L-82kid

I posted on here before because I was having tuning issues. I couldn't even burnout because my car would bog so hard it would almost stall out.

heres my original post:

http://forums.corvetteforum.com/show....php?t=1987187


Well I've been trying to tune my carb ever since and can't get it dailed in to run perfect all the time through the whole powerband.

Yesterday I had it running pretty good. It ran strong through the gears and even would spin the tires good. I drove for about 10 miles just cruising. the idle was fine about 850rpm and I firgured I had it set up fine. But then i turned the car off and when I turned it back on it ran like **** again. off idle under light acceleration it surged and almost stalled out, it was like I was running out of gas but i had 3/4 of a tank. then once it got to 2000rpm it would run fine. I tested it from 2000rpm up and it ran good. but every time I slowed down to a stop and started moving again it would stumble under light acceleration. i got home and tried riching up the idle with the 4 screws but it started running rough so I got mad and gave up.

Here are my settings:

750cfm speed demon mech sec.

4 idle mixture screws 7/8 of a turn out
76/83 jets
float bowls at the first line
I had to adjust the two butterfly blades to get the idle down to 850rpm. I kept them even.
makes 12in hg of vac at idle. (comp cam mag 280 cam)



Has anyone here had problems with this carb?
Does anybody know any good carb tuners in CT, I'm from just north of New Haven?

I don't have the money to buy a new carb and really want to get this one running perfect but I guess I don't know what I'm doing here lol.

I have loud exhaust and a rough cool sounding idle but my car runs like a dog lol.

corvette744

What size power valves are you running in it.8.5,did you spray carb cleaner around base of carb and manifold to check for vaccum leaks.Re those the factory jets for the 750.

enkeivette

BG makes some of the best carbs out there, I love my Mighty Demon. You just don't know what you're doing. Do some research, learn, don't complain.


I'll give you a quick run down of what you NEED to do, and do it in THIS ORDER. If you don't, your car will run like sh*t. Don't be lazy:

Clean you spark plugs with brake cleaner. If they don't come clean, replace them.
Take the carb off the intake manifold.
Set the butterflys so that the transfer slots look like little squares. (They should only be exposed as much longitudinally as they are wide)
Set the floats to the middle line.
Get the engine to operating temp.
Then, set the idle with the distributor. That's right, give the car as much timing as it needs to idle around 800. My car needed 18 inital. (Make sure to limit total to 36 with the right size bushing, you'll have to do the math) DONT SET THE IDLE WITH THE BUTTERFLYS.
Buy a cheap hand held vacuum guage.
Plug off all vacuum ports and hook the vacuum guage up to MANIFOLD vacuum (not ported).
Set the idle mixture screws evenly to pull the highest vacuum possible at ide.
Record that number, subtract two from it, install a PV of that number. (Ex. I pull 6.5 inches of vacuum at idle, thus my PV is a 4.5) Edit: Unless you have plenty of vacuum at idle, the rule is at least 2 inches less. But if you have a good amount of idle vacuum, the standard 6.5PV should be fine.
Then test drive it.
If there is still a bog during initial acceleration, install larger squirter nozzles. Go up 3 sizes at a time until there is no bog. If you get to size 40 and still have a bog, change the pump cam to a more aggressive one.
Then read your plugs. Jet accordingly.

That should get you started. And you can do it ALL yourself.

corvette744

I agree i think he will find he has the wrong or blown power valve in it-or the squirters are way to small.Probably 32 and needs 38-40 size.Great description on how to basically dial a carb in.Their are a few more tricks but dont think you need that....

enkeivette

To test the PV, unscrew it from the metering block, press your lips firmly on the side opposite the spring and suck. If the spring moves inward, the PV is fine.

breathial

I think your problems are more basic.

Simply stated, IMHO, the carb is too big for the engine you have. The Demon carbs are under-rated, in terms of CFM. Compared to a Holley, it'd be more like 800-850 CFM...

I'd suggest stepping down to a 3310-1 or 3310-2 carb, by Holley. They can be had for cheap, are easy to tune, and will work perfectly on your engine.

Again, JMHO.

gingerbreadman1977

i agree it could be an operator problem not carb problem. take it to a recommended tuner and have it sorted out. i have a demon 750 and bolted it straight on out of the box and it ran like a dream straight up.

V-Twin

There is no such thing (considering realistic carb sizes)

Darcy

I have a Speed Demon (670 I think, matched for the ZZ4)on my ZZ4 and I love it, although being born in the EFI age, a carb was all new to me.
The dumbest thing I did was not follow the instructions and set the initial idle with the screws on the side.
Be sure to set the inital idle with the big phillips screw that is hidden in the bolt hole where the stud for your air cleaner is. This step is after you have set the butterfly's properly with the square hole in the gap.

You will get it figured out, but you will need a vacuum gauge and patience. But if you like to rev your engine in the driveway just to hear the exhaust, this will not be an issue :-)

-->Darcy

427-390

How is the timing on your car? In the infamous words of Lars, "90% of all carburetor problems are actually timing problems" Before you do anything, confirm that you are at 36* @ 3000 RPM with the vacuum disconnected, and work out from there.

