brass counterparts. Air bleeds are appropriately sized.

These new calibrations are said to hold a flat fuel curve. Even at higher rpm the main circuits sustain constant BSFC (Brake Specific Fuel Consumption) values and air-fuel ratios. The idle circuits, which often carry the responsibility for part-throttle cruising, demonstrate fine non-boosted drivability.

On the bottom side of the base plate, machined, right-angle channels are visible. These introduce a source of boost pressure to four machined grooves on the throttle shafts. This innovation seals the shafts and prevents pressure escaping from the carburetor. For security, screw-in vacuum fittings replace the common pressed-in types.

Other interesting properties of these carburetors introduced by Demon in the late nineteen-nineties are easily recognizable. These include billet metering blocks and base plate with the distinctive Idle-Eze air valve as well as replaceable air bleeds, and idle-feed restrictors. To prevent galling in the bores of the aluminum base plates, the throttle plates are mounted on Teflon-coated throttle shafts.

Introduced in late 2013, their part numbers are as follows:

650cfm 5282020BT

750cfm 5402020BT

850crm 5563020BT

For further information contact:
Demon Carburetion

To learn about boost referencing blow-through carburetors read the story below.


  1. 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, it recorded around 12lbs boost and lifted a ring land off a piston and trashed the engine. 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.

  2. 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, the throttle plates are set correctly (i.e. when they are in the idle position, the transfer slots appear as small squares when viewed from beneath), and 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 stop. Similarly, if it’s too lean it will stop. An easy way to determine which condition exists on a rough-running engine is, when it’s idling and begins to fade, 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 having access to a tuning guru, playing with the sensitive air-bleed circuits, is easiest.

    Often the biggest menace of the blow-through carburetor system occurs when a right-angle carb hat is attached, which can cause the boosted air to behave badly–air doesn’t always react well to an abrupt turn. According to some tuners, the more expensive Holley Sniper system–though it makes no more power–offers better drivabilty. Hoping this helps.

  3. 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

  4. Neil, it could be the carburetor or the fuel system, as your trouble occurs at full throttle. We’ve provided information on addressing idle / bogging issues in several stories on this website. 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.

  5. Hello Vic,

    I have a 2000 Demon 750 RR carb that came with down-leg boosters (still new) but would like to use 14-hole annular boosters if possible for a blow-through SGL turbo 437 cube Pontiac. I understand only Demon boosters will fit my old Demon. Also, I’d like to use a Vortech full carb enclosure instead of the carb hat. Any help greatly appreciated.

    • Martin,

      The full carburetor enclosure is fine as is changing to annular boosters. I don’t know if Demon boosters are similar to Holley’s kind–it’s likely they are. Finding a knowledgeable person at Holley’s technical line could verify.


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