Demon Carburetor issue

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69dodgedart360

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Hey everyone, I have a 10:1 compression 360 with an airgap manifold, tti headers, ported x heads, comp xe284 cam and a speed demon 750cfm carb that's having some sputtering issues between 1500-2000rpm when in drive at about 25mph. If its left in drive and I floor it at that rpm range it sputters, doesn't accelerate and almost freezes and then after a few seconds takes off. If it is left in 1st it handles it a little better but still has a hesitation. I am thinking this is too much fuel and am thinking I need to jet it down. Any recommendations on what jetting would be a good starting point? I think it has 72 and 76's and a 4.5 power valve but will double check.

Thanks
 
Check that the accelerator pump is giving a good, solid shot. Maybe try a 6.5 power valve. Holley 750's are 72 jet and 6.5 PV stock. Also, check that your distributor is advancing as it should.
 
Step one Put demon carb on swap meet table. Step 2 Buy new holley carb set a few screws problem solved.

Sorry to be a dink, i had an 825 mighty demon and couldnt get a stumble out of it, i dumped it and bought a holley problem solved.

Do you know what kind of vacuum you are making with the car at idle in drive? start there, to determine PV size. What does it do from a dead stop when you nail it?
 
What your describing is lean, not too much fuel. I'd like to see the answers from the others questions before I comment further.

Also, is this a newly built issue, or has it run fine before and this has developed recently?
 
From a dead stop in drive or put in 1st there is no issue it seems that when this is going from part throttle to WOT quickly, if I ease into from part throttle to WOT there also is no issue. I can power brake it from a dead stop too with no problems. This issue was a problem always and not something that came up recently.
 
I will double check vaccum tomorrow but the cam has a lot of duration for an automatic so I know that it is not much. Also, at freeway speeds 65mph at 3000rpm if I floor it, it does a slight hesitation and then takes off as well. I just want to be able to floor it from part throttle and take off and get rid of the hesitation. New msd cap, rotor, and plugs have been changed earlier as routine maintenance.
 
All the Speed Demon's I looked up in a quick search shows they used at least a #75 jet in the primaries. I'm not sure you'd have to go that far, but I think that at least a #74 would help. The fact that this is mainly from a cruise situation leads me to believe that the primary jet is the culprit and not anything to do with the accelerator pump system. I am also assuming that this is a vacuum secondary carb that you have? Once the drivability side is corrected, I think the secondary jetting may also have to be addressed. Any other info you can give us will help, and while I don't think it's related to your current issue, I hope you're at least running the initial timing in the teens.
 
If the carb has never been apart...
1. Take a vacuum reading in gear at idle, and diving around at light throttle. You need to know what vacuum the power valve is feeling in real driving situations.
2. Take it apart and clean it very carefully. Make sure you remove air bleeds and jets, shine a light down the passages, and verify there are no "hanging chads" from the machining of the metering blocks. Make sure you use brake clean and shop air to blow out those passages and the tiny holes.
3. Install a power valve that is lower than the idle level by 1 number or so. Reassemble, reinstall. Test drive it preferably with that vacuum gage again. see what it's doing and if it's still doing it.

It sounds lean to me too, but not the accelerator circuit - the power valve or air bleeds may be incorrect or have "stuff" in them. I have yet to see a brand new Demon that didn't get shipped with aluminum chips and gunk in it and I've had a couple that had machining flash left partially blocking a passage in the metering block(s). That's why I just don't recommend or use them if possible. But you got it - it's a decent design - so I'd try and work with it.
 
Before you change setting that may be just fine!
Make sure the float bowl is set properly. a float bowl set just a little bit to high/low can cause a lot of problems.

then continue with the rest of suggestion above.
 
Fisher said:
Step one Put demon carb on swap meet table. Step 2 Buy new holley carb set a few screws problem solved.
Click to expand...

I'm with this. Demon had some issues later in their production. When is this carb dated?

I'd add get yourself a mech secondary carb.
 
What is your fuel pressure?

Improper pressure will make your carb do strange things. To high and it may stumble cause its loading up. To low and it acts like there is a lean condition.

Check the pressure first then move up to the carb. All the tuning in the world wont matter if that is not set right.

Good luck.
 
I assumed in my earlier post that yours was a Down-leg booster model. If so, here are the as delivered specs...


