Quote:
Originally Posted by Z Man
Thanks guys! My issues are a pig rich idle and off idle. have been swapped between 60 down to 52. Power valves have been swapped between 4.5 & 6.5.
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How is your throttle response immediately off-idle? Do the RPM's jump up crisply? or is there a stumble? Any black puffs of smoke out of the pipes when you rap the throttle? If so, then your accelerator pumps (one per carb) might be too rich, either via the duration cam (which makes the pump shot last longer or shorter) or via the squirter size (which changes the overall volume of fuel being delivered during the pump shot)
If the throttle response is good, and there's no black smoke when you hit it, then the accelerator pumps are probably ok, and your black plugs are probably either
A) Too much fuel from the idle mixture screws,
B) Too little spark advance at idle (the flame is still burning when the exhaust valve starts to open, leaving unburned residue in the combustion chamber), or
C) The spark plug heat range is too cold.
If you are confident that your overall tune is good (idle mixture is set at the point where maximum idle vacuum is created, and initial timing is set where you want it) then try going up one heat range on your plugs, and see if that helps to keep them cleaner...
DanEC's post above offers some excellent pointers on tuning in the idle on a dual carb setup...
Quote:
Originally Posted by Z Man
Holley offers 2 smaller carbs, 390 CFM with vacuum secondaries and 450 CFM with mechanical secondaries. I have zero experience with the mechanical secondaries. Any thoughts? Thanks!!
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My thoughts about this are: Stay away from mechanical secondaries in a dual carb setup. Actually, I agree with Patrick above - Stay away from mechanical secondaries
altogether, unless your car is primarily a drag racer.
When the day comes that the double pumper that came with my car finally craps out, I'm gonna have a well tuned vacuum secondary carb on there faster than you say "less is sometimes more"
From a performance standpoint, my personal opinion is that two carbs are overkill to begin with, unless your car revs to 15 grand. But, I understand the aesthetic, and the originality aspect of having a dual carb setup on a Cobra...
Now, some of the old-school popular wisdom I've read and heard several times, suggests that twin carbs actually flow at about 85% of their combined max CFM rating, so a pair of 390s = 780cfm, x .85 =~ 663 CFM in the real world, which would flow adequately to feed a well tuned 427 inch engine up to about 5300rpm... which might be a little light.