BarrY Grant 4 Corner Idle Question.."Mighty Demon"...

[Viet Nam Vett]

I have the BG 750 Mech Sec Double Pumper. I'll make this short and sweet. You BG owners will know what I'm talking about.

The Secondary Plates are covering the transfer slots in the sec Barrles as per the directions for under 1000 rpm idling. The manual says nothing about adjusting the rear Idle Air screws in this configuration. If I move them in and out I do get some reaction but not like the front screws.

The question is........Do I screw them all the way in and froget them...??? Or try and adjust them into the mix..?? Since the idle air transfer slots are not exposed..adjusting the jet screw seems worthless. I called BG but I was number 7 in the "Q" line to be answered. Didn't feel like waiting 2 hours..

 

[IH2LOSE]

Mark with carb off and flipped over in your hand. the long angled slot with the butterflies open is the idle circuit. the front and the rear when closed should look like a square not a rectangle.

Going forward once the rear is set, its set and done, not to be touched again. Check the accelerator pump/pumps and set them so there is no slop.meaning as soon as you hit it your on the pumps. Don't leave it as the factory had it as they are loose and give an off idle stumble.

These things are sensitive do an 1/8 in let the engine have a chance to settle in, Not like the Holley's 1/4 to 3/8 to get a reaction .

Set your fuel bowls 1/2 glass, turn it off walk away ,come back start it again let it run see if its still half glass.

Does you Mighty demon have the idle ease system in it? This help with low vacuum in a cam-med up motor.

Are you using a vacuum gauge. Adjust for the highest idle vacuum, Makes it easier then trying to hear the motor .

Remember screwing in leans the idle.
screwing out makes it richer.

All thought this is a racing carb it looking for 6 to 7 1/2 psi

Whats happening with the carb now what are you trying to do are you having a problem.

Let me know. Call me i think you still have my cell

 

[Viet Nam Vett]

Mark with carb off and flipped over in your hand. the long angled slot with the butterflies open is the idle circuit. the front and the rear when closed should look like a square not a rectangle.

Going forward once the rear is set, its set and done, not to be touched again. Check the accelerator pump/pumps and set them so there is no slop.meaning as soon as you hit it your on the pumps. Don't leave it as the factory had it as they are loose and give an off idle stumble.

These things are sensitive do an 1/8 in let the engine have a chance to settle in, Not like the Holley's 1/4 to 3/8 to get a reaction .

Set your fuel bowls 1/2 glass, turn it off walk away ,come back start it again let it run see if its still half glass.

Does you Mighty demon have the idle ease system in it? This help with low vacuum in a cam-med up motor.

Are you using a vacuum gauge. Adjust for the highest idle vacuum, Makes it easier then trying to hear the motor .

Remember screwing in leans the idle.
screwing out makes it richer.

All thought this is a racing carb it looking for 6 to 7 1/2 psi

Whats happening with the carb now what are you trying to do are you having a problem.

Let me know. Call me i think you still have my cell

Heh Larry, Don't have your cell #.PM Me

The carb has been set to the factory specs and adjustments you mentioned. I havw the manual..and the video. This was all done. The motor is running fine.There are no problems with the carb other then the rear Idle Air adjust screws as mentioned. The Accelerator linkage has been set..no slop. The front throttle plates are set to the spec.

The fronts are set so the slots are visable... The rears are closed with no slots visable... That's. why I'm wondering about the rear idle adjustment...??? The manual says noting about adjusting the rear jets when the rear's are closes for Below 1000 rpm idling.

Yes I do have the idle ez adjustment. But ..I can't get the carb to go below 800 rpm. With the idol ez closed it will not drop below 800 rpm.

The Motor was set with a Vacuum gauge. Everything is correct. The manual is just not clear on the rear jets when the rear throttle plates are closed.

 

[Kid_Again]

'shoulda bought a Holley DP


HEY, I'm not complaining (now that I fixed the Holley on the SB)

 

[JohnZ]

Dunno about the BG carb, but on a Holley, the idle mixture fuel is delivered into the venturi through a tiny round hole, not through a slot; the slot is the transfer slot, which aids the transition from the idle system to the main metering system (along with the accelerator pump). The arrow in the photo below is pointing to the transfer slot (which should appear as a tiny square, not a slot, with the throttle blades in the closed idle position), and the round hole directly to the right of the slot, centered on the throttle blade, is the idle fuel delivery orifice, served by the idle mixture adjusting screw. The hole further up (down in car position) and to the left is a manifold vacuum port.

