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 Back Suction A/F Control 
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Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1744
Back Suction: Reducing pressure in float bowl causes leaner A/F.

This can be done several ways, but the principal is the same in each case. A source of low pressure, venturi vacuum or intake manifold vacuum is connected to the bowl vent to reduce bowl pressure.

The connection requires a T with one leg to the bowl, one leg to the atmosphere (inside the filter) with a restriction, usually near 1/8” to 3/16” works OK, and the third leg with a valve in the hose to the vacuum source, the PCV hose or ported or hot intake vacuum.

The bleed to atmosphere provides normal bowl vent pressure when the valve is closed and the valve controls the “back suction” which leans the A/F.

Opening the valve supplies vacuum to the T, thus reduces pressure between the bleed and the bowl, and thus reduces pressure in the bowl and reduces fuel flow, leaning the A/F.

The valve can be a ball or needle valve, either may work the best, depending on bleed size and engine characteristics. With a little tinkering this will give the driver total control of A/F.


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Tue Oct 28, 2014 1:13 am
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Joined: Mon Dec 29, 2014 1:26 am
Posts: 433
Ever heard of Arduino? --> http://arduino.cc/en/Main/ArduinoBoardUno

Jet the carb to be richer than intended, then let the little open-source computer PWM a solenoid to apply vacuum to the bowl to lean it out to the desired AFR. Someone fluent in C/C++ (i.e. not me) could whip up code to give it an actual fuel map, using multiple sensors to determine what the target AFR should be at any moment.

The Arduino is what I use for my BRPV controller, though mine's only in closed loop once it's in boost and more than a certain amount of throttle opening.

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Mon Feb 02, 2015 7:26 am
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