Bg Carb Issues
Thread Starter
Le Mans Master
Joined: Jan 2001
Posts: 6,938
Likes: 1
From: San Diego CA
Bg Carb Issues
I "dialed" in the BG Carb this weekend. Not happy with the results. it ran better out of the box.
Set up the butterflies as per instruction at .020. Adjusted the Idle-Exe as 1.5 turn from bottomed out as per instruction. Warmed up the car and adjusted the A/F screws all evenly as per instruction.
Thsi measure with vacuum guage measuring manifold vacuum and digital tach.
(A/F as measured from # of turns from seated/ closed)
Factory setup- LF-3/4, LR-6/8, RF-3/4, RR-3/4, Idle Exe max open. Resulted in 1020RPM with 15" vacuum
1st try- LF-1, LR-1, RF-1, RR-1, Idle Exe 1.5 open. Resulted in 850RPM with 13" vacuum
2nd try- LF-1 1/4, LR-1 1/4, RF-1 1/4, RR-1 1/4, Idle Exe 1.5 open. Resulted in 750RPM with 11" vacuum
3rd try- LF-1/2, LR-1/2, RF-1/2, RR-1/2, Idle Exe 1.5 open. Resulted in 1120RPM with 16.5" vacuum
Took it for a spin and it was too lean. Went back to factory at 3/4 turn from closed with Idle-Exe completely closed and it still is idling at 1100RPM seems that the Idle-Exe barely changes any RPM like maybe by 20RPM from full open to full closed.
Set up the butterflies as per instruction at .020. Adjusted the Idle-Exe as 1.5 turn from bottomed out as per instruction. Warmed up the car and adjusted the A/F screws all evenly as per instruction.
Thsi measure with vacuum guage measuring manifold vacuum and digital tach.
(A/F as measured from # of turns from seated/ closed)
Factory setup- LF-3/4, LR-6/8, RF-3/4, RR-3/4, Idle Exe max open. Resulted in 1020RPM with 15" vacuum
1st try- LF-1, LR-1, RF-1, RR-1, Idle Exe 1.5 open. Resulted in 850RPM with 13" vacuum
2nd try- LF-1 1/4, LR-1 1/4, RF-1 1/4, RR-1 1/4, Idle Exe 1.5 open. Resulted in 750RPM with 11" vacuum
3rd try- LF-1/2, LR-1/2, RF-1/2, RR-1/2, Idle Exe 1.5 open. Resulted in 1120RPM with 16.5" vacuum
Took it for a spin and it was too lean. Went back to factory at 3/4 turn from closed with Idle-Exe completely closed and it still is idling at 1100RPM seems that the Idle-Exe barely changes any RPM like maybe by 20RPM from full open to full closed.
Team Owner
Joined: Feb 2003
Posts: 30,183
Likes: 313
From: Lake Norman NC
I have heard nothing but tuning problems with Demons unless they are sent to Lars. My dad had a Speed Demon on his 74, and had the same problems as you did Cali. He now runs a Holley Street Avenger with no problems. As for the Mighty Demon on my car..runs great, plugs look good, and no hesitation, but it was sent to Lars. Bottom line, I would wait for lars to get back in town and send him your carb. 
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
what is your float setting?? How about this for a guess..it almost sounds like your 2nd try was where you should have been but your float is giving too much gas and making it run rich??? and what you did in try #3 was to lean it out to get highest vacuum but the system isn't balanced... what is your float seting and what would happen if you changed it to reduce fuel flow at the #2 settings???
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
Originally Posted by Cali,68,L-79
Okay just warmed the car up, front float is a **** hair over the half line and the rear float is between the middle and top line of the sightglass.
Please advise.
Please advise.
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
idle ease should be out 1 1/2 turns from bottom is it???? and what does the vacuum do when you go from the closed idle ease to the 1 1/2 turns out?
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Race Director
Joined: May 2004
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I had a similar type problem a while back.
Nothing I did would make it work right, untill....
Out of desperation I took the idle air bleeds out and drilled them bigger.
This made it worse!
I then cut tiny pieces of wire, shoved it into the drilled bleeds
and put this contraption back. The gasket keeps the wires in place.
It works GREAT now.....and I keep forgetting to order new idle air bleeds.
Will do tomorrow....
You'll be surprised at the amount of air that goes through the idle circuit, which decreases the booster pressure and screws up the fuel flow.
There should be some people who can explain that WAY better than I can.
The idle circuit is a great thing, but a bad thing!!
Nothing I did would make it work right, untill....
Out of desperation I took the idle air bleeds out and drilled them bigger.
This made it worse!
I then cut tiny pieces of wire, shoved it into the drilled bleeds
and put this contraption back. The gasket keeps the wires in place.
It works GREAT now.....and I keep forgetting to order new idle air bleeds.
Will do tomorrow....
You'll be surprised at the amount of air that goes through the idle circuit, which decreases the booster pressure and screws up the fuel flow.
There should be some people who can explain that WAY better than I can.
The idle circuit is a great thing, but a bad thing!!
