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Old 10-18-2017, 12:01 AM   #11
msires
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Carbureted engines have an 'accelerator pump'. It is a diaphram that pumps fuel directly into the carb when the throttle is advanced. Its purpose is to eliminate any lag time between advancing the throttle and engine response. Without it, when the throttle is advanced there would be some lag before the increased airflow increased the fuel sucked into the carb venturi. The fuel pump(s) don't pump fuel into the venturi, they pump it into the float bowl, where it is sucked into the venturi through the jets. So if you 'pump' the throttle before starting, you are pumping fuel into the venturi. On a downdraft engine (car), that isn't a big deal since it will just sit there until the engine is cranked an a vacuum forms to suck the gas into the engine. On an updraft engine (airplane), the fuel runs out the carb, into the airfilter, and other places fuel was never supposed to be and sets up ideal conditions for an engine fire.


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Old 10-18-2017, 12:13 AM   #12
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I'm curious about what opening and closing the throttle actually does before starting a carbureted engine.
There is an accelerator circuit in almost all aircraft carburetors, that kicks extra fuel in as you advance the throttle to full. With the engine running, that prevents stumbling.

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Old 10-18-2017, 03:01 PM   #13
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Carbureted engines have an 'accelerator pump'. It is a diaphram that pumps fuel directly into the carb when the throttle is advanced. Its purpose is to eliminate any lag time between advancing the throttle and engine response. Without it, when the throttle is advanced there would be some lag before the increased airflow increased the fuel sucked into the carb venturi. The fuel pump(s) don't pump fuel into the venturi, they pump it into the float bowl, where it is sucked into the venturi through the jets. So if you 'pump' the throttle before starting, you are pumping fuel into the venturi. On a downdraft engine (car), that isn't a big deal since it will just sit there until the engine is cranked an a vacuum forms to suck the gas into the engine. On an updraft engine (airplane), the fuel runs out the carb, into the airfilter, and other places fuel was never supposed to be and sets up ideal conditions for an engine fire.
I quoted the full post because it is worth reading twice. I also bolded the most important statement. Pumping the throttle rather than using the primer has and will continue to cause engine fires.

If you feel the need to pump the throttle, ONLY do it while the prop is spinning (while air is getting sucked up into the carb).

My hot start procedure which I really only use when I fuel up or stop for a quick bio-break, is:

Pull primer plunger out, but do not depress.
Call "Clear!"
Hit the starter.
If she doesn't start in a few turns, she gets the primer squirt.
If she does start, set RPM to 1000, and very slowly depress the primer plunger.

Last edited by Domenick; 10-18-2017 at 03:05 PM.
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Old 10-18-2017, 07:33 PM   #14
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There is an accelerator circuit in almost all aircraft carburetors, that kicks extra fuel in as you advance the throttle to full. With the engine running, that prevents stumbling.

* Orest
So it's a type of rough priming?
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Old 10-18-2017, 09:20 PM   #15
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So it's a type of rough priming?
Kind of, sort of. And that is why it is dangerous to pump the throttle with the engine not running.

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Old 10-18-2017, 11:23 PM   #16
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Default Hot Start Procedure Carbureted O-320

There is one other thing that makes the position of the throttle important. Carburetors have more than one "circuit" for supplying fuel to then engine. There is a main "jet" that supplies fuel during the higher-power settings, and an idle "jet" that supplies a richer mixture that then engine needs at low r.p.m.s. Which jet in operation depends on the position of the throttle plate At idle, the port is downstream of the edge of the throttle plate so the engine vacuum pulls fuel from that port. When the throttle is more fully open it uncovers the idle mixture port so less fuel is sucked out of it. When you just "crack" the throttle for a start you are pulling fuel from the idle port, which automatically enriches the mixture. If the engine is "flooded" (you've put too much fuel in by over-priming), then opening the throttle wide (and applying the mixture cut-off) helps to clear out the excessive fuel.

In a carbureted car, you don't have a mixture control, but holding the accelerator down (without pumping it) defeats the idle circuit and can clear a flooded condition.

By the way, the joke among mechanics is that carburetor is French for "leave it alone".
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Old Yesterday, 02:18 AM   #17
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By the way, the joke among mechanics is that carburetor is French for "leave it alone".
No doubt with French (or Japanese!) carburetor's. I used to enjoy overhauling carbs on big ole 'Merican engines. Not much use for that these days.
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