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  1. #1
    Senior Member BasicQ's Avatar
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    Default 750 Fuel Demon to 750 Street Demon

    Received another Demon carby in the mail yesterday.

    like the others I have the purpose of this one is to take it from the factory calibration and recalibrate for a better all round performing carby. To be something calibrated a lot closer to suit how a lot of us would use our cars, 90-95% street driven and 5-10% strip. New calibration should be able to provide 12.5-12.7:1 WOT AFR and cruise around town, to car meets and to the strip and back from 13.5:1 through to at least 14.7:1. Previous Demon carbies tell me when I bolt it on with factory settings cruising around town will see AFR’s in the low 12’s and possibly dip as low as high 10’s. It will deliver WOT in the 12’s but the calibration of where 99% of most cars not being raced spend their time suffers.

    recently LXCHEV started a thread “Ultimate street carb choice for warm 383 SBC” where he explained that he had a race type carby on an almost 99% street driven car. He had enough of the rich cruise conditions that had several attempts at fixing but still blacken the plugs. He had the epiphany or at least was honest with himself that he actually had a street car as that was where it spent most of its life and that’s the driving he wanted to enjoy. He decided it was time to move on from the race fašade and its side effects to maintain the look and get a carby targeted for his intended purposes. From the last post I read he is chuffed with the 650 Street HP he now has and has reported far better drivability with barely losing any peak hp. That is my aim here, to change this carby from a blubbering black plug creator keeping the local servo happy to something a lot more enjoyable to drive without affecting WOT fuel delivery. What I will ultimately achieve here would also be possible with the carby he had.

    Something to note with Demon 750 carbies is the venturi size. Every other brand of Holley style carburetors has 750cfm venturi measure at 1.375” or thereabouts. Demon 750 are 1.400” venturi. They flow more but how much more I don’t know. The factory jet calibration is pri 75 sec 83. What is interesting is when I opened this one up the primaries are 78 and secondary jets 86! I can only assume these sizes were arrived at due to whatever engine this was on was dyno tuned. I have a theory as to why so large but without knowing the engine combo it came off the theory can’t be solid. More on that later.

    An identifier tip for Demon carby size is a two digit number on the main body, secondary side near the air cleaner ring. This one being designated a 750cfm with a 1.400” venturi has a 40 cast into it. Those designated 850cfm have a 56 cast into them for a 1.560” (some were 1.562”) venturi and 650’s have a 28 for you guesses it, 1.28” venturi. Early on there was an 825cfm that had a 42 cast into the main body for 1.425”. I have seen Demons advertised as 750’s before where a 28 is spotted in the photos. Some poor bugger that bought it ended up with a 650cfm.

    D0B5EA99-ACE8-4279-A26C-BAEDDB33AC3D.jpg

    Attachment 27144

    first step is to put it on my engine and go for a drive as received for the heck of it to see what AFR’s it produces. After that put factory jetting in and see what happens there. Continue lowering jetting and observe cruise through to WOT effects. I have a bunch more jets, air bleeds and emulsion bleeds on their way in the mail along with Demon brand rebuild kit. I have enough spares with me to get the tuning underway so I will start that process soon enough.

    Calibration as received:

    Pri MJ 78 (factory 75)
    Sec MJ 86 (factory 83)
    MAB 39 pri& sec
    IAB 70 pri & sec
    IFR 37 pri & sec (top position)
    Emulsion from top: .031, blank, .031, blank, .031 (there are only 3 emulsion holes but I have converted to as if 5 hole to compare to other brands).
    PVCR .059
    Pri squirt 31 pink cam
    sec squirt 31 pink cam

    Other than noted all specs check out to factory delivered specs.

    03BEE530-6947-4A94-8169-104ACC8D1ED6.jpg
    Last edited by BasicQ; 05-12-2020 at 06:20 PM. Reason: Add squirter size, add PVCR size

  2. #2
    Senior Member LXCHEV's Avatar
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    Watching with great interest!

    I still need to get myself setup with the air:fuel gauge - then I can continue to progress with the fine tuning (and learning) on mine. Really starting to enjoy reading these type of threads.

  3. #3
    Part of the furniture gtrboyy's Avatar
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    Yep I'll be checking in on this thread with interest as got a Demon carb on f280 355 stroker yet very few details about the carb or it's tuning & doesn't help that I grew up or learned most of what I know when efi got popular.

    Been long time since I've actually messed around with a decent carb..not since having triple webbered 202 decades ago.

  4. #4
    Do you ever leave? EH179's Avatar
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    Interesting thread.

    I can already see some time being spent on the idle and transition circuits.

