I usually dont post like this but I got frusturated enough so I will.
I installed a Speed Demon 750 (mechanical) on my 383 stroker (450 hp). The car has a 5-speed with 4.10 gears. With th eDemon, I had a big flat spot off idle, it back fired, and it produced horrible gas mileage. I then sent the carb to a well respected carb professional to have it properly tuned and dialed-in for my application.
When I re-installed it there was no improvement - additionally I realized that the "Idle-Eze' adjustment screw simply did not work. I had another qualified guy look at it and he could figure it out.

After a $450 Street Demon carb and $300 to the carb guy I gave up and put on a Holley 770 Street Avenger (vacuum secondaries). Right out of the box the Holley made my car run perfectly! Great throttle response w.o./ the flat spot, better street manners, better performance and better gas mileage!

While there is probably a good reason to use a Speed Demon for some application - I will never go down that path again.
Has anyone else had a similar experience?

By the way, I have a "professionally" tuned Street Demon 750 for sale if anyone wants it.

Jeff

who did the work on the carb?

Hmmm...

Sounds like you bought TOO big a Demon.
Did you get a recommendation based on cam duration?
You proved my point...replacing a double pumper Demon with a VAC second Holley. The vac Holley will only supply what the engine actually needs.

Bad street combo.....mechanical secondaries, and standard transmission.
In my opinion, no double pumper belongs on a street car. (one that is driven regularly, not a race car with tags). If the Holley was a D P it would act the same way. The vac. sec. carb will get much better mileage, so that was a good move, but don't beat up on the Demon to much, it just wasn't on the right combo.

Wrong! I have a 750 holley mechanical secondaries and it performes flawlessly!! no flat spots, excellent throttle response and i have it on a 383 also i have put several 625 road demons on 1 of my toys and a freinds and had tons of problems with both?! like loading up after shut down slight hesitation at take off etc...and these were vac secondary demons!!! just my 2cents

Squirters too small,,,great big ole lean bog, it sounds like to me anyway.A DP will work on a street manual trans if it will work on anything,but I've always used them on autos too. I know Bill has had great success with a Demon carb, but I've heard too many stories like this to even try one. Lean causes the backfire, rich will just make it accelerate slowly.

Hmmm...

Sounds like you bought TOO big a Demon.
Did you get a recommendation based on cam duration?
You proved my point...replacing a double pumper Demon with a VAC second Holley. The vac Holley will only supply what the engine actually needs.


Bill, I'm not so sure the 750 was too big. I ran a sd750 on my old 420hp 355. It was the first year in production (just after they stopped the claw carbs) and the only things I had to do was install the idle air bleed restrictors, which helped the rpm drop from park to drive and transition stumbles, and also up the squirters to .035's along with shimming the pump springs. All of these "fixes" were done to overcome what was a "too big" carb on a 355. On a "450hp" 383 I can't imagine it would be any worse.

I had a 650 speed demon last year on my old 350, recommended size by BG. I had problems from the get go. I was on the phone to the bg techs many times and followed their instructions. I even got to the point where they had me drilling out the (forgive my ignorance) I believe they are called the air bleeds. I never got it to run right, in their defence I was learning but just could never get it right. I gave up and tried a 3310 holley. I was able to get it to work great, then I picked up an old holley 650 dp. I rebuilt it myself and it was awesome. With my new 400 I built this winter I decided to try and skip the grief and called Pro systems. They built me a 780 dp that is, from this rookies standpoint "awesome". It seems to me all carbs are not created equal, along with all carb tuners. Anyone can mess with a carb and so call tune it, but only a knowledgeable guy can tune it right, in my opinion anyway.

my 850 Speed Demon help me make 586HP/600TQ. No complaints.

Thanks for all the input. I am happy with my change to the Holley. BG and my engine builder recommnded the Speed Demon 750. Before I sent the carb out to be tuned, I asked the carb guy if I should just punt now on the Speed Demon and go with one of his tuned Holley's that he normally uses and he said "No, I can tune the Demon to work with your combo". Obviously it did not work out for me. I would rather not name the carb guy - he actually came highly recommended to me by several guys on this forum and Team Camaro.
In any event, I am happy now with waht I have.

