Just to summarise and recap - engine is a 383 SBC, formerly with an Edelbrock Performer 750cfm carb (part no. 1407) and now sporting an Edelbrock Thunder 650cfm carb (part no. 1805).
The Edelbrock Performer and Thunder carbs have mechanical secondaries that open up from roughly 60% throttle opening but they have a flap above them to control when the air starts to flow. On the performer, the door has some weights on it so it opens for a certain air flow (i.e. throttle demand). The secondaries will only be active when the engine needs them. The only way to adjust this is to add or remove weight.
On the Thunder series, the air flap is spring loaded so by altering the tension of the spring, you can tune when the flap opens and the secondaries become active. I've seen the standard setting reported variously between 2 and 2.5 turns of spring tension. I've learned enough so far to never assume anything is set up correctly out of the box so the first thing I did was to unwind the tension and set it myself to 2 turns. At this point, some pictures will probably help.
Here's the carb on the engine with the air filter removed.
Flap closed:
Flap open:
Adjustment - The spring tension adjuster (flat blade screwdriver required) is clamped down with a torx fixing. Loosen the torx screw and you can then adjust the tension. Note that you need to have the flat blade in and hold it firmly before you loosen the torx otherwise the spring will just unwind and you will lose whatever setting the spring was at.
So what effect does adjustment one way or the other have? If you loosen the spring, the flap opens sooner so the secondaries become active quicker. "Great!", I hear you say. "Lets have it on the loosest setting and get more power!"
Sadly this is not the case. Fundamentally, air flow in a carburettor always reacts quicker than the fuel. Changes in fuel flow will always lag behind the air and that won't change. That's why there are things like accelerator pumps to cover the "hole" in fuel supply when you stomp on the throttle pedal. The air rushes in but the fuel flow takes a moment to catch up. The Eddy carb has a primary accelerator pump but no secondary pump (like a Holley double pumper). That's where the air flap comes in.
At the stock setting of 2 turns on the tension spring, if I press the throttle sharply I get a momentary bog or stumble. The engine briefly dies for half a second then picks up. Looking at my AFR gauge, I can see its a lean condition. This is because the engine tries to suck in a load of air/fuel and the spring allows the air flap to open, the air rushes in immediately but the fuel isn't flowing yet to keep the AFR sensible and there is a lean spike, then engine falls flat on its face until the fuel starts flowing, the AFR returns to where it should be, and the engine comes back to life and accelerates happily. If I press the pedal slowly and steadily, the air flap opens slowly, the fuel has time to be pulled to the boosters and the problem goes away, but obviously this is rubbish to drive, having to nanny the throttle pedal all the time.
No amount of fiddling with the primary accelerator pump shot can solve this as it doesn't affect the air/fuel in the secondaries. See my previous posts for pump shot nozzles, actuator linkage adjustment and pump spring upgrades. None of this worked - this is what the adjustable air flap is for. By tightening the spring, the door takes longer to open and while it does so momentarily chokes the secondaries, making the mixture richer. This gives time for the air flow to establish itself and all the clever passages that meter the fuel to start working properly. The lean spike is therefore eliminated, or at least greatly reduced. I like to think of it as the opposite of the accelerator pump. The pump shot gives you more fuel, conversely the air flap reduces the amount of air, but both methods are aiming to stop the mixture from going lean when the secondaries open quickly. The end result should be seamless transition as the secondaries come on. Anyone who says they can feel the secondaries kicking in are probably experiencing some sort of bog or stumble and need to do some tuning work. You should barely notice it.
That's my simplified explanation anyway.
As far as results go, with the stock setting of 2 turns of tension, I was getting spikes up to 18:1 on the AFR gauge before settling down to 12.7:1. Obviously this is not good, but the one crumb of good news is that my mixture is ok at WOT once things have settled down. I started winding on the tension and ended up at 3.5 turns which brought the lean spike down to 15:1. Cruise seems to be anywhere between 13.5:1 and 14:1 depending on rpm so a momentary blip to 15:1 is barely noticeable but is still there. The engine doesn't die but you can still feel it hang momentarily after changing gear.
The only problem I have now noticed is that if you floor it from below 2000rpm in 5th gear, the engine can bog rich. I think this is because the air flow is not enough to overcome the increased spring tension on the air flap and it stays in the "choked" position. The engine wants more air but it can't get it until the rpm rises, the air flow increases and then the door opens. In lower gears this doesn't happen because the revs rise so quickly. On the open road when you're going along at 50 in 5th and want to overtake someone, its extremely annoying because I have to either roll gently into the throttle or change down a gear and this completely ruins the car. Whats the point of having a massive V8 with mountains of torque down low if you have to keep changing gear all the time?! I backed off the air flap tension and this problem goes away, but then the lean spike on flooring it and changing gear returns.
Conclusion and lessons learned
Since doing this investigation and later on moving to a Holley carb, I have learned a few useful things. Part of the reason this Edelbrock carb didn't really work for me was my engine spec and the nature of my car.
I am not saying Edelbrock carbs are bad. What I am saying is they are better suited for a particular application. Both my Performer and Thunder carbs were very good for cruising around once I had got them set up properly. I got good fuel economy at around 18-20mpg and after my tweaks to the idle channel restrictor I had very good low speed manners and a sharp part-throttle response. They would be ideally suited to a heavy car with long gears, or automatic transmission, stock or mild cam and dual plane intake designed to give high torque straight off idle
What the Eddy carbs are perhaps not best suited for is a very light car with short gears, manual transmission, a big cam and single plane intake designed for higher rpm. A solution exists for cars like mine and its called a Holley Double Pumper.
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