Thread Starter
Le Mans Master
Joined: Jan 2001
Posts: 6,938
Likes: 1
From: San Diego CA
I will attempt to make the floats even but damn if you don't need a big wrench turn for a little float level adjustment, for example I had to turn that 5/8's wrench 3/4 of a turn to get the 2ndary float from top of the line sightglass to just between the top and middle line.
Guys the BG carbs have three lines, bottom, middle, top. LARS says to put it just under the middle. I'll adjust tonight.
As far as that airflow thru the Idle-Exe the adjustment only made 40RPM change total from total open to total close. I never noticed any difference in Vacuum when adjusting the Idle-Exe screw.
The venturi boosters do not flow until throttle is applied so it's not them.
Guys the BG carbs have three lines, bottom, middle, top. LARS says to put it just under the middle. I'll adjust tonight.
As far as that airflow thru the Idle-Exe the adjustment only made 40RPM change total from total open to total close. I never noticed any difference in Vacuum when adjusting the Idle-Exe screw.
The venturi boosters do not flow until throttle is applied so it's not them.
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
Dave, i have been thinking about this and i'm not far behind you on installing my speed demon, so this is of an interest to me... so lets do some critical thinking on what you have told us.... and yes you can be critical of my thinking skills
A. butterflies at .020 both primary and secondary so idle will be above 1000rpm (as opposed to primary at .020 and seconday just visible for idle under 1000 rpm)..So we know we are going to have a high idle over 1000 rpm.
B. we know the floats were out of balance allowing a high float level allowing more gas into idle equation - more gas, more rpm in the idle circuit..
C. full idle ease adjustment only accounts for 20 -40 rpm?????
i wonder if floats out of balance effects this?????
D. in all of your trials turning the screw out ( increasing the idle mixture) decreased RPM (while making system run richer)
E. " 3rd try ...1/2 turn settings (very lean) idle ease 1.5, 1120 rpm @16.5.... your comments... runs to lean on test drive"
So... why not set float levels at middle setting, idle ease 1.5, and try and get the highest vacuum you can at 1,000 rpm. (based on the .020 transfer slot settings) perhaps try TUNING TO DESIRED RPM not the highest vacuum reading. what if on your previous 3rd try you had made the mixture richer, the idle would have come down some (just like in the rest of your tests , presumably vacuum also) ...when you went to test drive it would of had a richer mixture and might not have been too lean? let me know what you think? bob
A. butterflies at .020 both primary and secondary so idle will be above 1000rpm (as opposed to primary at .020 and seconday just visible for idle under 1000 rpm)..So we know we are going to have a high idle over 1000 rpm.
B. we know the floats were out of balance allowing a high float level allowing more gas into idle equation - more gas, more rpm in the idle circuit..
C. full idle ease adjustment only accounts for 20 -40 rpm?????
i wonder if floats out of balance effects this?????
D. in all of your trials turning the screw out ( increasing the idle mixture) decreased RPM (while making system run richer)
E. " 3rd try ...1/2 turn settings (very lean) idle ease 1.5, 1120 rpm @16.5.... your comments... runs to lean on test drive"
So... why not set float levels at middle setting, idle ease 1.5, and try and get the highest vacuum you can at 1,000 rpm. (based on the .020 transfer slot settings) perhaps try TUNING TO DESIRED RPM not the highest vacuum reading. what if on your previous 3rd try you had made the mixture richer, the idle would have come down some (just like in the rest of your tests , presumably vacuum also) ...when you went to test drive it would of had a richer mixture and might not have been too lean? let me know what you think? bob
Thread Starter
Le Mans Master
Joined: Jan 2001
Posts: 6,938
Likes: 1
From: San Diego CA
Originally Posted by bobs77vet
A. butterflies at .020 both primary and secondary so idle will be above 1000rpm (as opposed to primary at .020 and seconday just visible for idle under 1000 rpm)..So we know we are going to have a high idle over 1000 rpm.
Well maybe I can eyeball it from above and turn the 2ndaries down.
B. we know the floats were out of balance allowing a high float level allowing more gas into idle equation - more gas, more rpm in the idle circuit..
So... why not set float levels at middle setting, idle ease 1.5, and try and get the highest vacuum you can at 1,000 rpm. (based on the .020 transfer slot settings) perhaps try TUNING TO DESIRED RPM not the highest vacuum reading. what if on your previous 3rd try you had made the mixture richer, the idle would have come down some (just like in the rest of your tests , presumably vacuum also) ...when you went to test drive it would of had a richer mixture and might not have been too lean? let me know what you think? bob
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
Originally Posted by Cali,68,L-79
Although I hate to adjust it, I mean I just took it off yesterday and have to pull it again today Well maybe I can eyeball it from above and turn the 2ndaries down.
Well maybe I can eyeball it from above and turn the 2ndaries down. just out of curiosity why did the factory setting seem off to you?
Thread Starter
Le Mans Master
Joined: Jan 2001
Posts: 6,938
Likes: 1
From: San Diego CA
I'd perfer an idle of 800-850RPM. I kept both buterflies at .020 becuase LARS said to but that's before they made the idle-exe. I can eyeball it form on top.
