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 Where's this dirt coming from? MC-2150 
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Joined: Fri Feb 08, 2013 2:37 pm
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Unintended Consequences 101: (...or......I hope this makes sense.....)

Examine the hose routing schematic in the post above. The Solenoid Valve Bowl Vent is connected to the Vapor Separator and the VS has or is a Tee with connection to the PCV and the Charcoal Can. The PCV purges the vapor from the CC and the accumulated liquid from the VS. The PCV vacuum is applied to the SVBV through the VS Tee. Consequently, If the SVBV leaks (as yours did) the PCV vacuum reduces the pressure in the float bowl and reduces fuel flow, leaning the A/F.

The Solenoid Valve is supposed to be open when the key is off and open when the key is on. Engine running, valve closed. Engine off, valve open so vapor can flow to charcoal can.

The SV closed when key is on makes it so the air filter is the only source of float bowl pressure when the engine is running, so the carb bowl vent pressure is the same as the static pressure inside the air cleaner, which is the pressure acting on the air bleeds inside the carb choke horn and throat. This provides for consistent metering when conditions change, such as the filter becomes restricted.

You are fortunate to have a carb with this type of auxiliary bowl vent because you can use it to intentionally reduce bowl pressure to control the A/F. Some aircraft carbs use variations of this principle, called “Back Suction” to reduce the fuel flow with increasing altitude to maintain a constant A/F. If you read NACA TR 49 you may already understand this.

To give active control of the A/F while you are driving, connect the PCV to the bowl vent with a valve between the PCV and the vent to provide control of the back suction. Use a WBO2, such as an Innovate MTX-L, to watch the effect of changing the valve opening. You may need to restrict the internal vents some to achieve the desired effect. Perhaps block one. I have done this with both needle valves and ball valves, and they both have advantages. Obviously, the ball valve is quicker to adjust.

This is also a handy way to increase fuel delivery in a draw through carb installation when used in conjunction with a manifold pressure referenced fuel pressure regulator, so fuel pressure rises with boost to balance increased pressure in the bowl, like a blow through. One installation of dual AFBs on an 8-71 BBC ended up with fixed orifices in the line from the manifold to the bowls and in the bowl vents. Started with a needle valve but ended up with it in one position and never needing to turn it. In that one the same fixed orifice provided a lean cruising A/F and the correct very rich A/F for high boost. I can't believe how lean a cruise you can run with Roots superchargers because the turbulence and heat vaporizes the fuel. Still wonder if it's better to be lucky or good.


Back_Suction_A_F_Control_about633.html ... <<--- ... A post with some description.

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Mon Feb 02, 2015 5:24 am
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Joined: Sun Jun 23, 2013 1:41 pm
Posts: 366
Location: Phila. Penn.
Tuner, It makes sense and is very interesting. The waggy actually is a little lean at WOT. At least that's what I think because if I backpedal to 7/8 throttle it almost always picks up a little. Not that this beast goes to WOT very often... Once its all running back to normal, the first suspect is restriction by the replacement 2 stage power valves. But what you are suggesting is an interesting option.

The vapor seperator conection to manifold vac is actually very restricted. The port to the PCV line has just a pin hole into the main can itself. My guess is that when parked, relatively warm fuel vapor travels from the bowl to the cooler separator can and condenses out. Although I've not found a definative illustration or photo, it looks like the with the tiny bleed is oriented up, so as to draw only vapor when running. I need to look through some earlier FSMs, as the 1984-87 one simply neglects to cover the evap recovery system.

I'd like to hook up my Innovate WBO2 to the waggy. I think for it to be worth anything, I need to put a bung into the pipe before the cat. Also might need to keep the air injector feed to the heads shut or it will be diluting the exhaust. There's another air line to the cat itself that IIRC runs all the time so the cat would be safe. Reality is that once the wagoneer is running reliably, I'll go back to working on the barracuda. My distributor and carb projects are awaiting. :D


Mon Feb 02, 2015 10:47 am
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