General theory question on Dual Carb setup - Team Camaro Tech
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post #1 of 7 (permalink) Old Mar 15th, 10, 04:44 PM Thread Starter
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Russ
 
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General theory question on Dual Carb setup

On a dual carb setup, how does each carb distribute their fuel to the cylinders? Is the front carbs fuel going to the front 4 cylinders (1,3,2,4) and the rear carb feeding the back 4 cylinders (5,7,6,8)? I was just curious how that worked. I replaced my spark plugs yesterday, and noticed 1,3,2,4 all looked tan (normal for my car), but the rear 4 all appeared extremely rich. Both carbs are pretty much tuned and jetted identical. I've never seen my plugs look this way before. Would you suspect something on the carbs, or just a coincidence? I'm typing this all on my phone, so sorry for any typos. thanks, Russ

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post #2 of 7 (permalink) Old Mar 15th, 10, 05:00 PM
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Re: General theory question on Dual Carb setup

The style of distribution will depend on the type of manifold and system.
For a dual-carb. blown engine, the chance of one of the carbs causing symetrical cylinder differences is quite different than a common Edelbrock dual-4 set-up.
If there is a resonable common runner distribution then I would suspect something like a vacuum leak at base of one carb, the blower or intake, or a carb that is quite out of tune would be necessary to cause a large variance in plug readings ( and thus cylinder firing differences).

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post #3 of 7 (permalink) Old Mar 15th, 10, 05:25 PM
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Re: General theory question on Dual Carb setup

Quote:
Originally Posted by Blown69RS View Post
On a dual carb setup, how does each carb distribute their fuel to the cylinders? Is the front carbs fuel going to the front 4 cylinders (1,3,2,4) and the rear carb feeding the back 4 cylinders (5,7,6,8)? I was just curious how that worked. I replaced my spark plugs yesterday, and noticed 1,3,2,4 all looked tan (normal for my car), but the rear 4 all appeared extremely rich. Both carbs are pretty much tuned and jetted identical. I've never seen my plugs look this way before. Would you suspect something on the carbs, or just a coincidence? I'm typing this all on my phone, so sorry for any typos. thanks, Russ
Russ..I run a big block with an 8-71 roots/dual 850 dp,s.
I see this with extended idle times on the street.
You would think that 3 lobe rotors want to screw the a/f forward..maybe so at high rpm but mine definetily fattens up the rear cyls idling around.
I just replaced a blower to manifold gskt that showed some fuel saturation at the rear for that reason.
I keep my holleys jetted equally for wot..simply leaned out the rear primary idle mixture screws 1/16th turn in a attempt to correct.
I do believe it is a low speed anomoly with a blower.Raw fuel can rest in cavitys and ledges and richen rear cyls at idle rpm.
Not sure if this is your issue.,.just some food for thought.

George
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post #4 of 7 (permalink) Old Mar 17th, 10, 09:26 AM Thread Starter
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Re: General theory question on Dual Carb setup

thanks for the quick responses John & George. I appreciate your help... I've been running this thing on the street for 5 years now, but still learn something new every day it seems.

George - I know it'll vary with everyone's individual setups, but could you tell me what level of vacuum you have at idle? (with RPM) how 'bout when put into gear (vacuum, RPM) It's a yearly battle at some point in time with mine. With the blower restraint, NOS plates, carbs,,, I'm just begging for vacuum leaks all the time.

I seem to forget every year from fall to spring what the car had ran like the year before. Currently have idle set around 900-1000 with 8-10 vacuum. When in gear (TH350), it's around 550-600 with 4-5 vacuum. Still got some tweaking to do before cruising season,,, always the most stressful time for me. You can't get the cars out and around here until the weather gets nice enough and the roads are cleared of all the sand and crap,,, but then you have only a month or two to have everything running right for the first shows/cruises of the year.

thanks again for you help and input,
Russ

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post #5 of 7 (permalink) Old Mar 17th, 10, 05:10 PM
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Re: General theory question on Dual Carb setup

Quote:
Originally Posted by Blown69RS View Post
thanks for the quick responses John & George. I appreciate your help... I've been running this thing on the street for 5 years now, but still learn something new every day it seems.

George - I know it'll vary with everyone's individual setups, but could you tell me what level of vacuum you have at idle? (with RPM) how 'bout when put into gear (vacuum, RPM) It's a yearly battle at some point in time with mine. With the blower restraint, NOS plates, carbs,,, I'm just begging for vacuum leaks all the time.