I ran an 850 V.S. Demon on my 540 with a Dual Plane LS-6 Intake and definitely had driveability issues. I then tried an old but good 850 double pumper that I borrowed from 540HotRod. The driveability and performance and fuel mileage all improved. I tried and didn't have a great deal of success with the Demon, but I have had no problems with the old 850 DP and now 1000 HP DP from Holley. I managed to get 18.2 mpg last year going to the Corvette Forum Cruise In with the Holley 1000 Double Pumper, so yes they can get great mileage and run very well on the street.

I love my Demon. Nice to have a carb without a bunch of plastic parts also ....

I usually dont post like this but I got frusturated enough so I will.
I installed a Speed Demon 750 (mechanical) on my 383 stroker (450 hp). The car has a 5-speed with 4.10 gears. With th eDemon, I had a big flat spot off idle, it back fired, and it produced horrible gas mileage. I then sent the carb to a well respected carb professional to have it properly tuned and dialed-in for my application.
When I re-installed it there was no improvement - additionally I realized that the "Idle-Eze' adjustment screw simply did not work. I had another qualified guy look at it and he could figure it out.

After a $450 Street Demon carb and $300 to the carb guy I gave up and put on a Holley 770 Street Avenger (vacuum secondaries). Right out of the box the Holley made my car run perfectly! Great throttle response w.o./ the flat spot, better street manners, better performance and better gas mileage!

While there is probably a good reason to use a Speed Demon for some application - I will never go down that path again.
Has anyone else had a similar experience?

By the way, I have a "professionally" tuned Street Demon 750 for sale if anyone wants it.

Jeff

you go from a double pumper to a vac sec. carb and you blame the carb...
the vac. sec. is covering up your poor choice in carb size on the double pumper.... a holley dp would do the same thing.... a smaller dp would've been the answer..

I had a speed demon 650DP on a mild 383, ran great out of the box minus the float setting. I can't wait to get another one.

You know, I've read on several boards about people having horrible issues with Demon carbs, and others that have never had a carb run so good. I really wonder whats up with that. I have an 850 Mighty Demon that is awesome on my 470 BB. Can manufacturing be so bad that one is junk, and the next one works? I can see being disappointed because one doesn't meet the expectations, but it seems that the ones that do have problems, are so severe they are barely driveable. Anyone have a clue? This is just too strange.

Okay let me state the facts as I see it: (my .02 cents)

Demon:
Fairly priced
No plastic junk
Billet base plate and metering block
Site glass for float position
Factory adjusted
Comes with video/DVD to help you adjust even though baseline is set at the factory. (with the instrustions included anyone should be able to adjust)
Instructions included to help with adjustment

Holley:
More expensive
Plastic parts
None billet parts unless aftermarket
No video/DVD included for adjustment
No site glass for float adjustment
No instructions for adjustment

I don't want to beat up on either manufacturer as I have had good results with both. I think the Demon is a fine piece of work out of the box w/ little tuning if you choose the right carb. Just me, but I would choose the Demon hands down evey time...

Hopefully I will not ruffle too many feathers with what I am about to write. I was talking to a friend today about this.
Also keep in mind I am no expert by any means.

I think people over estimate what they need and because a Demon (from what I have read) flows more cfm than it says it does, picking the right carb is a little bit harder.
Say a person thinks they want a 850cfm carb, chances are they only need a 750cfm carb. Now they go and buy a 850 Demon (for all the above reasons= nice parts) which really flows (again from what I have read) 950 or so. Now the carb is probably 2 sizes too big. This person will not be happy at all.
I had brought a 850 dp Demon and sent it back after thinking about it and am now going to buy a 750 Demon as I think my 750 dp Holley may be a little small, but not small enough to buy a 850 Demon.
I hope I have picked the right one, because they are a very good looking piece. So I hope it’s the right one for me.

As this is a very inexperienced carb persons opinion, I Hope did not ruffle too many feathers.

