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Old 04-04-2008, 08:03 AM
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Lean problems with BG Speed Demon carb

I just bought a new Demon carb - first time buyer always had Holley's & Edelbrock's - and these past two days I've been working and finding time to tune my new carb to perform well with my car. Crazy thing is that after about ten minutes of driving my A/F meter reads big-time lean! I have years of experience tuning my past Holley's and Edelbrocks but I couldn't figure out what was wrong with this new Demon carb. A call to BG's tech line gave me a question about what sort of manifold do I have. After I told the guy RPM Air-Gap he asked if I had checked to make sure that the two vacuum lines on the passenger side in the front of the carb were seated and sealed onto the manifold and not just only sandwiched on the gasket. I took off my Demon carb and noticed how the carb's vacuum lines passages on the underside of the carb where actually awfully close to the edge - unlike any Holley carb. Then I noticed that the top part of the manifold on the passenger side has less metal on that side making a "wall" than on the driver side "wall" of the manifold. Being that the manifold is aluminum and the Demon carb's base is too I guess that the BG tech guy is saying that maybe after the metals get hot they move which allows a "air-leak" into the carb down there giving me the very lean A/F meter readings after ten minutes of driving.

I've never heard of this before. The annular boosters in this 750 Speed Demon carb for low end throttle response are something else!!! I just ordered a 3/8" thick aluminum open spacer to cover up my manifold irregularity on the passenger side which is right under the Demon carb's vacuum line passageways. The carb spacers have the same amount of metal on both sides unlike my RPM Air-Gap. I'm hoping that will fix the problem.

I would think that two aluminum materials - the manifold and carb base would get bigger and would seal each other as the hotter they get! The telephoned tech guy at BG didn't seem too helpful. Maybe he thought that I was a carb beginner? He asked me what type of manifold I had and after I told him he said to make certain that the underside of the BG carb was completely covered by metal and not just the over-hanging gasket. I asked him about any other advice and he said that I'd have to start the carb over at it's baseline. Well the EZ-Idle screw I have bottomed out clockwise so it shouldn't be effecting the motor. All four of the idle mixture screws are turned 1.5 turns counterclockwise. Both float levels are good - visible through the huge glass sights. I had to richen up the jets to get it to run properly which fixed it - until the motor gets up to operating temperature. I haven't modified anything else on the carb. The accelerator circuit - untouched - seems to be working fine.

After about a minute of warming up the cold motor the car pulls away and runs very well. It idles well at the light and takes off from idle to cruise while accelerating well also. Then when the motor and carb get to operating temperature I can sit at the light and watch my A/F meter go from the green lights (where I like it) move to the yellow lights and then within say thirty seconds it works it's way into the lean red lights! (Please remember that if I switch back to my tuned Holley 750cfm VS carb it runs like I like it the entire time - so I know it's not my current motor but something on the BG carb.)

I wonder if when the BG carb's base heats up it is warping? Hopefully this weekend I can put the spacer on the manifold and try the Demon carb again. I will post my findings. Has anyone ever experienced something like this before?

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Last edited by Sixtyninemercury; 04-04-2008 at 09:36 AM.
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Old 04-04-2008, 11:19 AM
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I have PM'd the guy on this forum who says he is the BG tech - I can tell that he has read my PM but hasn't returned me a PM yet with any advice.
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Old 04-04-2008, 11:42 AM
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I have a Speed Demon on an Airgap but never had the problems you describe just a blown power valve that took me too long to figure out.

My four mixture screws are just one turn out and the Idle Eze is open 1.5 turns, the baseline setting.
My fuel level is just below middle on the sight glass.
Do you have an open plenum, I use a one inch four hole spacer, makes throttle response really crisp ?

Can you really trust that A/F meter ?

Idle Eze Notes.
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Old 04-04-2008, 11:51 AM
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Quote:
Originally Posted by malc
I have a Speed Demon on an Airgap but never had the problems you describe just a blown power valve that took me too long to figure out.

My four mixture screws are just one turn out and the Idle Eze is open 1.5 turns, the baseline setting.
My fuel level is just below middle on the sight glass.
Do you have an open plenum, I use a one inch four hole spacer, makes throttle response really crisp ?

Can you really trust that A/F meter ?

Idle Eze Notes.
Is that manifold on a SBF? My manifold is a square-bore. I think it would make a difference in the casting where the carb meets it. My mixture screws are a half turn more rich than yours. My float levels are a little bit higher than the middle line of the sight glass which I know is a bit richer than they suggest. I have no spacer on the manifold. With my EZ idle screw bottomed out the carb is not letting in any un-metered air into the motor. Yes my A/F meter is a good reference point - like I said when I noticed the problem I changed the BG back to my 750cfm VS Holley and the meter ran true in the greens (slightly rich) where I like it for best acceleration.

