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

I would have tested somebody else's Demon before I'd made another purchase. Perhaps someone could have loaned one to you.

SD are real good carbs. I've run a Road Demon, Speed, and a Mighty Demon. Yes, they were different to tune compared to a Holley (notice I didn't say it was difficult, just different), and I love 'em all. JIMO, of course.

I have for some reason never managed to make my 750 Speed Demon bog, stumble, or backfire. The only problem I had was the idle stinks (literally). I drilled out the air bleeds and it helped a lot but still not perfect ...Other than that this carb is wonderful.

Demons are known for rich idles. I have a 750 mighty demon on my 350, and i couldnt be happier. Now with a 750 speed demon mechenical secondary, of course your mileage isnt going to be great. A vacum secondary is going to get better mileage for sure. And really the traits youre listing are all pretty comon with carbs that are too big for a motor. The 770 street avenger is much smaller than the 750 speed demon. I talked to demon and their rep told me their 750 carbs are more along the lines of a 900cfm carb as rated by holley {dont kill the messenger here im just telling you what i was told by a tech} The difference lies in the CFM rating. Where Holley dry flows their carbs, demon wet flows theirs. Im glad you got your problems worked out though and id be happy to take that P.O.S. Demon off your hands for $50 :)

I have heard WAY too many of these Speed Demon stories. Some have a happy ending afters tons of tweaking and drilling, but I'll keep buying Holley's, opening the box, bolting them on and driving away!

lol nova guy!i'm in for $52.50!
(sorry moderator,too much evilbay time logged) :)
Eric B

Thanks for the input. I am sure both carbs can be made to perform just fine but I am a Holley guy from here on out.
The Speed Demon is on EBAY - I am at $250 so far. If I had not have gone EBAY I would have sold it for $10!

Jeff - You should be mad at the carb guy not the SD carb. IMO you can't tune a carb for an engine combination without the darn thing being on the engine. Too many variables, cam, timing, gears, compression and such all come into play. The best thing that could have come from the money you spent was to have it set back to factory spec's and my guess is that didn't happen...

Greg - the only thing wrong with Demon carbs is guys try to tune themlike it's a Holley, it's not. I'm far from a guru with these thing and spent long hrs trying the things that always worked for me with holleys and got no where... Took a step back set the Demon back to factory spec's and followed the video and written directions that came with the carb. The thing worked like a charm, I'm not talking performance tuned for best 1/4 mile times but it idled, and was responsive and the car was very driveable.

If anyone has a new BG carb they think is a piece I'll be glad to pay the shipping if you are planning on tossing it! :D

ebay is overrated!$53.00!LMAO!:)

DjD, I hear what you are saying and I am sure you are correct. The thing is, why would you buy something that you have to tune right out of the box, even if you were an expert at tuning it?

I guess I am just lazy but I like my stuff to work right out of the box! :-)

I bought the 670 Street Avenger and my ONLY problem is that I should have ignored the 'free advice' and gone with the 770. I have tried Edlebrock and can't seem to get it exactly right and I have had to many bad experiences with the Quadrajet. I too took the Holley out of the box, put the power spring in, and drove it. No messing with anything.

If anyone has a new BG carb they think is a piece I'll be glad to pay the shipping if you are planning on tossing it! :D

Dang it!! Dennis beat me to it.

OK, I'll pay $5 over what DJD will ;)

Seriously though. Carbs (Holley, Demon, Edey, etc.) all get their fair share of blame when an engine isn't running right -- typically the carb is the first thing to get the finger pointed at it.

But I'll have to admit all the "trouble" I went through with my SPeed Demon turned out to not be the carb after all. I had a weak ignition system and as it turns out, a rich mix is much easier to light off than a lean one . . . now that my ignition is back on track I am very close to stock jetting again.

Sometimes the wrong carb for the application is picked and the carb manufacturer gets the blame . . . not saying that this is the case here, but you never know.

Anyway, glad to hear your engine is running like it should :D

DjD, I hear what you are saying and I am sure you are correct. The thing is, why would you buy something that you have to tune right out of the box, even if you were an expert at tuning it?

I guess I am just lazy but I like my stuff to work right out of the box! :-)

Im really not trying to be an ass here but.....

LOL, Come on man, You have a performance engine!!! That motor has so many different variables and things different from the next guys setup that its impossible for a carb to perform at its best with out tuning. I dont care what carb you get, its going to need some tweeking. That 650 speed demon would work on anything from a high winding 302, to a moderate performance 400 and you expect it to bolt on and go just like that????? !!!!

