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Content tagged 'lean'

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  1. Someone mentioned the HACman system for leaning the carbs for altitude.  It seemed like a cool idea.  Digging deeper I learn that there once was an automatic leaning setup, but it is no longer offered.
    Digging even deeper, the explanations seem to be a little obscure, perhaps so we'll want to buy something we could easily cobble together on our own.  I hate to ruin the Hacman's day, but c'mon!  My understanding is that one takes the low pressure from the (initially plugged) tap into the Bing carb's throat, mixes it with another pressure source for some reason taken behind the air cleaner screen, and uses this "mixed" pressure to influence the flow through the jets of the carb via the float bowl.  Basically, you're messing with the float-bowl vent pressure.  Lower float-bowl pressure means less fuel flow through the jets.   Elegant and brilliant, no?  Well it seems to me that the float-bowl by default vents to the local pressure inside the cowling, whatever that is, so any pressure drop thru the air filter element is irrelevant.  Plus, if we only want a leaner mix, couldn't we just admit a teensy amount of air into the carburetor throat?  Take out the throat plugs, install hose barbed fittings, connect the two tygon tubes into a Y-fitting, then route the one tube to your panel, and install a brass needle valve from the hardware store, the other side exposed to the cockpit pressure.  Closed would be full rich, and then at altitude open the valve to admit some air directly into the carb throats to lean the mix to get the correct EGT.  Here, you're not even messing with float-bowl pressure.
    Now I've done no calculations, so it's possible that you wouldn't be able to lean the mix enough with this simple scheme.  In that case, we just use two needle valves, one of which connects to the carb throats as before,  and a second, which connects to engine compartment pressure.  We connect the two pressure sources together with another Y-fitting, the third leg of which then goes back to connect to the float-bowl vents (another Y required).  Now we're doing the HACman thing on the cheap.  We set the needle valve connected to engine compartment pressure to some nominal opening.  We will immobilize it after initial tweaking, maybe mounting it behind the panel where we don't normally even think about it.  We leave the valve connected to the carb throats mounted on the panel.  The panel-mounted needle valve is closed for full rich, and again is opened a bit if we want leaning, making the pressure applied to the float bowls something between engine compartment pressure and carb-throat pressure, depending on by how much we open the needle valve.  We would only have to play with the cabin-pressure needle valve in the beginning, in order to set an appropriate sensitivity for the panel-mounted valve.  Clearly it's not set right if full leaning at 10kft requires only 1/8 of a turn, or 5 turns.  In this case we reach behind the panel and adjust the other valve.  Again, EGT is our guide.