DEMON Carb tuning.
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Joined: Jul 2005
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From: Yuma Az
DEMON Carb tuning.
I need some advice on tuning a mighty demon 750 carb. First off I have a zz 383 (crate motor) with msd HEI ignition. Im not sure at what rpm to idle the car at, right now its about 900-1000 and a lot of the time when I turn the car off I get a backfire through the carb. I believe this is because the idle is too high. I have played with the idle screws, and from what I can tell, the driver side one affects the throttle linkage and the passenger side one controls the secondary butterflies. Whats the best way to adjust the two? Also how do I know that I have the right air fuel mixture?
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Joined: Jul 2005
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From: Yuma Az
Any way to fix it? How do you know the correct idle, do you set it right above where the engine starts to choke, and how do you avoid that off idle stumble without just idling the car higher?
Instructor
Joined: Feb 2006
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Correct idle for your engine should be arond 800RPM, what i am talking about is that often i cannot turn the idle lower on the carb at all, sadly i have not found any solution to this, curently im running a 800cfm mighty demon at 1000RPM idle and i cannot get it any lower even if i unscrew the idle screw.
Racer
Joined: Apr 2006
Posts: 477
Likes: 43
From: Millbrae California
demons are tuned on the rich side, the 4 idle mixture screws are close at 1/2 to 3/4 turn out from being bottomed out
just crack the secondary throttle speed screw to avoid sticking the throttle valve and adjust the speed with the primary screw NOTE the demons have a idle air bypass screw that is accessed thru the air cleaner stud hole. they use this to add extra air when the engine has a big cam. if you need more info go to the barry grant web site on how to tune a demon
i tune any carbs air/fuel mixture with a exhaust gas analyzer and/or wide-band air/fuel meter but you can get close by trial and error so far every mighty demon we have tuned is real close for power and cruise mixtures for a performance engine
just crack the secondary throttle speed screw to avoid sticking the throttle valve and adjust the speed with the primary screw NOTE the demons have a idle air bypass screw that is accessed thru the air cleaner stud hole. they use this to add extra air when the engine has a big cam. if you need more info go to the barry grant web site on how to tune a demon
i tune any carbs air/fuel mixture with a exhaust gas analyzer and/or wide-band air/fuel meter but you can get close by trial and error so far every mighty demon we have tuned is real close for power and cruise mixtures for a performance engine
East bound and down...
Joined: Dec 2004
Posts: 987
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From: Jonesboro Arkansas
Originally Posted by PNK
i cannot get it any lower even if i unscrew the idle screw.
Instructor
Joined: Feb 2006
Posts: 144
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Yes ive been turning the screws for both the primary and secondaries, also adjusted the floats and dieled in the timing, but on almost any other carb i can shut off the engine completely by just turning the idle screws out enough.
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Thread Starter
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Joined: Jul 2005
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From: Yuma Az
the primary screw makes a big differance in butterfly position. I can see it move when I turn the screw, the secondary makes a big differance in idle when lowering the rpm but can hardly see the butterfly move, the idle doesnt seem to go up as much with the secondary screw as with the primary. Is this normal? Also what could be causing my engine to backfire up through the carb whan I turn it off?
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Joined: Jul 2005
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From: Yuma Az
not sure right now, I had a garage set it up, but am not too confident in their work. So I plan on getting a timing light and timing it according to LARS paper this weekend. About 14 degrees initial if I remember right?
Pro
Joined: Sep 2002
Posts: 546
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From: Pinellas Park FL
14 to 16 is recommended by Demon customer support. I had to play with initial and total before I could get the carb to perform ok. Once you know the timing is correct, then you can start the process of tuning the idle circuit.
Drifting
Joined: Oct 2004
Posts: 1,496
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From: Lafayette Louisiana
Do you have the video and instructions that ship with new carbs? If not, see if it is available online. I can fax you copies of it plus Lars' paper. The high idle that can't be adjusted down is indicative of a lean condition.
