gingerbreadman1977

its a long post but if your interested in demon carbs or would like to help me out please read along......

I dont really have any problems with my carb , in fact i love demon carbs but i do have quite a few questions about them. i would call myself a novice with carbs but with some understanding of how to pull it apart , clean and make adjustments. knowing about all the circuits, passages, bleeds and parts is beyond me at the moment but im learning all the time.

my carb is the 750 DP which i believe is a mighty demon (see there is my lack of knowledge coming out already) and i have 2 of these same carbs on different chevs with similar set ups but it has honestly taken me too long to learn this much so far.

i have set my transission slots to .020 or like squares as the manual says. i did my best but one of my transission slots is a little longer then the other so i have just had to choose a middle spot as good as i could.is there any way to adjust the butterflies so i dont have this or is it not really a problem ? please see the picture below and more story ....



standard my carb comes with 75 jet in the primary and 83 in the secondaries. i have changed to a 73 in the primary but i think i need more changes and this is why ....
after adjusting the slots and installing the carb,setting my floats etc etc my best vacum is at only 3/4 a turn out on the mixtures. i ran out of light this afternoon and did a test with pcv,brakes and all other vacums hooked up and i had 9 vacum. tomorrow i will block all vacums and see how much i can get. my point is BG recommends mixtures should normally be 1-2.5 turns out ? i know i wont get there.

BG also states get your vacum reading, divide it by 2 and that should be your powervalve size. go the the next lowest if its in between. once again my carb has a 6.5 standard but there are no numbers on my powervalve matching this . there is three numbers on it stamped but they are not together,there is a 2 and a 5 and another 2. how do you know what you have?

so in short my questions are
where is the powervalve size marked?
my idle ezy does nothing when i turn it, why is that?
what is a good vacum reading to shoot for?
my cam is only a 490/490 on a 383 with 108 Lc so does that hurt me?
even though my transfer slots seem a bit different is it ok?
i know there is alot of you guys out there with the same carb as me so what jet sizes and powervalves are you running?- i know everyone has different size cams motors and all that but i just want to get an idea as i payed for a dyno tune once and wont do it again.

anything i have said wrong or you think is a good tip please dont be shy, im listening.

baxsom

power valves are marked on the face, you should see numbers in a verticle line like 6 and then a 5 below it, that would indicate a 6.5

gerry72

I would avoid fiddling with the carb until you get a chance to buy and read one of the books widely available out there on how to tune and modify a Holley carb.

For that you will glean such wisdom as playing around with the jets and power valves will have no affect whatsoever on the idle circuit. That you shouldn't try to tune the main circuit until you have the idle circuit in the zone.

Without knowing the overlap and the intake closing point, your compression, and ignition statistics, it's hard to say if your vacuum level is appropriate. I suspect it is well below what you should be seeing due to tuning issues, though.

As for the transition slot, you don't want to go beyond the .020 blade setting. Doing so can over-expose the transition and induce a lean stumble when transitioning from the idle circuit. What BG wants you to do is set the throttle blades, then control the idle speed with their EZ Idle screw. As to why it's not responsive, I do not know.

The idle mixture screws being only 3/4 of a turn out may or may not need addressing. The indication is that your idle fuel circuit is a bit too fat. You address that by decreasing the size of the idle fuel restrictor in the metering plate. You can also address this with the idle air bleed, but that is for fine-tuning the circuit, which is not called for in this case. The number of turns isn't as important as is the sensitivity of the adjustment screw. The better your emulsion in the idle circuit, the more sensitive the screw will be in that a quarter turn one way or the other has a significant affect on the idle.

Again, I strongly encourage you to get the academic side down before you start to fiddle with the carb.

Grumpy 427

Not trying to bash here, but sell that carb and buy a holley you will be much happier. I had an 850 mighty demon and could not get it to idle. Sold it and bought a holley 850 dp Hp problem solved out of the box. There have been tons of issues with Demon carbs, mine was full of aluminum shavings out of the box, it took me a couple days to go through the whole carb and remove all the loose flashing left behind.

baxsom

wow, i had quite the opposite experience. my brand new holley leaked out of the bowl gaskets and acellerator pump gasket and had a warped baseplate while my brand new speed demon needed has ran well enough that i have left it alone after an idle speed adjustment. and just for the record, my demon didnt have shaving one in it.

nitronick

Any adjustments on the mixture screws should be made with the idle adjustment screw backed out and the plates completely closed so that it is drawing fuel from the idle circuit only. Start at 1 1/2 to 2 turns out on the mixture screws to get it started and running and then make your adjustments using dwell.

Graemeinvette

My Speed Demon ran straight out of the box, float bowls were as clean as they could be, no sign of any debris anywhere else. It replaced an almost new Edelbrock Performer and the difference in pick up and performance were huge.