Last edited by Sixtyninemercury; 04-04-2008 at 12:01 PM.
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Old 04-04-2008, 12:01 PM
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Mine´s on a SBC.
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Old 04-04-2008, 12:06 PM
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Quote:
Originally Posted by malc
Mine´s on a SBC.
I will show you a pic of my SBF's Air-Gap manifold - on the passenger side you can notice how there is a straight groove cut into it which might be letting in un-metered air into the plenum from the vacuum grooves underneath the carb that are cut into it's baseplate which are close to the base's edge.

http://www.putfile.com/pic/7931705
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Old 04-04-2008, 12:11 PM
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The pic is way too small to see any groove.
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Old 04-04-2008, 12:23 PM
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Quote:
Originally Posted by malc
The pic is way too small to see any groove.
It's between the rear bolt hole and the front bolt hole on the passenger side. You can see the radius where the straight groove starts just after the rear bolt hole if you are eye-ing from there towards the front bolt hole.

Here is the start of the radius circled in red: http://www.putfile.com/pic/7936403

In other words the wall thickness of the passenger side is thinner than it should be compared to the thickness of the wall of the driver side.

Only other thing I can figure out is maybe the base is warping once it gets hot but I didn't see any leakage of fuel anywhere?!?!

Last edited by Sixtyninemercury; 04-04-2008 at 12:30 PM.
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Old 04-04-2008, 12:27 PM
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There´s more meat on yours than on my Chevy Air Gap.
Here´s mine.

And this is yours.
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Old 04-04-2008, 12:29 PM
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are u using a wideband afr gauge or a narrow band gauge also is it a heated o2 if its a narrow band.

reason i ask is if its a narrow band o2 they are only accurate around 14.7 afr at 15.5 afr the thing can read dead lean, same goes for the opposite direction.

also if ur using a narrow band and its a non heated type at idle the sensor may be droping below operating temp witch will also cause a lean reading.

if u are using a single wire o2 sensor i suggest going to a 3 or 4 wire heated o2 both are the same the only diff between the 2 is the 4 wire has its own ground wire so it dosent rely on getting ground from being screwed into the ex system

also narrow bands should not be relied apon to much for anything other then wot
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Old 04-04-2008, 12:31 PM
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I don´t think those slots on the underside of the carb´s baseplate are too near the edge.


I use four holer gaskets MRG-55.
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Old 04-04-2008, 12:36 PM
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[QUOTE=malc]There´s more meat on yours than on my Chevy Air Gap.
Here´s mine.

Without your spacer you might have my problem - I'm not sure - this is what the BG telephone tech guy suggested to me - when I get my online-ordered 3/8" aluminum spacer I will try it and find out. I have a 2" four hole phenolic spacer that I put on top of my manifold and it covers up the straight groove that I am talking about - but not before the spacer - there is not as much meat on the passenger side compared to the driver side with no spacer - carb on top of manifold.
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Old 04-04-2008, 12:39 PM
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Quote:
Originally Posted by Boosted89RS
are u using a wideband afr gauge or a narrow band gauge also is it a heated o2 if its a narrow band.

reason i ask is if its a narrow band o2 they are only accurate around 14.7 afr at 15.5 afr the thing can read dead lean, same goes for the opposite direction.

also if ur using a narrow band and its a non heated type at idle the sensor may be droping below operating temp witch will also cause a lean reading.

if u are using a single wire o2 sensor i suggest going to a 3 or 4 wire heated o2 both are the same the only diff between the 2 is the 4 wire has its own ground wire so it dosent rely on getting ground from being screwed into the ex system

also narrow bands should not be relied apon to much for anything other then wot
Yes mine is heated. This motor has had many carbs on it - Edelbrock and Holley - and this is the first time I've ever had this crazy experience on my A/F meter. I am experienced enough to tell how it drives after it warms up and the meter is just confirming what I am feeling through the gas pedal.

It is not the meter!
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Old 04-04-2008, 12:41 PM
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here u can see the diff in how the 2 diff types read
narrow band

and wideband


the output form the narrow band is for a very small afr range as u see that lean reading may not be lean at all
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Old 04-04-2008, 12:42 PM
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I have never had mine installed without a spacer.
First off I had a half inch to give me clearance for the brake booster vac. under the rear floatbowl.
Be patient the BG tech guy will eventually get back to you, he´s pretty good.
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