Tuning a Demon Carb

[ohno50]

Looking for some advice. I just purchased a 2007 West Coast Cobra with a 460 Ford Racing Crate Engine. It has a big old Demon Carb (not sure of the part number) on top of it and I have a feeling that its running a bit too rich. It hesitates at times under acceleration. I'm a bit new to the carb thing as I've only really worked on EFI stuff before.
Question that I have is are there any quick tips you guys can give to set this thing up properly or what to look for. Or is this something that needs to be brought to someone who knows what they're doing to tune it properly.
Thanks in advance.

[Danielg]

Engine timing is critical with all Demon carbs. They do not work at your typical 12 deg advance. They must see a minimum 18deg to function, they require that much.
I have them on two cars and love them, I hated them at first because they hesitated etc. I locked out the timing full advance using a start retard msd for easy cranking. works flawles and the engine response is crazy.
Unfortunatly when people buy demons no one tells them about the 18-22 deg they need, I also was not informed, or I would have bought something else. but I end up messing with them and couldn't figure it out and a sharp tech a BG told me about the timing. Problem solved.
Daniel

[jhv48]

First of all, what makes you think it's running too rich?

My demon would hesitate on throttle tip in because the idle circuit was a tad too lean. Open the four corner idle screws about a quarter turn and see if that helps. Might be all you need.

If that doesn't work. You'll be taking it off to check the transfer ports exposure/throttle blade opening.

First try the easy stuff.

[ohno50]

Thanks for the advice guys.
The reason why I was thinking its running too rich is the the smell of pure gasoline when I lean into the fun pedal. That was just my guess, like I said I'm an EFI guy so the whole carb thing is new to me. I definitely have hesitation though when I accelerate, even after its warmed up and been running for quite some time.
Just as another point, idle is pretty steady at 1000 rpm

[*13*]

What size carb is it?

[ohno50]

not sure...still trying to get the info from the original owner

[*13*]

Theoretically that could be your hesitation problem. You really need to find that out then see what jets & nozzles have been installed

[ohno50]

will do...thanks

[jhv48]

Try dropping the idle to about 800 and then, if it still hesitates, open up each of the four idle screws another quarter turn.

[Zoom This]

Danielg correct. When I tossed my 850 Holley for a 650 Demon 5 years ago it required 19 degrees initial advance and I set my motor at 32 total advance by 2200 rpm. If you set up the idle mix screws correctly and check to make sure the throttle blades are positioned correctly to the transition slots, your half way home. Rich mixture is caused by both the main jets as well as the idle circuit. You really need to spend time on the dyno with a sniffer up your exhaust to dial in a good A/F ratio at light throttle cruise and under full throttle. 14.5-15:1 is good for light throttle cruise at around 2500 rpm. 12.5-13:1 for high speed.

You may still get that big old carb to work on your 460 and I guess it is probably an 850. A lot of CFM unless your turning at least 6500 rpm for peak HP.

When I tossed my Holley I absolutely could not shake the light stumble at throttle tip in. Following a set of exact Demon carb set up instructions my 418W has been happy ever since, as well as the driver.

Bill

[elmariachi]

Personally I would remove the carb, get it up on the bench and take a reading on what's inside. Look for junk in the bowls, make note of the jetting, and replace the power valve (a very common and inexpensive culprit in carb problems.) Make sure your float levels are good, replace gaskets and then take a go at tuning using Demon's literature. Also, some Demons have the idle-eze, see here:

Demon Carburetion - State of the art carburetors for the race and hot rod industry.

As Zoom said, tuning with an O2 sensor or sniffer helps cut to the chase.

Don Gould at 4secondsflat.com is a Demon expert. For $10 he'll send you his tuning guide, a must-have for getting the most out of Demon carbs. He also has a Demon baselining video on YouTube.

[DAVID GAGNARD]

Originally Posted by Danielg

Engine timing is critical with all Demon carbs. They do not work at your typical 12 deg advance. They must see a minimum 18deg to function, they require that much.
I have them on two cars and love them, I hated them at first because they hesitated etc. I locked out the timing full advance using a start retard msd for easy cranking. works flawles and the engine response is crazy.
Unfortunatly when people buy demons no one tells them about the 18-22 deg they need, I also was not informed, or I would have bought something else. but I end up messing with them and couldn't figure it out and a sharp tech a BG told me about the timing. Problem solved.
Daniel

Never heard of this before........maybe the case for a big block Ford engine,but my 351-W with a Demon runs perfectly at 12 degrees intial timing,no hesitation,no bog,no nothing,just flat out goes...
My 331 stroker runs prefectly at 10 degrees intial timing, no hesitation, no bog, no nothing, this one really hauls the mail from idle to 6500+rpms....

on both carbs,all I did was change jets by reading the spark plugs, adjust the float levels as per directions and use a vacum gauge to tune the idle circuit...both have not been touched in years once I got them tuned to my engines.........

David

[Silverback51]

99 times out of a 100 it's a lean condition that causes a stumble, not a rich condition.

[Chaplin]

Originally Posted by DAVID GAGNARD

Never heard of this before........maybe the case for a big block Ford engine,but my 351-W with a Demon runs perfectly at 12 degrees intial timing,no hesitation,no bog,no nothing,just flat out goes...
My 331 stroker runs prefectly at 10 degrees intial timing, no hesitation, no bog, no nothing, this one really hauls the mail from idle to 6500+rpms....

on both carbs,all I did was change jets by reading the spark plugs, adjust the float levels as per directions and use a vacum gauge to tune the idle circuit...both have not been touched in years once I got them tuned to my engines.........