I never could understand what Barry Grant has to gain by undersizing their carbs .......in other words, an 850 that flows 950 ,a 750 that flows 850 a 990 that flows 1050......in my opinion its all marketing B.S. to make their stuff look more glamorous than everybody elses. I 've seen their lower end carbs and they look just as cheap as Holleys LOWER end carbs and I know I've never seen plastic parts on Holleys better carbs either. As far as all the little perks that come in the package with BG stuff did it ever occur to you that you pay more for this stuff when buying BG carbs......they dont give anything away. Look at most of the competitors in carburated drag classes(not brackets) and see what they prefer. Younger racers or folks who havent been gearheads since Nixon was in the oval office just dont know any different so they go out and buy the B.G. stuff.........its like Comp. cams with all their marketing.......do you really believe their cams are so revolutionary that nobody else has a clue? ,I mean how many ways can you redesign a cam lobe in the last ten years?......and I run comps stuff! Look ,Holleys high end stuff is very good and i'm sure B.G.'s is too but make sure you compare apples to apples ...........that being said you dont find any B.G. stuff in my shop.................

what ever the truth is i heard enough people complaining about the demons that that i went with holleys, pro-system holleys but holleys.it is weird though . people either seem to compare them to FI they like them so much or they are ready to toss them in the garbage. the other problem seems to be support. none of my local shops carry them , they tell me they can't get any support on them. interesting.

You know, I've read on several boards about people having horrible issues with Demon carbs, and others that have never had a carb run so good. I really wonder whats up with that. I have an 850 Mighty Demon that is awesome on my 470 BB. Can manufacturing be so bad that one is junk, and the next one works? I can see being disappointed because one doesn't meet the expectations, but it seems that the ones that do have problems, are so severe they are barely driveable. Anyone have a clue? This is just too strange.
I can think of a few reasons... quality control being a MAJOR issue. Everything is mass-produced these days. It's not just BG it's holley too. It's not just carbs either, it's the quality control of aftermarket heads, intake manifolds, camshaft and timing chain accuracy etc etc. Then when you combine this with how well your rings are sealing, how well you honed, torqued, the type of oil you use.... Heck no two motors are the same. Just cause your demon doesn't work well you can't put it down to BG.... you can't really put it down to anything unless you have some major time on your hands to examine your engine with a fine tooth comb. Same goes for a holley.

I see carb choice as more and more of a guessing game these days. Just my thoughts.

From the engine builders I have talked to they say the same thing.
BG makes great carbs, they are just the hardest to get set perfect.
QuickFuel makes best out of the box performance carb.\
Holley make good carbs and are the easiest to tune.

I had a tough time getting my Speed Demon 750(mech. 2nd) set up correctly,took about 8 weeks,major PITA.On the same engine the 3310 got about 10-11 mpg and went 8.0-8.1 in the 1/8,the Demon gets 15mpg and a best of 7.86 on the exact same set up.The biggest problem I had trying to set it up was timing,thought it would be the same as a Holley.Absolutely not.Went from 12 initial to 21 initial and got a fair idle,put in the air bleeds and it had a good idle,put in some a little smaller air bleed and it is perfect.Had a tip in flat spot that was cured by playing with the accelerator pump cam adjustment.Tech line told me Demon 750 = Holley 850 btw.I still use both brands,and also have a Carter and a Weber that get used some,the Holley is much simpler to set up,but any new carb I buy will be a Demon.I have a stack of Holleys in the garage that I know will be good to go at any time,but I also know that the results won't be as good.

My $.02 from my experience.

I have a Holley on the Corvette and a Demon on the Chevelle. Both work absolutely great for the application. I did have to pay to have the Holly dyno-tuned for the Corvette and I did it my self with the Demon. I suspect that the experience I got from here and the Corvette helped. Both absolutly scream from idle to flat WOT. I get over 20 MPG on the road with the Corvette and I plan to drive the Chevelle 1500 miles on a vacation to California with my wife in a couple of weeks if the weather indicates no rain. I am expecting at least 15 MPG average on the trip with about 18 MPG on the feeeway.

I agree that a vacuum secondary carb is better for street use, but a double pumper can be made to work fine. HOWEVER...HOWEVER...a double pumper cannot be "tuned" or "set up" on a bench, regardless of who the tuner is. You have to put the carb on the car, and do very tedious and painstaking testing with pump cams and squirters. If you wanted to invest all the necessary time doing that, I bet you could make the Demon double pumper work as well as the Holley vacuum secondary. I doubt there's anything wrong with the carb.

Here is so tech info about Demons.
http://www.carcraft.com/techarticles/56919/index.html

tuning w/air bleeds
http://www.carcraft.com/techarticles/81958/index.html

Basic setup for Demon:
http://www.omniracing.net/html/4_corner_idle.html