# 1402010VE

Fuel:Gasoline

Number of Barrels:4

Carburetor Flange:Square bore

Choke:Electric

Secondary Type:Vacuum

CFM:750

Fuel Inlet Dual

Primary Jet Size:76

Primary Power Valve (Hg):6.5 Hg

Primary Discharge Nozzle Size (in):0.031 in.

Secondary Jet Size:83

Primary Outer Air Bleed Sizes Included:70

Primary Inner Air Bleed Sizes Included:39

Secondary Outer Air Bleed Sizes Included:70

Secondary Inner Air Bleed Sizes Included:39

Carburetor Finish:Silver

Ford Kickdown:No

Booster Type Down-leg

Throttle Linkage Type:Universal
 
69dodgedart360 said:
Hey everyone, I have a 10:1 compression 360 with an airgap manifold, tti headers, ported x heads, comp xe284 cam and a speed demon 750cfm carb that's having some sputtering issues between 1500-2000rpm when in drive at about 25mph. If its left in drive and I floor it at that rpm range it sputters, doesn't accelerate and almost freezes and then after a few seconds takes off. If it is left in 1st it handles it a little better but still has a hesitation. I am thinking this is too much fuel and am thinking I need to jet it down. Any recommendations on what jetting would be a good starting point? I think it has 72 and 76's and a 4.5 power valve but will double check.

Thanks
Click to expand...
not to sound like a smart ***!!! but I would use it for a wheel chock or a door stop!!! i've had too Many bad experiences with them and have lost too Many races because of them,, if you had the time I would tell you all about it.but that's just me go back to your Holley or an edelbrock send ether of them to rustyratrod (Rob)and he will make it like new again!!!=D> sorry if I have rained on your thread Artie
 
I will take all the demon carbs nobody wants.
How many years are on it? when was the last time it was rebuilt?
 
OldManRick, that this the correct carburetor that I do have, and when I first got it, I jetted it down from those specs but have lost the paper where I wrote down what jetting is in it, I want to say 72 primaries and 76's in the rear but not sure. I also sprayed it out with carb cleaner when I first bought it as well due to all of the horrible things I have heard about metal shavings. Unfortunately, I am a broke college student so I am stuck with this one and cant buy something different, but have had good luck with it so far. My Brother took my tools so I couldn't check timing or vacuum but I decided to start with pump cams. Also, I checked the float levels, they are where they should be so that is good. I had the stock pink cam in it so I just tried a few and I think I fixed it!
White cam in hole 1: Kinda helped but still sputtered bad
Blue cam in hole 1: Helped and the engine noticeably liked the extra pump shot, but down low seemed like it was too much fuel.
Green cam in hole 1: Sputtering down low is taken care of! no issues! Just goes!

I always tried hole 1 because my car idles at 700 in gear.

I am grateful for everyone steering me in the right direction and giving me things to check out. THANKS! I didn't have a chance to run on the freeway so I don't know if that is fixed but will try later and report when I get back from school.

Thanks again!
 
So from a dead stop at WOT it takes off and from cruising and part throttle driving it is a lot better than before. Fuel pressure at idle is about 6-7 psi and I am using a Holley Red Electric Fuel pump with no regulator. At freeway speeds at 2800rpm if i floor it, there is a slight hesitation, but not as bad as before and after the slight hesitation, it takes off. If I do this over and over between varied rpm (2200,2500,2800,3000), sometimes the hesitation is there, sometimes it is not. I also had only 1/4 of a tank of gas in it so I want to try again with a full tank. I am going to get my vacuum gauge and timing light today from my brother and then check that tomorrow after class. I have also never changed springs in my MSD distributor and feel like that might be a good tuning part to have for fine tuning like what I am trying to do.
 
I'm glad to see you working with it to get where you need to be. Your own, hands on tuning is the best way to learn what your combo wants/likes.


I'm not recommending anything different right now, but a few thought are...

1) The Holley/Demon style carbs come metered with a bias toward the lean side. Meaning that they respond better when they need to be richened vs leaned out. My personal experience is that more then 2 numbers less on primary jetting usually get's into a lean condition. Mainly because the air bleeds are tailored for the factory delivered configuration.

2) For a streetable setup, you rarely have to tinker with the pump cams. More then a good adjustment and/or a small nozzle change is usually all that's required.

