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CC Advertisers
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11-14-2006, 07:28 AM
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CC Member
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Join Date: Apr 2006
Location: Cincinnati,
OH
Cobra Make, Engine: 1965 GT350, 1967 GT500
Posts: 82
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Not Ranked
Dual Quad dilemma...too much carb?
Guys,
Does anyone have a suggestion regarding the use of an original BJ-BK 600 CFM dual quad setup for a good, practical driver? I drive a '67 GT500 soon to have a mild 428 w/Edelbrock heads. The stock dual quad set-up is too much carburation for the motor. How can I avoid dumping too much fuel into the motor and causing a rich condition and premature wear etc.?
I guess I could re-jet the carbs?
Any ideas would be welcome.
MC
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11-14-2006, 07:30 AM
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CC Member
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Join Date: Dec 2001
Location: Louisville,
KY
Cobra Make, Engine: I'm Cobra-less!
Posts: 9,417
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Not Ranked
Vacuum secondaries I assume?
I just dyno'd my 428FE with a pair of 725cfm Demons. A/F ratio was great all across the band.
But yes, you could jet down along with some other tricks.
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11-14-2006, 08:01 AM
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CC Member
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Join Date: Feb 2002
Location: Dadeville,
AL
Cobra Make, Engine: Sold my EM.
Posts: 2,459
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Not Ranked
In case you didn't notice this thread: Harm of overcarburetion?
__________________
Tommy
Cheetah tribute completed 2021 (TommysCars.Weebly.com)
Previously owned EM Cobra
"Never attribute to malice that which is adequately explained by stupidity." - Hanlon's Razor
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11-14-2006, 10:05 AM
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CC Member
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Join Date: Jan 2004
Location: Shawbury,
Posts: 325
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Not Ranked
I run dual 600cfm Edelbrocks on my small block. I disconnected the secondaries, not because of overcarburation, but rather because the flapper valves would open too soon and cause a flat spot during accelleration. Anyway I wonder the secondaries open at all with a nominal 1200cfm carburation on a small block.
The plaugs are rather white than black, because of the general lean jetting of Edelbrock carbs.
I think you can't go wrong with dual 600cfm vacuum secondary carbs on a 428.
-Simon |
11-14-2006, 11:48 AM
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CC Member
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Join Date: Jul 2001
Location: Southwest,
WI
Cobra Make, Engine: Shell Valley, Mopar thingy (small block of course)
Posts: 2,215
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Not Ranked
Add a little weight on the conter weights on the flapper valves (drill and peen some lead in) to help with them opening too soon.
__________________
Brent Dolphin
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11-14-2006, 12:03 PM
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CC Member
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Join Date: Dec 2004
Location: Prince Frederick,
MD
Cobra Make, Engine: Unique 427 S/C 427 FE S.O. 484 cu in
Posts: 952
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Not Ranked
Quote:
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Originally Posted by SFM5S159
Guys,
Does anyone have a suggestion regarding the use of an original BJ-BK 600 CFM dual quad setup for a good, practical driver? I drive a '67 GT500 soon to have a mild 428 w/Edelbrock heads. The stock dual quad set-up is too much carburation for the motor. How can I avoid dumping too much fuel into the motor and causing a rich condition and premature wear etc.?
I guess I could re-jet the carbs?
Any ideas would be welcome.
MC
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They came with two fours stock. Why do you think they are two much? |
11-14-2006, 02:30 PM
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CC Member
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Join Date: Apr 2006
Location: Cincinnati,
OH
Cobra Make, Engine: 1965 GT350, 1967 GT500
Posts: 82
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Not Ranked
They did come stock, you are correct, but when I run a calculation the numbers seem to show a smaller amount of carb required.
I.E. 428 x 6000 (rpm) / 3456 = 743 CFM, even if I go to 6500 RPM, we get 800 CFM or so.
The dual quad set-up is cool to look at and talk about, but I think that a well thought out single four would perform better.
