Vacuum Secondary vs Mechanical Secondary

[Z Man]

Thanks guys! My issues are a pig rich idle and off idle. I do not have a BJ/BK, etc pair. Just 2 equal 1850s. I have machined the choke housing off, progressive linkage is correct, float levels are right, main jets have been swapped between 60 down to 52. Power valves have been swapped between 4.5 & 6.5. Every combination and the plugs are black in 5 minutes. I am relatively new to carb tuning and learning more than I ever thought I would need to! I love it, but it's getting frustrating.

[Gaz64]

Two 600 vac secondaries are fine, twin 390s would be a little small.

Your idle feed restrictions need to be smaller, but first thing to check is the idle air bleeds are clean of any foreign debris.

A blown power valve diaphragm will also give you this condition.

Gary

[DanEC]

You should be able to tune the idle circuit with the mixture screws provided your cam isn't too big and you have a reasonable idle vacuum reading. Are you tuning the idle mixture on both carbs together to maximize idle speed - it's a rather tedious back and forth process?

I start by closing the front throttle blades until the cam is just barely making contact with the idle screw. Then open both carb idle settings about 1-1/2 turns equally and start the car to make sure it has a reasonable idle speed. If it needs to increase or decrease, then make adjustments equally on the idle speed screws for both carbs. Then I use a tach and a vacuum gage to start tuning the mixture screws on one carb at a time to maximize the readings - go to the other carb and repeat - go back to the first carb and readjust - then to the second carb again. There is going to be a sort of dead zone in the mixture settings that don't seem to make any difference in the idle speed or vacuum readings and I tend to set final adjustments towards the lower side of that zone to attempt to be slightly lean - but I'm not sure it really works that way or makes any difference. The system just may not allow such fine adjustments. After you have the idle mixtures optimized, turn both idle speed screws equally to adjust for the curb idle speed that you want. As long as your idle mixture circuits respond - roll off in rpm on the rich side, and roll off in rpm on the lean side - then all should be good with that.

However, the moment you crack the throttle the transition circuit comes into play (not the accelerator pump circuit) and it is pre-set in Holleys by non-adjustable air bleeds. And if you have a big cam and a high idle speed for it, the transition circuits may even be contributing at idle and if so, other than lowering idle speed, there isn't much you can do about that without extensive carb mods. These transition circuit bleesds are set as a compromise by Holley to work with just about anything that you might bolt that carb on from a 350 up to a 482 stroked FE. That is what is probably rich on your car. There are means of modifying the bleeds to lean out the transition circuit - you can do some Googling. They run from inserting pieces of wire in them, to drilling and tapping for replacement bleeds, to relocating the bleeds. Or some of the premium Holley knock-offs like Quick Fuel had replaceable bleeds - but Holley has bought most of them out.

But no matter how much you work on one of these, a carb set up is never (well, that may be too strong) going to idle as cleanly as a modern fuel injection system.

[moore_rb]

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.

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...

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!!

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.