  5. #5
    Senior Member 46Crab's Avatar
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    You have got me interested as I often wondered what the Demon carbs are like.

  6. #6
    Senior Member JaseQ314's Avatar
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    Watching with interest.

    I have an old BG 750 Silverclaw on my 308 (was potentially going to be a 355 once otherwise it would be a 650), black plugs and fuel diluted oil are the normal and a consistent idle is something I can only dream of. Many thanks to you guys on here I have improved it over time but it really needs to be either replaced or really dug into and calibrated properly.

    Good luck with the project BasicQ.

  7. #7
    Senior Member BasicQ's Avatar
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    Doesn’t look like I will get this carby on the engine for any testing this weekend as it’s raining now and I am off for an organized cruise tomorrow and I will stick with a known tuned carby for that.

    There was still however some preliminary adjustments to be made before the first bolt on and start up to set some baselines. One of these adjustments before first start up is to set the throttle blade to transition slot exposure.

    First pic is how the primary blades were set as received. Hopefully this is not how they were when on the engine this carby came off. A lot of transfer slot is exposed, so much that the timed spark port can be seen on the left throttle bore. I couldn’t imagine there was much mixture screw authority with this setting.

    9167A11F-3DD3-48FA-AFD2-CC01DC12C3EF.jpg


    Set now to close to the recommended ‘square’ that would provide better transition function. They may not be perfect squares as I like to go a little bit either way so the screw head slot runs east/west and that makes it easier to return to this known setting without taking the carby off and flipping it over to reset

    D5FD19E3-2704-4D8B-B45E-AB7AF2D93EC7.jpg
    Last edited by BasicQ; 05-12-2020 at 09:48 PM.

  8. #8
    Senior Member BasicQ's Avatar
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    Next was to set the floats to a baseline.

    What I have found that works for start up is to remove the bowls and set to a measurement. I turn them upside down and for the primary the gap between what is the top of the inside of the bowl or roof, to the top flat part of the float is set at .500” or 12.7mm. For the secondary .450” or 11.4mm.

    [img]https://i.ibb.co/9N9m0qR/91-ED4729-F1-C4-4-B6-A-8271-E4-A0-AE13-F439.jpg[/img]

    The difference is because most engines sit on an angle lower at the back than the front. Some manifolds correct for this and some don’t but the carby always tilts so the secondaries are lower than primaries. Float level in the secondary is higher to account for where the fuel level would be lower at the metering block due to the angle. This isn’t anything scientific or working within tight tolerances, just a starting point and setting baselines.

    287ECA2E-9901-4B32-B8D6-A3DBD22DD937.jpg

    Switched to the app and all images display.
    Last edited by BasicQ; 06-12-2020 at 08:27 PM. Reason: Trying to get images to upload/removed question regarding image uploading

  9. #9
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    I never had much luck with Demon carbs . One King Demon i tried and tried and couldn't get fuel out of it and gave up.
    Some seem to run ok but I've watched a lot of different ones on a chassis dyno and fuel curves can be a bit all over the place.

    One thing I have learnt over the years , you say the Demon has a larger venturi than a similar Holley . But perhaps the different booster venturi restrict the airflow .
    A perfect example is to look at a 750 Dominator Holley . Has similar specs to a 1050 but they have a skirted booster venturi to restrict the airflow .

  10. #10
    Senior Member BasicQ's Avatar
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    Quote Originally Posted by Gregaust View Post
    I never had much luck with Demon carbs . One King Demon i tried and tried and couldn't get fuel out of it and gave up.
    Some seem to run ok but I've watched a lot of different ones on a chassis dyno and fuel curves can be a bit all over the place.

    One thing I have learnt over the years , you say the Demon has a larger venturi than a similar Holley . But perhaps the different booster venturi restrict the airflow .
    A perfect example is to look at a 750 Dominator Holley . Has similar specs to a 1050 but they have a skirted booster venturi to restrict the airflow .
    There is also cases where two carbies from the same manufacturer will have the same venturi and throttle plate diameters, one has down leg boosters and the other the physically larger annular but both will be rated the same cfm. Quickfuel Q series is just one case. That series the 750cfm rated carbies have 1.390” venturi and 1.687” throttle plate. They are used with both downleg and annular boosters but Quickfuel still call them all 750. It was the 950HP that showed the number is as much a name as a true flow specification.

  11. #11
    Senior Member BasicQ's Avatar
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    Something to note when setting float levels and the bowl has a sight glass with increments.

