bhmyers

I have a 750 Demon and can not get it tuned right. It still runs rich costing me HP. I talked to a pretty well know race car driver that has a shop by where I live, and he told me to take it off and throw it away. He said the Holley 750 HP out performs the BG Demon. Does anyone else have this type of problem?

dman535

I poorly tuned Holley will run just as bad as any other poorly tuned carb.

Where is it running rich ? What have you done to it ?
What are the stats on your engine ?

Sigforty

What is your set up a 750 may be to big for you. I have a 650 Crate speed demon on my ZZ4 and it runs great. Have you tried reading Lars tuning papers?? You can find his papers in the tech section of the corvetteforum or on http://www.corvettefaq.com

bhmyers

I considered that, I have a 386 CI (stroked @ .040)

LAvetteman

How could a carb that is similar to a Holley, but more refined be worst than a Holley??? :rolleyes:

GrandSportC3

I never had a Demon carb.. but I do have a HP series Holley.. and it's the best carb that I've ever had... (80528 HP Series double pumper - mech. secondaries)

LFZ

see!!! i wasn't the only one! i had the same problems you had, and my conclusion was to violently bash my speed demon with an aluminum baseball bat and throw it in the river down by my mechanics garage. best answer to your problems! :thumbs:

Bob Turner

I just bought a Holley 750 vac sec since the Demon 850 I have is too complicated (idle circuits, 4 corner idle, that stupid .040 idle transfer slot and whatever else) for my liking.

I am NOT bashing Demons; I feel that they are good carbs but I am going back to Holley. Simple as that.

Bob :cool:

PRESSURERAT

If you throw it away, please throw it in MY trashcan!! ;)

gkull

Your local race car driver wishes that you would throw it in his can.

I at first was pissed at the performance versions of the Edelbrock 650 and 750. Because out of the box they just blew black smoke. All it takes is a little thought and reading

Alwyn678

I will take it if you are gonna throw it away...........I will also pay for shipping.........i also have a Holley 750 I will trade you for it :thumbs:

PRESSURERAT

In my experience the Demon is a good carb but people tend to go a little large with cfm's . In my opinion, a 600cfm Demon will do the job of an off the shelf stock Holley 750. The idle bleeds will tend to make the bigger size carbs run rich on smaller engines. Have you contacted Barry Grant? There are a finite number of things which will cause a carburetor to run rich, and solutions for all of them. There is not much to an idle circuit and one of BG's tech people should be able to talk you through it. If you could get a smaller Demon that would probably be better yet. A 750 Demon is jetted big enough to support a pretty hot 454 so keep that in mind when working with it!!

Corey_68

After running a Demon 850, you couldn't pay me to own another Holley. :nonod:

OkieVett

I cannot say enough good things about my Demon carb, did require some tuning

bhmyers

Has anyone installed an o2 sensor to help get these demons dialed in just right? I was thinking this may be my next move....

gkull

Yes, the O2 sensors would be the way to really dial in the Demon.

I would buy the HP series book on how to hot rod Holley carbs. all of the ideas apply to BG Demons.

I drilled .125 holes in each primary throttle plate. That got it lean enough to where with a vacuum gauge I could set the four corner idle A/F ratio screws all the same about 1 1/2 turns out. I drilled the squirters out & set the arm for the fastest squirt. I now use 75/81 jets with a 14 X 3 open element K&N filter. 12 degreees advance initial and 800 idle in gear close to 900 in N. My higher idle rpm is required with a large cam and 11.2 compression.

bhmyers

I adjusted the idle screws to about 3/4 of a turn and was trying that. The thought of drilling something on my carb scares me, I just dont know enough about them. I can still smell gas. Im running 72/82 Jets

Robert N

While I know littel about carbs and nothing about the Demons, the statement that most people run a carb too big for their engine is correct. Personally, I would start by jetting down the carb 2 sizes. You need to get a baseline. Fine tuning the carb with the idle mixture screws, use of the O2 sensor, vac guages is the proper way, but that cannot be done if the jets are 2 large/small.

theandies

I don't care what anyone says, a Lars tuned Q-jet ROCKS.
Thank you Lars....You ARE the man :yesnod: :yesnod: :yesnod:

gkull

It doesn't get much simpler than this!

Quote:The thought of drilling something on my carb scares me.


Some times during your life you have to take that leap of faith!:) .125 drill is a 1/8th drill bit. When I was doing it I thought: "What if it's too big of hole, or I screw it up!"

All you have to do is remove the screwed up throttle plate and install a new one or two. I destroyed things much more costly than a stupid little throttle plate.

Remove the carb. Drill a hole in each primary plate towards the front of carb about a 1/4 inch from the edge. After drilling clean up the ruff edge on the punched through side. I just used a larger drill bit to dress it up.

I think what most people are missing about carbs is the primary jetting has nothing to do with the idle A/F mixture.

To set the idle screws: Turn each of the four corner idle screws in till they have a light touch. Then count the turns out on each one to, "TWO" turns out. It's very important that they are all the same. Attach your vacuum gauge. Some people are good enough to adjust carbs without a gauge. They do it by the sound of the motor.

With your car at operating temp turn up the idle speed to 1100 rpm. On our Demons adjust the passenger side idle speed screw. That"s the secondary throttle plate. So your vacuum gauge is not bouncing so badly. (I've never done a mild cam!:)) Look at the gauge. Turn each of the screws in 1/8 of turn and see if your idle rpm went up or you could hear the motor smooth out also maybe a rise on your vacuum gauge. Do them all another 1/8th. Keep on going in small increments. Motors like mine actually shake when then are out two turns and as you get near the correct A/F the motor is smooth and the idle speed even starts going up a couple hundred rpm. So it is often necessary to continue to turn the idle speed down to the original 1100 setting.

Once you go past the correct number of turns out your going to loose rpm and vacuum. So back up to the best setting. Then turn your idle speed down to your correct idle speed. Mine is 900 in "N" and it drops to 800 in drive. The milder the motor the lower the rpm. You're done.

Always have all four screws the same number of rotations!

Always use the secondary throttle plates on a mechanical secondary double pumper type carb for setting idle speed. That way your not screwing up the transfer slot setting and air and fuel is coming more equally to all cylinders. It also helps on the transition from running on the primary only to when the secondaries start to open because your already getting flow through there.

Happy Vetting!
:auto:





[Modified by gkull, 3:44 PM 10/8/2003]