I seem to forget every year from fall to spring what the car had ran like the year before. Currently have idle set around 900-1000 with 8-10 vacuum. When in gear (TH350), it's around 550-600 with 4-5 vacuum. Still got some tweaking to do before cruising season,,, always the most stressful time for me. You can't get the cars out and around here until the weather gets nice enough and the roads are cleared of all the sand and crap,,, but then you have only a month or two to have everything running right for the first shows/cruises of the year.

thanks again for you help and input,
Russ
Hey Russ..viewed your car..nice Pro street.
We have different combos..so my idle rpm/Hg is a moot point.
You did ask though.I idle in park at 1000 rpm with some blower roll.Hg hunts around 7-9.In gear with a 2400 stall,850 rpm..limited roll and about 5-7 Hg.
If you are idling in gear at less than 600 rpm..the cam must be mild or you are pig rich compensating for a diluted intake charge with a larger cam.
You have a 400 rpm drop going from park to drive?What convertor are you running?
Anyway..the tune up starts in the distributor.Assuming a static cr of 8/1 and a boost friendly performance blower grind..initial timing should be around 20* btdc and mechanically limited to a total of 32-34* all in by 3000 rpm max.
Shoot for this blower friendly spark curve then move to the carbs.
Like I mentioned prior..I do run the rear carb primary idle a/f a tad leaner than the forward carb with extended idle times.This is atop a big block..whether your 383 responds to this will require some testing on your part.Its a safe test though..a tad lean in the rear on the idle/transition ckt will not affect your wot a/f ratio that should be in the 11.8/1 range with a Roots on 93 pump swill.

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post #6 of 7 (permalink) Old Mar 17th, 10, 11:16 PM Thread Starter
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Re: General theory question on Dual Carb setup

Quote:
Originally Posted by fatblock View Post
If you are idling in gear at less than 600 rpm..the cam must be mild or you are pig rich compensating for a diluted intake charge with a larger cam.
George,,,
It's a fairly mild cam,,, it's this one:

COMP Cams 12-564-4
COMP Cams Nitrous HP Camshafts
Hydraulic flat tappet
2,400-6,500
Intake Duration at 050 inch Lift 230
Exhaust Duration at 050 inch Lift 244
Duration at 050 inch Lift 230 int./244 exh.
Advertised Intake Duration 274
Advertised Exhaust Duration 292
Advertised Duration 274 int./292 exh.
Intake Valve Lift with Factory Rocker Arm Ratio 0.487 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio 0.501 in.
Valve Lift with Factory Rocker Arm Ratio 0.487 int./0.501 exh. lift
Lobe Separation (degrees) 113


Quote:
You have a 400 rpm drop going from park to drive?What convertor are you running?
it's the B&M Holeshot 2400

I tend to play around with things and have them off and on throughout the season. I'm guessing I have a slight vacuum leak somewhere right now. I'll be spraying it down this weekend to try to find it.

On a side note, I'm fairly sure I got some bad gas in the car just the other day also. Once I drained the cell and lines, and put in new stuff, it began to clear up some.

Depending on the mood of the week, and the mood of the car, the idle gets tweaked a few times a year. On average, I'd say I usually idle around 800-900 in park and 600 or so in gear. I'll get it back there sooner or later. I think I've just became hyper sensitive to every little change,,, maybe it's not that big of deal right now.

At one time I used to have quite a bit of "blower surge", up and down 800-1200. It sounded kinda neat, but got to be irritating. That's when I ended up getting it to idle smooth, and also ended up with a "in gear" idle down around 450-500. It seemed awful low, but it really was running good and I left it. I switch back and forth on my favorite sound though, I'll tweak it for a smooth idle for a while, then I'll go back to the intentional surge. I guess what I'm saying is I can't leave anything alone, and end up causing myself grief from time to time.

Did the cam specs and convertor choice stand out and scream any problems to you?

thanks much for all your insight.

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post #7 of 7 (permalink) Old Mar 18th, 10, 04:34 PM
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Re: General theory question on Dual Carb setup

Russ..The nitrous grinds work well with forced induction..
I am not suprised that you dump 400 rpm when shifting into gear with a 2400 Holeshot.
I ran one myself..and the same result.
I am not vendor bashing here..just real world results.That converter is tighter everywhere and not a big step up from a stock stall.
Best suited for stock to mild performance builds with short duration and high manifold vacuum.
Imo..that off the shelf converter is not well matched to your camshaft.
I would take your dyno sheet..be honest with your goals and contact a reputable converter company for suggestions.

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