Edit: I just read your original post where you mentioned that you followed Lars' paper to a tee and had driven the car 6000 miles after that. I would go back and double check the timing as a new baseline starting point. FWIW, I have a bone stock BB car with A/T and 3.08 gears and can easily spin the tires with little effort.

454Can-Am

Have you pulled the carb apart............................ I never could get mine tuned up good and when Lars worked on it , he found a small spaceship in the front bowl !!!

It turned out to be a small piece of deburring media that got left in the carb at the factory. I guess it blocked jets and bound the float up causing havock and making the car running different all the time.

Once Lars worked his magic............ its worked like a charm ! Rick

enkeivette

What is it with everyone judging performance by how easy it is for them to spin the tires? I can chirp the tires going into 3rd with my bone stock neon, but that car still has a Hampster engine!

BTW, if you have an idle eze crack it all the way open, see if it makes a difference.

hugie82

If you had it running good and then you shut it down, than it ran like crap I would start with a run a fuel presure gauge and drive it for a day or two. You may have a pressure changes from outside temp., bad ground (if electric), tank venting, ect.... WATCH OUT, I SAID SOMETHING BAD ABOUT MY CRAPPY DEMON Jr. and LARS JUMPED ALL OVER ME FOR BEING A ROOKIE CARB. TUNER

Amelio

There are lots of good suggestoons here and if I may I will add a few. First off like mentioned check your basics such as timming, also clean your plugs or replace if they need it. Run the car and pull the plugs again if they are black as night you are way too fat. What are you usin for fuel delivery? electric or mechanical pump and you do have a fuel pressure regulator if it's electic correct? Do you know where your fuel pressure is? You will need roughly about 9psi or so, double check that. I also agree with the abouve stated comment about BG's being underated compared to a holley. I do not have hard proof but I have exsperience. I always used holleys, on the old 350 I had in my Vette I used a Holley 830 anular discharge, this was bolted on a 355 with aluminum heads Victor Jr. intake fairly large hydrollic flat tappet cam and about 10:1:1 pistons The car ran great! I have since rebuilt the engine Same heads but full port and polish solid flat tappet cam and bumped up the pistons to 11.6:1 I wanted to get a new carb I have a friend that works at BG or did at the time I told him I wanted the 850 mighty demon but he said it would be way to big. I let him know I would be sparaying the car with a 250-300 shot and needed the larger cfm for the big end. He and a few other BG reps told me I needed the 750 mighty demon, I don't know exactly how but supposedly they wet flow them????? and it flows close to 850cfm

I bought it an bolted it on the car right out of the box and it has been working great! I have not tuned the car yet as the engine is still fresh and I am working on other projects but the car runs really strong and I have not put the squeez to it yet.

Hope this helps....good luck

L-82kid

Thanks for the the help. I am def. a rookie tuner.

my carb is the older version so it doesn't have that idle eze screw down the center.

The PV is stock out of the box for the 750cfm speed demon. I think 6.5?

Fuel system is stock tank, lines, and mech. pump. I have a little fuel pressure gauge on the feed lines to the primary float bowl.


I have mine at the first line so I'll try raising it to the middle one.



I start with this as an initial setup:

the butterfly blades at .020", with 76/83 jets, and idle mixture screws out 3/4, stock 6.5PV

The Idle is at 1200rpm-1400rpm! so if i advance the timing the idle will come down? or Do I retard it, I thought retarding it causes it to come down.

I found my notes on how my timing was setup last time:

12deg @ 800rpm initial
36deg @ 3000rpm less vac.

L-82kid

I had my carb float bowls off when I rejetted and didn't see anything weird in there, but I haven't taken the whole thing apart to check it out.

enkeivette

Those jets seem big for your app. Do you have annular boosters or downleg boosters?

So let me get this right, the car was idling at 800 and now it's idling at 1200 on its own? If this is so, check the timing to see if the dist moved on its own. Did you clamp it down tight enough?

When you set the idle at 800 last time, was the engine at full operating temp?

LiveandLetDrive

oh boy, not again

mxscooter

Howdy
I have the exact same carb, and I had the exact same problem you're having. I hated it at first, however with some patience you'll understand how simple it is to tune.
If it was working fine before then do the easy stuff first - yank it and check all the fuel passages, including your fuel supply line to the carb. Is there an obstruction holding a float valve open, or blocking a passage? I had a turd barely holding one of my valves open and it would run fine and then like crap, and then fine and then like crap lol.... And then yet another errant turd ended up blocking the rear fuel intake port - it was teflon tape that bubba used further up the fuel line! Also, become friends with your float adjusters and how they work, then after that you should get jiggy with you 4-corner balance screws. I originally thought my carb was too big for my 355, but after fooling with it I have gotten it to run perfectly, it's smooth as glass.

78 Vette

You need to disassemble it and clean out all of the passages they are full of metal. I speak from experience and not from just one carb. I have seen a pallet of B.G have 2 good carbs out of the bunch. The rest were full of metal and some had casting flaws. . Carbs don't work then not work there is a reason. Clean it good