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Old 08-14-2007, 05:29 AM   #1
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On-Demand lean burn with carb

I was thinking, I have a carbed vehicle. Would it work to have an adjustment knob that went to my mixture screw on the carb, and then an Air/Fuel ratio gauge in the exhaust and manually adjust for lean burn on the fly?

Any comments on that?
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Old 08-14-2007, 07:47 AM   #2
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No. Carbs usually have holes drilled in to control the AF mix, or they use needle valves. The set screws you can adjust from the outside only affect the idle.

The only carb I'm aware of that would allow you to even attempt this is the Holley, and then only with extra parts. You can order an external adjustment plate that replaces the drilled hole plate with one that has a set screw controlled needle valve that will let you rejet the carb without tearing it down. You would have to work out a way to adjust from inside the car from there.

I am by no means a carb authority, there may be a carb out there that you can set up to run like this. If you want this kind of tunability, I'd suggest replacing the carb with a throttle body injection setup. You can get one sized to your engine with an O2 sensor, and if you get one aftermarket you can control the fuel/air mix through programming. You could also play games with the O2 sensor, perhaps connecting the wire directly to ground through a knob-style potentiometer on the dash. You'd want to have one that would let you control the voltage so you can keep it at a point between 0 and 1 volts, with enough knob that you can adjust within 1/100 of a volt. The computer reads the voltage to determine the A/F mix, so with a separate A/F reader (conveniently plugged into the O2 port on the exhaust you'd have to add) you would be able to directly manage the fuel/air mix.

Only bad thing, is forgetting you are super lean when you hit that long hill, and melt the engine down.

Hope this helps.
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Old 08-14-2007, 07:49 AM   #3
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That would only adjust the idle mixture though. Another thought would be to apply an adjustable amount of vacuum to the float bowl chamber through the vent. A little more tricky, but it would be effective throuout the carbs operation range.
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Old 08-14-2007, 06:05 PM   #4
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I guess I have quite a bit to learn about carb's. If I have this correct, the jets are what determine how much fuel gets into the engine in proportion with the air coming in. So when I am messing with that mixture screw it is only idle I am messing with which coorelates to not doing anything as I idle less than a minute during my whole commute.
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Old 08-14-2007, 09:07 PM   #5
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Quote:
Originally Posted by beatr911 View Post
That would only adjust the idle mixture though. Another thought would be to apply an adjustable amount of vacuum to the float bowl chamber through the vent. A little more tricky, but it would be effective throuout the carbs operation range.
From his post I think beatr911 knows about this.

If you are serious this is the way to do it. This is called back suction mixture control. Stomberg used it on many small aircraft carbs. It is the principle Bing or Rotax used on altitude compensating carbs. I and another guy modified this for a manual mixture control for ultarlight engines. Another guy is marketing it.

http://www.greenskyadventures.com/bing/HACmanorder.htm

The principle is to vent the carb float bowl through an orifice and TEE into that line between the carb and the orifice and feed partial vacuum from near the venturi through a needle valve. This may require drilling and tapping in the carb. You don't need the $159 kit from Greensky. A $5 valve from ACE hardware worked fine. Yes I flew with this with no trouble.

I know I am not giving quite enough information but only the principle. I need to turn in though.
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