 

[BarryK]

Mark

look on my website tech articles page. I have a paper on the BG carb set-up and it includes setting the 4 corner idle screws and the idle transfer slot.

http://69.253.166.197/page1/page65/page65.html
scroll about 1/2 way down the page and look for a file called :
BG Carb set-up.pdf.

hope it helps you a little

 

[Viet Nam Vett]

Dunno about the BG carb, but on a Holley, the idle mixture fuel is delivered into the venturi through a tiny round hole, not through a slot; the slot is the transfer slot, which aids the transition from the idle system to the main metering system (along with the accelerator pump). The arrow in the photo below is pointing to the transfer slot (which should appear as a tiny square, not a slot, with the throttle blades in the closed idle position), and the round hole directly to the right of the slot, centered on the throttle blade, is the idle fuel delivery orifice, served by the idle mixture adjusting screw. The hole further up (down in car position) and to the left is a manifold vacuum port.

Sorry John,
I used the wrong terminology.. I ment what you have said above. As stated the BG has the same set up as the fronts but also uses it in the rear..along with the 2 Idle Fuel/Air mixture screws.

I spoke with BG about 1/2 hour ago. There tech guy says that even though the rear throttle plates cover the transfer slots as per the manual...they still suggest trying to adjust them and or end up with as close as possible the same setting as the front.

No where is this mentioned in the manual . The tech guy told me that maybe it's time for a Manual re-write. This adjustment only applies to the BG Mighty Demond . It does not apply to other BG Vacuum Secondary Carbs.

Don't ask me why....???? It's there sand box...I'm just playing in it.

 

[Viet Nam Vett]

'shoulda bought a Holley DP


HEY, I'm not complaining (now that I fixed the Holley on the SB)

Man you gotta lot of room to talk with your Holly Fuel Gusher....That thing is an Kam-A-Kazi's dream... Just turn up the fuel pressure and drive into the nearest target....

 

[Viet Nam Vett]

Mark

look on my website tech articles page. I have a paper on the BG carb set-up and it includes setting the 4 corner idle screws and the idle transfer slot.

http://69.253.166.197/page1/page65/page65.html
scroll about 1/2 way down the page and look for a file called :
BG Carb set-up.pdf.

hope it helps you a little

Thanks Barry,
The PDF on your site is for the Seed Demond. The Carb I have is the Mighty Demond. The Secondaries are set differently then the PDF by Lars. As you can see by my replies in this post ...they didn't make them selfs clear on the rear jet adjustment since the transfer slots are closed off at idle. It woulld indicate that adjusting the jet screws would be futile since the transfer slot is covered and the throttle palte and is closed in the Barrle bore.

 

[IH2LOSE]

Sorry John,
I used the wrong terminology.. I ment what you have said above. As stated the BG has the same set up as the fronts but also uses it in the rear..along with the 2 Idle Fuel/Air mixture screws.

I spoke with BG about 1/2 hour ago. There tech guy says that even though the rear throttle plates cover the transfer slots as per the manual...they still suggest trying to adjust them and or end up with as close as possible the same setting as the front.

No where is this mentioned in the manual . The tech guy told me that maybe it's time for a Manual re-write. This adjustment only applies to the BG Mighty Demond . It does not apply to other BG Vacuum Secondary Carbs.

Don't ask me why....???? It's there sand box...I'm just playing in it.

Mark so was the manual I was reading from correct that both are equal.

Did he go into the timing thing with you ,or does he think getting them equal will do the trick and wake up the rear set screws to act as lively as the fronts?

 

[IH2LOSE]

PS mark I am at the car now tring to knock down the idle

Now at 900

With the TV cable set the way I had it with the car in park it actually loads the motor as if its in gear thats why there is no change in the idle from park to gear, Now the motor is unloaded in park and netreaul and see if it still shifts correct

 

[Viet Nam Vett]

Mark so was the manual I was reading from correct that both are equal.

Did he go into the timing thing with you ,or does he think getting them equal will do the trick and wake up the rear set screws to act as lively as the fronts?