David

This is true, but is more of a function of the cam profile than the carb itself. Demon makes different carbs depending on your cam profile. If you have a big cam with a large amount of overlap, Demon recommends running as much initial timing as you can in order to clean up the idle. On my old Demon carb'd engine, I ran 17 degrees initial plus 18 mechanical for a total of 35 degrees. It really did help clean up the idle and the lower rpm. Probably applies equally to Holleys as well.

[*13*]

I ran a Demon on a BBC & had no issues with timing either. My initial "guess" would be that the carb is too big & they have installed oversized jets &/or nozzles to compensate. Is it a Vacuum Secondary carb, or double pumper?

If you do a carb size calc online you'll get a pretty good idea of the size Carburetor you need. I ran a 600 double pumper on a big block chevy that was in a big old cadillac with 2.92 rear gears. That little carb gave the motor a nice tight bottom end like nothing else would(I also ran a 750 & an 850 on that motor). Since it rarely saw much over 5k rpm, if it saw that, there was really no need for more carb. A Cobra is obviously a different beast. However, if you examine your driving habits & install the smallest carburetor the engine can reasonably run, given those circumstances, you will always have a little more snap in your acceleration & the possibility for better fuel economy.
Keep in mind that if you go too small your engine wont be able to suck enough air/fuel through the carb at high RPM & you will not have the power up in that range that the engine "could" produce. It's all a matter of how the car will be driven.

[DAVID GAGNARD]

This is true, but is more of a function of the cam profile than the carb itself. Demon makes different carbs depending on your cam profile. If you have a big cam with a large amount of overlap, Demon recommends running as much initial timing as you can in order to clean up the idle. On my old Demon carb'd engine, I ran 17 degrees initial plus 18 mechanical for a total of 35 degrees. It really did help clean up the idle and the lower rpm. Probably applies equally to Holleys as well.

My 351-W is in a street car,maybe 350hp, hydraulic flat tappet cam,carb didn't need much tuning to get it right on......
The 331 stroker is in a road race car, cam is 620+ lift,solid roller,not sure exactly how much overlap this cam has, would have to pull the cam card, but, tuning the carb took all of one day at the track with few changes.....
One thing I've noticed is the Demon carbs run better with the floats a little higher than how you would normally set them on a Holley.....
The thread starter may be "over carbed" with his combo, I've seen this all too many times........

Holley and others use this rule of thumb for calculating cfm of a specific engine combo,as follows:
max rpms X cubic inches, divided by 3456 X vol.efficency....then add 50 cfm to that final number......
On my 351-W, I'm dead on with this formula, on my race car, I'm about 40 cfm short.....but it runs like a bat outa hell........
David

[mdg]

Isn't Don the carb and ignition expert?
FBO Ignition, Distributor tuning and Carburetion Professionals, FBO Ignition systems, Ford Ignition, Mopar Ignition, Ford Distributors, Mopar Distributors, Ford distributor curving, Mopar distributor curving, HEI Ignition, ignition tuning, ignition t

[lovehamr]

Man, all the talk about rich and timing etc for an off idle stumble is mostly silly. Does it have to have the correct (enough) advance yes, as with any carb. But Silver is most likely correct about the off idle stumble being a momentary lean condition. For the OP you must remember that the carb is a vacuum referenced metering device and each circuit in it has a job in a specific load range and RPM. This is why advance can be so touchy, up to a point increases in advance also increase vacuum. In EFI fuel is constantly controlled via pulse width from the CPU so any momentary lean or rich condition can be compensated for immediately. With a carb when you tip into the throttle the main fuel circuit cannot compensate for the increase in available air until the vacuum signal starts to pull extra fuel from the main/power circuits though the booster venturies. This takes a second or so. To make up for this momentary lean condition the carb is built with an accelerator pump circuit which operates solely off of mechanical movement of the throttle arm giving a squirt of fuel just as the carb is about to go lean. If the lever/spring adjustment is too loose or there is a blockage or other problem with the acc. pump system then the carb will go lean momentarily and if severe enough the engine can die all together. My advice is to go to the Demon web site; look at carb identification, then download the correct manual and read it cover to cover so that you understand your excellent carb's operation. Specifically, set-up, adjustments (I say Acc Pump) and trouble shooting.

Start on the right at chapter one - ID your carb. Good luck man and feel free to ask other questions as well.
Demon Carburetion - State of the art carburetors for the race and hot rod industry.

[elmariachi]

i can thank a Street Demon 850 and a Mighty Demon 750 for everything I know about carbs on Cobras. Jacking around with those two for 18 months was great training for the 5 minutes it took me to set up my pair of BC/BD Holleys. I personally think Barry Grant made a decision many moons ago that more timing (and subsequently more vacuum) versus less makes it easier to mask carb tuning quirks and maximize performance and that became the company line. I like the BG 4 corner idle, and the idle-eze was a great alternative to drilling the butterflies. But my car NEVER ran as good with either of those as it does now with the twins, both of which are going on 50 years old. And where BG had me trying to set it at 22-24 initial timing, I am back at 18 initial with the twins and it runs like it should.

I agree with Lovehamr, get the manual, baseline it on the bench and start from scratch.

[ohno50]

Thanks for all of the help everyone. I found out some more info....
850 Demon Carb w/ mechanical secondaries. Due to the duration of the cam, I am guessing that its a "Mighty Demon". The engine is a Ford Racing 460 Crate Engine (P/N M-6007F460FT).
Answering the question on driving style; I don't drive it super hard, I'll trounce on it now and then but the laws here in Ontario are pretty strict when it comes to muscle/performance cars and aggressive driving and the car is already a heat score when I'm just sitting at the stop light idling!!
I think I'll save up my $$ and get a proper dyno tune and see what that gets me. I imagine they'll say the carb is too big for what I need but I'll deal with that when it happens. Still a blast to drive though, as you all know there's nothing like having a 550hp go kart!!!