Now on with the lessons

Rock'n Old

My Road Demon is the best investment I have done to my -69, 350.
It was like I got a new car. Mount and go!

(Just to say that not all Demon carbs are filled with shavings)

MotorHead

Any carb that is near the correct size for the motor and functioning properly can be tuned to get amazing results. Both in performance and gas mileage.

I would get a book on how to tune a Holley for starters and a wideband O2 sensor is really the only way you "see" what your tune is doing at idle, cruise, part throttle and WOT.

There is allot to it. I have been at it for a long time and still only know a minimal amount. If you really want to find out a few things other than changing jets and squirters go to Innovate site and to their forums. I have a 825 Race Demon that runs perfect on my 650HP 427ci and with OD tranny I get over 20mpg on the highway.

And please no more Holleys are crap or Barry Grants suck, if you have functioning carb and know what you are doing you can get either to work just fine.

stingraymyway

Sounds like a huge amount of carb for your Vette.
I went with the Speed Demon 650 CFM on a edelbrock 2116 performer intake, GM vortec heads, crane cam.
"Out of the box set up" was WAY too rich. Even fouled plugs. Checked the power valve. It was an 8.5, suppose to be a 6.5. Changed that and dropped the primary jets from 70 to 67.
Now it runs much smoother, no fouling/flooding. And I'm getting 18 MPG driving around town.

Not sure why your idle-ease isn't doing anything. Should make a difference. I think I'm about 2 turns out.

MotorHead

Holley and BG carbs are always too rich out of the box, by design, built in safety factor, better rich than lean and ruining someones $15,000 motor

gingerbreadman1977

thanks for everyones replies . it all helps even the negative ones. i should have mentioned in my earlier post that both carbs run quite well but im thirsty for information and would like to be able to fine tune better with the help of some others. its not hard on here to figure out the good advice from the bad. I dont really have any solid awnsers to my questions thus far but ill try to reply to all your comments as best i can .

Baxsom i couldnt find the kind of numbers that should have been there. i even unscrewed the powervalve for a closer inspection.all i found was the 5-5-2 scattered around the perimiter that i mentioned but i dont believe they were related. i have had both these carbs since new so i know they havent been tampered with. next time i have it apart ill double check.

gerry 72 i am actually in the process of reading a few books about holley / demon carbs. in fact im always trying to read more and learn more of late but my time is scarce and i cant do it as much as i would like. reading and learning about carbs is like an investment and takes days,weeks,years to become an expert so for me asking a few simple questions on here cant do any harm. If i had to read all the books on things before i actually did anything i would not have started my car since 1989. i know what your saying and i take it on board but im more hands on .

grumpy you are in fact grumpy ..... but im not selling the carbs for another brand. there is fine tuning in all carbs. im just trying to learn more and find that ever elusive perfectly tuned carb. it actually runs pretty well but i know its far from perfect.

nitro nick your theory of closing the plates goes against what the demon instruction dvd and paperwork recommends. actually it says damage may occur to the plates or shafts if the plates are hard on the stoppers but ather than that i did start at 2 turns out like you said and went from there..ending up at 3/4 a turn out for best idle.

graemmeinvette and rocknrold your comments are first class. i like the sound of those lessons.

motorhead i always take on board what you say. you comment is the truth and i agree all the way including there is no point to bash either carb. they are both very good carbs .

stingrayman im not so sure about too big. both cars that i have these carbs on are reasonably similar. the vet is a crane powermax
HR-230/359-2S-12.90 IG
Operating Range: 3000-6500 RPM
Duration Advertised: 292° Intake / 300° Exhaust
Duration @ .050'' Lift: 230° Intake / 238° Exhaust
Valve Lift w/1.5 Rockers: .539'' Intake / .558'' Exhaust
Lobe Separation Angle: 112°
with a th350 3000 stall and 355 rear gears so i think a 750dp is perfect.

the other car is the 490/490 crane hydraulic cam in a 383 with 3.73 rear gears and 2500 stall in a th350 so i think its swallowing all a 750 dp has to offer as well. i could be wrong but does this extra info change your mind . you might be right i dont know.

nitronick

This is not a theory....Backing out the idle speed adjustment screw to close off the plates will not harm anything. I have made these same adjustments for many years when I was in the carburetor remanufacturing business. I have never had any damage occur when I worked on them. I have seen damaged plates and shafts when someone that didn't know what they were doing had slammed the throttle closed with their hands doing God only knows what.

When you are trying to make adjustments on your idle circuit, you need the engine drawing fuel only from the idle circuit. If the plates are opened, even the least bit, you will be drawing fuel from the main jets as well as the idle circuit. If they are opened too far, it will override the idle circuit and draw only from the main jets.