    There is tuning ability with float level as a raised float level also brings the fuel level in the main well higher and therefore easier for the boosters to draw out. It does often have another effect that can bring on booster flow earlier, later and richer or leaner. That effect comes about from the float level relative to the top emulsion bleed. The top float level line in the image below sets the float level above or submerges the top e-bleed. The middle line sets the float level at the e-bleed level and the bottom line puts the e-bleed above float level.
    This also ties in to what I was saying about carburetors sitting on an angle in the car, primaries high, secondary low. For the primaries the fuel level may be a little higher on the face of the metering block where the e-bleeds are compared to observed height in the sight glass due to the distance between them. Secondary opposite.

    [img]https://i.ibb.co/VMd33fQ/069883-E2-9-E00-4-C29-87-EE-12-ACABA46-C10.jpg[/img]

    The below is not my words but helps to describe the effect of each,

    The vertical location of the bleeds entering the main well influences the fuel flow in the following ways.

    1: Orifices above float level or between the well and the nozzle allow bled air to raise the pressure (reduce the vacuum) in the nozzle and above the fuel in the well. That delays the initial start of fuel flow from the nozzle to a higher air flow through the venturi and is used to control the point in the early throttle opening where the main starts.

    2: Orifices at float level increase low range (early throttle opening) fuel flow by carrying fuel with the airflow to the nozzle.

    3: Orifices below float level increase fuel flow by the effect of lowering the level of fuel in the well to the hole(s) admitting air. This is like raising the float level a similar amount (increases the effect of gravity in the pressure difference across the main jet) and also adds to the airflow carrying fuel to the nozzle. Locating the orifices at different vertical positions influences this effect’s progression.

    4: The "emulsion holes" influence is greatest at low flows and the "main air bleed" has most influence at high flows.


    Be sure to get float level right for tuning goals.
    Last edited by BasicQ; 05-12-2020 at 09:20 PM.

  12. #12
    Senior Member BasicQ's Avatar
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    A teaser to where the recalibration mods are headed. This is from a previous recalibrated Demon.

    Before

    [img]https://i.ibb.co/7tPP24w/20200417-110213.jpg[/img]

    After

    [img]https://i.ibb.co/Y7zq85S/20200417-110221.jpg[/img]

  13. #13
    Senior Member BasicQ's Avatar
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    That 'after' metering block is on the carby that is on my car now.

    That carby is what Demon call Mighty Demon 850 annular. It has a 1.562" venturies and 1.75" throttle bore. The same as a lot of 850s and the same as Holley 1000 HP and when Demon still had the 'Race Demon' series the 1000cfm in that line up. In that Race Demon series the same dimension venturies and throttle bore with annular was designated an 820 cfm. The carby I have on my engine now has identical specs throughout as that 820 Race Demon except it does not have removable sleeves being a Mighty Demon.

    I have just now cruised up the Princes hi-way for around 25 mins or 40k’s from Geelong to Werribee for a car meet and cruise. On the hi-way at 100kp/h revving at 3250rpm afr averaged .97-.99 lambda. If BP Ultimate still has a stoich of 14.4 then that is 13.9 to 14.2:1 afr. It would dip as low as .92 and get as high as 1.02 lambda depending on gradient. If I were going on a longer drive and wanted to save more fuel I only need to temporarily replace the 70 iab's for 75's to take away some transition fuel. This gets it over stoich.

    I could, if it were open, drive on through to Calder and run an 11.4 1/4 mile @118mp/h in the 3350lbs HQ and drive the hour home without touching a thing. There is a lot more impressive out there these days but to me that is the non roll cage equipped street car I want.



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    Last edited by BasicQ; 06-12-2020 at 08:32 PM. Reason: Add ‘40k’s’

  14. #14
    Senior Member BasicQ's Avatar
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    Monitoring afr’s on the way through Geelong to Queenscliff and the reverse on the way home returned these results on level ground steady speed. Total cruise distance including getting there and home was 183km. 28.5 liters used.

    60kp/h = .95 - .96 lambda av
    70kp/h = .97 - .99 lambda av
    80kp/h = .98 - 1.0 lambda av
    100kp/h = .97 - .99 lambda av

    On the dyno WOT 12.4 AFR @peak hp.

    They are the numbers I am targeting with the 750 Mighty Demon.

    Why monitor AFR at kp/h? Because this is a street/strip car and those are the steady state throttle positions. If I am out cruising I am likely doing it at one of those speeds and therefore throttle positions. WOT is all that matters for the strip.
    Last edited by BasicQ; 06-12-2020 at 03:31 PM. Reason: Change fraction of tank fuel used to liters used.

  15. #15
    Do you ever leave? EH179's Avatar
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    Not too shabby... 6.4 km/litre- and that is very similar to where i am at. WOT is a very different story

    I know of a few cars that would be lucky to get 280 km from a 65 litre tank and 1 has Holley Sniper EFI.

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