Yes ..your manual is for your Vac Sec carb and is correct.

My Carb is set correctly except he wants me to try and retune the secondary jets. He did admit as mentioned above that the manule is not clear and needs a re-write.

The problem of the high idle is.... as you say. He started with the retard the timing and use the "Timed Port" instead of the Full Vac port thing. I just agreed with him and said ok I'll try it. But I'm just going to lower the primary throttle setting by 1/8 of a turn. He did say that was ok and not to go any lower.

He also said that since I have so much timing in at idle..(10*) initial and then Vacuum (Over 20*) is why my motor will not idle down. I didn't want to get into a debate with him. He wante me to run the motor on the timmed port which would give me no vacuum at idle..!! Hence a hot motor..!! When I tried to explain that I would heat the motor with no Vacuum..he said I was wrong....Doooooooooohh..!

I'm not retiming my motor or playing with the vacuum advance. It runs great .All I have to do is lose about 100 more rpm.

 

[IH2LOSE]

Yep

They want the timing to set the idle,and timed vacum for advance.

best of luck with it I am still with manifold vacum, keep me posted.

I think when talking with there tecnical support. There job is to have a proper carb regaurdles of how they get there or how it effect the motor.

PS I do have to tell you not every one agrees with the full manifold vacum for advance, But once they see how it works ,they are belivers

 

[JL66REDCPE]

All I have to do is lose about 100 more rpm.

Put an a/c system on it. That will get you the 100 rpm.

 

[Viet Nam Vett]

Yep

They want the timing to set the idle,and timed vacum for advance.

best of luck with it I am still with manifold vacum, keep me posted.

I think when talking with there tecnical support. There job is to have a proper carb regaurdles of how they get there or how it effect the motor.

PS I do have to tell you not every one agrees with the full manifold vacum for advance, But once they see how it works ,they are belivers

Well that's where I'm staying...Full Suck....:L on the diaphram. Even thr GM engineers from what I understand went with timmed Vacuum on the 427 Tri Power Motors back in the day. But I think that was for emmision control.

I'm sure John Z could straighten us out on that one.

 

[Viet Nam Vett]

Put an a/c system on it. That will get you the 100 rpm.

I figure I could pull your Big Block with a tow line...that'll be good for at least 300 rpm...:L

 

[JL66REDCPE]

I figure I could pull your Big Block with a tow line...that'll be good for at least 300 rpm...:L

Yep -- that would work too.

 

[Kid_Again]

Man you gotta lot of room to talk with your Holly Fuel Gusher....That thing is an Kam-A-Kazi's dream... Just turn up the fuel pressure and drive into the nearest target....

....just like a Phantom from the old days, follow the trail of black smoke

...god, now I don't cry in the garage (unless I'm having an emotional day :cry ) anymore...i HAVE to admit, it was a bit distressing having the carb running THAT rich but it did force me to fix it right away and bring the BIG garage fan from home and swap out the little guy i had at OCNJ


NOW i have to figure out how to route the flex fuel line from the pump to the regulator to the blower box without having you look at it and go

 

[JohnZ]

Well that's where I'm staying...Full Suck....:L on the diaphram. Even thr GM engineers from what I understand went with timmed Vacuum on the 427 Tri Power Motors back in the day. But I think that was for emmision control.

I'm sure John Z could straighten us out on that one.

That's correct - it was done for reduced emissions, and resulted in customer complaints about overheating; Chevrolet then issued a TSB instructing the dealers to connect the vacuum advance line to full manifold vacuum on all BB applications that used "ported" vacuum as-built.

As I pointed out in my "Timing 101" paper, almost nobody these days understands the concepts and reasons for distributor centrifugal and vacuum advance; BG techs suggesting going back to "ported" vacuum to cure their carb's limited calibration range just adds them to that list. :eyerole

 

[JL66REDCPE]

That's correct - it was done for reduced emissions, and resulted in customer complaints about overheating; Chevrolet then issued a TSB instructing the dealers to connect the vacuum advance line to full manifold vacuum on all BB applications that used "ported" vacuum as-built.

John -- To what port/device did Chevrolet instruct the dealers to connect the vacuum advance line to in order to get manifold vacuum on a 427 ported engine.