Carb setup question

[Aggressor]

I use a vacuum gauge to set carb idle mixture, watching for maximum vacuum and smoothness. This was done with good results on my Shelby Gt500 180 degree intake. Now I am attempting the same thing with the crossram having a large open plenum beneath each carb. With no direct dedicated porting to the two sides of the engine, is it possible to adjust the idle mixture on one side of the carb too rich and the other side too lean with the same net result averaged as they dump into a single plenum.

There is no vacuum crossover between the two sides of the crossram so each side of the engine is fed by a carb dedicated to that engine side. In essence you have two uneven firing 4 cylinder engines connected by a crank. Individually the carbs have to be balanced to feed evenly into the two large plenums and engine side to side the carbs must be vacuum balanced to each other. ?? If I am correct?

-Geary

[Brent]

Geary that set-up looks fantastic!!!
Kudos on the project.

I would check with someone at Holley or Paxton for a "blow through" suggestion on the idle bleed adjustment.

I normally start them at 1.5 turns out all the way around, let the car get warm, have someone put it in gear, set your idle speed screw, and then turn the bleed screws out 1/4 turn more and see if the idle increases. Making sure I get the best idle with the butterflies as closed as I can get them. Makes sure you never get into the transition circuit.

Had 2 4's on a sheet metal tunnel ram with a HUGE plenum and everything was the same.

Have fun!

[Aggressor]

Thanks Brent,

I composed the thread about 3:00AM and wasn't sure that it would make sense to anyone but me. Just a bit foggy-brained and propped up with caffeine at the time.

Somehow it all worked out today.

Now I'm jetting down the primaries another 6%. It seems like the wrong direction with the blowers but the plugs are sooting up too quick with no sign of a lean condition.

-Geary

[pbrown]

Set the T-slots at .020 (they should look square when looking at the blades). Then with idle mixture screws set at 1-1.5 turns out, adjust them all in or out is equal increments until you get the best idle. This will also be the point of highest vacuum. I would get two identical vacuum gauges and connect one to each carb.

If after all of that you left with your idle screws set at less that one full turn, reduce the size of the idle feed restrictors by .002 increments until you can regain that 1-2 turn range of the screws.

If the screws are out past two turns, reduce the size of the idle air bleeds.

Keep in mind that altering the idle air bleeds will change the fuel curve. Increasing them will cause the mains to come in later. Smaller and the mains will come in too early. A properly tuned carb will have the mains come in at about 2500 RPM.

Changing the main jets will do nothing for the idle since they aren't active at idle.

Get a good wide band AFR meter and you can get it perfect.

[Aggressor]

Thanks Patrick,
You seem to be well studied on Holleys and I appreciate your input. Looking back at my old setup, the Edelbrocks are just so easy to work with compared to the Holleys.

The jetting changes are the result of road testing up on the primaries. Real dry black soot generation with only the very spark tip clean.

With the new setup you have to think about what your doing at all times. I'm still not used to working with a pressurized setup. I tried to check a float level on Tuesday without disconnecting the blower hose and was lucky only to have a relatively small amount of gasoline sprayed around the engine bay. For several moments though, it looked like a whirlybird lawn sprinkler.

-Geary

[pbrown]

I've seen a few. What carbs do you have? Are they a blow through design. Carbs under boost typically work best with annular discharge boosters and smaller emulsion holes.

Many people try to increase the size of the air bleeds right off. I suspect they do that because it's easier to make an air bleed larger than an idle jet smaller. I modify all of my Holley type carbs for screw in air bleeds, e-holes, PVCR, and idle jets. You'll need a set of starting and bottoming taps (6-32, 8-32, and 10-32). I use brass set screws for bleeds and jets. Just drill them to the size you want. You drill out the old jets and tap the hole just far enough so the set screw goes in flush. Doing this make the carb infinitely adjustable.

I like to attack a carb tune starting with the idle, then cruise, then wide open. A dyno will really help but you can do the wide open tests at a track.

And again, a wide band tester would really help. I use an Innovate Motorsparts LM-1. The LM-2 will be shipping soon and has dual sensor capability which would help greatly with your engine config.

[Aggressor]

Patrick,

These are not modern 21st century blow-thru modifieds.The setup I'm using is as 60's vintage as you can get. The carbs I'm using are a pair of classic 1450 style (4776C) double pumpers. The original carbs for the Supersnake were 1850 style. My understanding is that most carbs are easily adaptible to the pressurized setup even without a carb surrounding pressue box. At the Macungee car show in 2000 I talked to "Dyno" Don Nicholson about my project. He introduced me to his 427 Oakdale group and I was told why I could not use my original Edelbrocks for blow-thru. After being convinced that I had to go to Holleys without exception, I walked about 50 feet to look at a Studebaker street rod. Yep McCullock blower with blow-thru AFB's. They just needed solid floats and marine style seals on the accelarator pumps. In some cases you may have to setup a boost referencced throttle plate for sealing the plate shafts but nothing major. I'm still a bit sketchy as to the details of how it works with the boost above the plates, but at idle and easy running I still have very good vacuum. The original car also had vacuum operated secondaries and large dual Girling style power brake setups. I may have lost a bit of bottom-end response but in general the car feels like her old self. I know the FE community doesn't like a vacuum advance on the distributor but I am a believer in using it. I checked with Craig Conley(Owns Paxton) about it and was told there's no reason not to install it.

-Geary