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Old 06-03-2011, 02:46 PM
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muscletruck7379 muscletruck7379 is offline
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lean decelleration with a carb

ok, so going off the idea that when under decel in a carb vehicle the vacuum shoots above what it normally is. even though the throttle blades are closed this vacuum still pulls on the fuel in the carb, a little through the idle circuit and a little through the boosters. ever shut off the key and turn it back on? cool flames huh?

a popular mod with the guys on mpg based forums is an injector kill switch to kill the fuel and take your time slowing down.

with a carb you cannot kill the fuel, but you can kill the ignition. what I am thinking is using something like a sprinkler valve hooked up to a larger vacuum port (pcv or power brakes) running off a switch that will kill the ignition and open the valve, basically causing the carb to see no signal, its not firing so its not running lean and without the high vacuum it will give the vehicle a lot less compression braking. could also be very useful on long downhill runs that you wouldn't give it any throttle anyways (common where i'm from).

the only thing that i can think would be a problem is it getting a rush of fuel before it gets spark, and making it backfire (like the key trick that caused the muffler to blow apart) but by adding a capacitor in the circuit to the valve, it would hold it open just a little bit longer. allowing it to start smoothly up. all run off a momentary switch (like on the shift ****)

any little bit of mileage can help these days!

any thoughts?
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1984 mercury capri, 5.0 conversion, my little monster...
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Old 06-06-2011, 08:47 AM
jimandmandy jimandmandy is offline
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Some import carbs and maybe a few domestics had an electric pilot jet that allowed complete idle fuel cutoff. With the throttle plate closed, virtually no fuel should be coming out of the other jets, so thats not much of an issue. I dont see any advantage in bothering to cut the ignition. The main problem is how to give it a signal that you are coming to a stop and not kill the engine, manually?
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Old 06-09-2011, 01:27 PM
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sounds like your talking about a feedback carb with the mixture solenoid, or the variable venturi? I was thinking about trying this out on my highboy with an eldebrock 4bbl and I don't think that either of those carbs were available over 350 cfm or so. you may be right about not needing to kill the ignition though, which would simplify wiring
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1984 full size bronco, -DIS, wide band megasquirted 300 and np435 w/ dual range aux tranny on the way! - finally updated! 12/29/2011
1988 f150 extended cab 4x4, mostly stock.
1984 mercury capri, 5.0 conversion, my little monster...
1961 ford falcon, all stock, in family since '68
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Old 06-10-2011, 08:47 AM
jimandmandy jimandmandy is offline
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No, its not any fancy feedback or variable venturi. I know what you are talking about with the variable idle jet. Our Dodge as that setup. The one that came to mind is the simple Solex 1-bbl used on old VW's. These had the electric cutoff idle jets was to prevent run-on or dieseling upon shutdown. Since Edelbrock are based on Carter designs, maybe you can find a Carter application that had such a feature.

The other possibility is a variable idle jet from a Carter like our 1986 Dodge. Wire it up on/off with a switch instead of feeding it a computer signal. Get fancier with a dashboard mixture gauge, run from an O2 sensor, and do the variable mixture from a manual control dial, leaning out when not under load to really increase gas mileage. No, this doesnt burn valves and all this nasty OWT's about running lean. Thats hogwash. Aircraft engines from WWII to the present run lean, and cool, when not called upon to produce full power. After all, you are only messing with the idle circuit, not the main and power jets, that are both vacuum controlled on your Edelbrock.
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Old 06-10-2011, 08:47 AM
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