Incase you don't have one, here's a pump graph. It can never hurt to have the info.
[ame]http://www.holley.com/data/Products/Technical/PumpCamGraph.pdf[/ame]

3) It is possible to mask a lean transition with more pump or nozzle. That should really be avoided if possible, because your steady cruise can still be lean. That can also raise your operating temp some if it was getting toward the extreme side. Reading a fresh set of plugs after some miles would show whether that's a issue.

Like I said, not recommending, just thinking out loud.

Good Luck, and keep us informed. :thumbrig:
 
That Holley Red pump isnt a very large volume pump. Sounds like you have a little bit of motor there, i am wondering if your fuel pump can keep up? I know Demon carbs have a different set up for the elmulsion tubes.? Spelling.. The one thing i did with my carb when i first got it, was i took it completely apart and went through it. I found a ton of loose aluminum and filings throughout the carb. I felt i had it all, maybe i missed some? The finish of the holes and ports in the carb left a lot to be desired.
 
Glad to see he is getting some good advice to chase down the gremlins rather that quit/giveup...
 
OldmanRick said:
It is possible to mask a lean transition with more pump or nozzle. That should really be avoided if possible, because your steady cruise can still be lean. That can also raise your operating temp some if it was getting toward the extreme side. Reading a fresh set of plugs after some miles would show whether that's a issue.
Click to expand...

Totally agree.

Big cams generally require a richer fuel mixture because their not efficient at low speeds. I believe your jetting it way down like you did was counter productive and the extra pump shot is just a Band-Aid fix. Generally out of the box jetting is pretty close. You rarely have to go more than 2 jet sizes one way or the other. Why did you jet it down so far?
 
Have you ever had the carb apart? The Demons were notorious for having machined metal shavings in them. They can be excellent carbs, but the quality control sucked. If it was mine, I would strip it, soak it in cleaner, blow it out vigorously with HIGH PSI compressed air and build it. It almost sounds like maybe the vacuum secondaries are opening too soon and drowning the carb with air. If it does the same thing after the rebuild, maybe adjust the secondary opening.
 
Any updates Anthony? Check out what jets are in the thing. Seems to me on my small block i used the #74s on primary an left the seconds at #85-#86 what ever it came with. After a year of farting around with that carb your dad talked me into dumping it an running an eddy haha never looked back after that. Then later ran a Holley an ran great as well. My demon is now sitting on the bench waiting for a swap meet or a sell haha
 
69dodgedart360 said:
Hey everyone, I have a 10:1 compression 360 with an airgap manifold, tti headers, ported x heads, comp xe284 cam and a speed demon 750cfm carb that's having some sputtering issues between 1500-2000rpm when in drive at about 25mph. If its left in drive and I floor it at that rpm range it sputters, doesn't accelerate and almost freezes and then after a few seconds takes off. If it is left in 1st it handles it a little better but still has a hesitation. I am thinking this is too much fuel and am thinking I need to jet it down. Any recommendations on what jetting would be a good starting point? I think it has 72 and 76's and a 4.5 power valve but will double check.

Thanks
Click to expand...

I would like to know a couple things.

what elevation are you at?

how much manifold vacuum do you have?

what is the initial timing?

what is the total timing?

what rpm does it full advance to total?


it wont run good with the ootb jetting and your cid combo.
secondaries are fine, but the primary should be changed to 73's or 74's, after that its on to the shooter size which will probably be around 31 with an orange cam...but one thing at a time.
 
69dodgedart360 said:
I have also never changed springs in my MSD distributor and feel like that might be a good tuning part to have for fine tuning like what I am trying to do.
Click to expand...

I tune the dizzy first before worrying about carb crispness issues!!

All the MSDs I have seen new come with the slowest advance springs OOTB!
With a somewhat vague description of your combo I would recommend with your combo. 12 to 16 initial advance.
Curve = springs all in by 2800 to 3200.
Limit 16 to 20 degree bushings for total of 32 to 36 total timing with no spark knock.
Then tune your vacuum advance for efficiency/mileage and no spark knock.
If you experience slight spark knock after dizzy is tuned and power is noticeably better you may need to richen carb circuits to quench spark knock, cooler plugs can also assist this. There are some really good articles on tuning distributors on the net maybe even here on FABO I haven't searched.
Tuning is like building an engine, one part at a time gets changed. Changing no two parts at a time so as not to confuse the results. It took me a month or better to get my combo tuned to really be crisp everywhere. Dyno time could shorten this tuning duration by a lot. Be patient methodical, scientific and the results will impress and empower you.
 
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