LARS
3 A/F screws were 3/4 turn out but the left rear was almost 1 turn. Now all are back to 3/4 out.
LARS
On your questions:
Yes, you always want to set your idle speed evenly between the primary and secondary idle speed screws after you've established the .020" transition slot exposure - keep them balanced and even.
BG claims that all heir carbs are tested. Alhough they make a good product, I have my doubts as to their testing program: About 50% of the BG carbs I work on right out of the box have severe leaks, imbalanced settings, and the mixtures are WAY off. One of our Forum members in the DC area brought me out a brand new BG carb for setup, and it had a broken float bowl screw seal that made the carb POUR fuel out of the primary bowl area. There's no way the carb ever could have been wet tested without this being detected. So I don't trust the factory settings at all - they're not even close.
Use the Idle-Eze screw if you find that you need to adust the idle speed screws up more than 1/4 turn open from the .020" exposure setting. If you need more idle speed, set the screws back to the .020" setting and adjust idle speed with the Idle-Eze. If you expose more than .020" of the transition slot, you will not be running on the idle circuit, and you will end up with an off-idle stumble.
I have found that idle mixture screws need to be in the 3/4 to 1/2 turn range on the BG carbs. Keep them all evenly set as outlined in my paper. Thay are sensitive as you get them into their range, so adjust them in 1/8 turn increments as you get close.
If you're running a stock exhaust system with cast iron manifolds, you can gain better performance by dropping the jet sizes by one or two sizes at all 4 corners. If you have headers with a free flowing muffler system, the stock jetting works pretty well.
Yes, you always want to set your idle speed evenly between the primary and secondary idle speed screws after you've established the .020" transition slot exposure - keep them balanced and even.
BG claims that all heir carbs are tested. Alhough they make a good product, I have my doubts as to their testing program: About 50% of the BG carbs I work on right out of the box have severe leaks, imbalanced settings, and the mixtures are WAY off. One of our Forum members in the DC area brought me out a brand new BG carb for setup, and it had a broken float bowl screw seal that made the carb POUR fuel out of the primary bowl area. There's no way the carb ever could have been wet tested without this being detected. So I don't trust the factory settings at all - they're not even close.
Use the Idle-Eze screw if you find that you need to adust the idle speed screws up more than 1/4 turn open from the .020" exposure setting. If you need more idle speed, set the screws back to the .020" setting and adjust idle speed with the Idle-Eze. If you expose more than .020" of the transition slot, you will not be running on the idle circuit, and you will end up with an off-idle stumble.
I have found that idle mixture screws need to be in the 3/4 to 1/2 turn range on the BG carbs. Keep them all evenly set as outlined in my paper. Thay are sensitive as you get them into their range, so adjust them in 1/8 turn increments as you get close.
If you're running a stock exhaust system with cast iron manifolds, you can gain better performance by dropping the jet sizes by one or two sizes at all 4 corners. If you have headers with a free flowing muffler system, the stock jetting works pretty well.
Last edited by Cali,68,L-79; Oct 18, 2004 at 08:12 PM.
Instructor
Joined: Jul 2004
Posts: 245
Likes: 0
Cali...
Check out this web link for a very good Demon tuning information. It is what I used for setting up my Mighty Demon.
http://www.4secondsflat.com/demon101.html
Jeff
Check out this web link for a very good Demon tuning information. It is what I used for setting up my Mighty Demon.
http://www.4secondsflat.com/demon101.html
Jeff
Thread Starter
Le Mans Master
Joined: Jan 2001
Posts: 6,938
Likes: 1
From: San Diego CA
Update. Had 1050RPM with 16" vac.
Adjusted the floats to 1/2 on sigtglass. Idle went down 50 RPM.
Idle -Exe 1.5 open
A/F screws 14/16, almost 1 full turn open
idle went to 953RPM at 15" vac.
Raining cats and dogs so test drive has to wait till tommorow but it sounds much better. I think I'm real close.
Adjusted the floats to 1/2 on sigtglass. Idle went down 50 RPM.
Idle -Exe 1.5 open
A/F screws 14/16, almost 1 full turn open
idle went to 953RPM at 15" vac.
Raining cats and dogs so test drive has to wait till tommorow but it sounds much better. I think I'm real close.
Instructor
Joined: Jan 2004
Posts: 246
Likes: 0
From: Marshalltown Iowa
Originally Posted by 406shark
Cali...
Check out this web link for a very good Demon tuning information. It is what I used for setting up my Mighty Demon.
http://www.4secondsflat.com/demon101.html
Jeff
Check out this web link for a very good Demon tuning information. It is what I used for setting up my Mighty Demon.
http://www.4secondsflat.com/demon101.html
Jeff
Call this guy... He once told me that the tough BG tech support issues were sometimes sent his way. Also, he knew about the Idle-eze last fall well before the info was released to the public. I feel he is a true insider and knows this product inside and out.