A few years back I had to take the idle feed restrictors down a few thousandths to get my Demon to idle properly. Too rich. But does look good.

Im really not trying to be an ass here but.....

LOL, Come on man, You have a performance engine!!! That motor has so many different variables and things different from the next guys setup that its impossible for a carb to perform at its best with out tuning. I dont care what carb you get, its going to need some tweeking. That 650 speed demon would work on anything from a high winding 302, to a moderate performance 400 and you expect it to bolt on and go just like that????? !!!!

Nova, I hear ya' too and no offense taken at all. As to your question whether I expect it to bolt on and go just like that, my answer is yes. It is certainly true that for OPTIMAL performance you need to tweak a little. If you are truely wanting optimal, then you need to re-jet from the spring to summer, summer to fall and fall to winter.

The thing is, the stories I hear about Demons aren't ones of not performing optimally, it's that they hardly run at all! I sure expect it to run fine out of the box. Yeah, I might have to tweak it to get that last .10 at the track, but when it stumbles off idle, or is so pig rich it burns your eyes out in out-of-the box tune, that is a problem. I also realize that plenty of people have bolted on Demons with NO problems, but you have to admit, there are a ton of stories out there of ones that haven't.

My same stock jetted 750 DP Holley has been on combinations that run from low 12's to low 11's with no tuning or twealking at all. All I have done to it are remove the choke flap, put in a notched float and jet extensions...that's it. I fully expect I could bolt it on to a 302 and it would run fine. Heck, I bought a $50 jet kit but haven't even cracked it open because I haven't seen the need.

It's not like I own stock in Holley or anything and I certainly have no axe to grind with Demon. It's just that there are folks out there reading this stuff right now that are thinking about a carb purchase and I try to let them know my personal experiences so they know what to expect from a Holley.

Here's how I see it... The manufactures try hard to provide a product that is out of the box setup to start your engine and with a little idle mixture adjustment driveable. That's all based on matching the right carb to your combo. In this thread Jeff went from a manual sec carb to a very forgiving vac sec carb. The vac sec carb in my mind is more streetable from the get go so it performed better out of the box.

There's a bit more and I touched on it in my first post. The BG carbs are not tuned like Holleys, read the BG instructions. If you bolt on a Holley and turn the throttle blade adjustment screw(s) to set the idle you'll have no real problem raising or lowering the idle without pulling the carb off the idle circuit. Don't try this with a BG carb, you'll be off the idle circuit in a heartbeat and your out of the box carb will run like crap. Per BG instructions, fire the engine up and use the rich lean mixture screws to find your idle. Leave the throttle blade adjusted as it came from the factory... Guess what? It works!! I fought a Mighty Demon for a long time and finally gave up, set the carb back to factory spec's and followed the directions...

Now there is more to this if you have a big cam and are not running enough initial timing and there are engines that won't idle without messing with the air bleeds and don't expect to not have some off idle flat spots without playing with the accl pump and squirters... On vac sec carbs you also may have a wot bog if the sec's open too soon... So the out of the box thing is really more about idling and being able to drive the car enough to start tuning the thing to run properly.

Through experience I have found that both Holley and BG carbs work well but I've also found that they don't always come out of the box with the factory settings like they should. Did someone else get the carbs before and send them back? Did the factory QA miss and are these carbs being assembled by lazy workers? I don't know but I've seen both BG and Holley with the floats way off, the throttle blades and idle mixture not set to spec right out of the box... If you have read the Holley basics articles I have in the techref area you'll see I always pull the bowls and dry check the floats and verify the factory settings on new carbs as well as used. In the long run it seems to make life easier...

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

DLT - I think the author knows what he's saying but worded it wrong in this one... http://www.omniracing.net/html/4_corner_idle.html

He has you set the idle (throttle blades) with a piece of paper as a feeler gauge and an additional 1/8 turn of the screw. If that equals, I think it's .020" of the transfer slot being uncovered thats a way to do it without pulling the carb and turning it over. Then he has you set the idle mixture screws at 1.5 turns from seated. That may be the factory setting, I'm not sure. Then he has you lean or richen the mix with the idle screws (throttle blades) and this is where I think he's meaning idle mixture screws... That's what he should have refered to, to be inline with the BG instructions. Of course even this requires you have enough initial timing for the carb to work with your cam of choice...