In my case for the sake of originality I need to maintain the dual qual set-up.
Mike C.
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11-14-2006, 05:30 PM
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CC Member
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Join Date: Dec 2004
Location: Prince Frederick,
MD
Cobra Make, Engine: Unique 427 S/C 427 FE S.O. 484 cu in
Posts: 952
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Not Ranked
Just put the two fours back on. When I was in High School my daily driver had two 600 Holleys vacuum secondaries on a lowriser 427 with an automatic. No problems.
I have the same set up on my 427 Medium riser. No problems.
It will run fine. They are vacuum secondaries and they don't open unless the engine needs it. Good luck
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11-15-2006, 12:02 AM
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CC Member
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Join Date: Jun 2003
Cobra Make, Engine: ERA 427 w/496 Side Oiler, roller, dual quads
Posts: 417
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Not Ranked
SFM5S159, first of all, I'm not sure what size the Bj/BK carbs are, but I believe they're a bit smaller than 600 cfm; could be wrong, but 535 cfm comes to mind. Doesn't matter, vacuum secondaries only open when the engine can use the extra fuel/air, and only open as much as the engine can use; so even if they're rated at 600 cfm, they won't supply that much cfm on your engine. Run those puppies!
Dan
__________________
Do you know why they call it "PMS"? Because "Mad Cow Disease" was taken. --Unknown, presumed deceased
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11-15-2006, 03:14 PM
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CC Member
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Join Date: Apr 2004
Posts: 1,031
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Not Ranked
Mike,
I just sent a PM. These systems can be adjusted to be very smooth.
Dan
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Dan Case
1964 Cobra owner since 1983, Cobra crazy since I saw my first one in the mid 1960s in Huntsville, AL.
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11-26-2006, 02:19 AM
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CC Member
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Join Date: Mar 2006
Location: Tacoma,
WA
Cobra Make, Engine: West Coast-450 HP 460
Posts: 347
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Not Ranked
Mike,
Your forgetting the efficiency factor in your calculation. Most engines run 80 to 85%, with real good intake, cam profiles, heads and headers you can reach 90 to 95% but thats about it untill you go to forced induction.
Your 743 CFM X .90 = 669 CFM. 743 CFM X .95 = 706 CFM
Go to www.csgnetwork.com and plug in the numbers. It does the calculation for you. Two 500 CFM Edelbrock's, progressive linkage, lots of power with no flat spots.
Ray |
11-26-2006, 07:39 AM
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CC Member
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Join Date: Apr 2005
Location: Tucson,
AZ
Cobra Make, Engine:
Posts: 5,391
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Not Ranked
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11-26-2006, 09:23 AM
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Senior Club Cobra Member
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Join Date: Jul 2002
Cobra Make, Engine:
Posts: 15,712
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Not Ranked
Dual fours on my 427, work good.
As for 'reading plugs' bear in mind a Multiple Spark Discharge type system (typical MSD) tends to make the plugs run a lighter color (even white) regardless of other factors (like a rich mixture). So it can be a bit misleading.
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11-27-2006, 06:21 PM
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CC Member
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Join Date: Jul 2001
Location: Southwest,
WI
Cobra Make, Engine: Shell Valley, Mopar thingy (small block of course)
Posts: 2,215
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Not Ranked
Also, high end motors can and do achieve better than 100% VE. From the old cross rams to modern engines. As a matter of fact, Hughes Engines has a couple dyno tests on street engines on thier web site with better than 100% VE.
__________________
Brent Dolphin
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11-28-2006, 12:33 PM
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CC Member
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Join Date: Feb 2006
Location: Naples,
FL
Cobra Make, Engine:
Posts: 183
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Not Ranked
That "Calculator" is missing some things. A dual plane manifold needs a larger CFM carb than a single plane on the same engine, because only half of the carb can be drawn through by any one cylinder. Whoever wrote that calculator program seems to be unaware of that.
I wouldn't trust it.
Kevin
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