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Old Nov 10, 2012, 02:33 AM
Brutus1967 is offline
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I convince things to work
The Netherlands
Joined Nov 2009
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Originally Posted by turk1 View Post
I didnt see any example so I cant say anything.Can you put a scetch drawing about it?The fuel jet and throttle valve placing is the only key factor.
Pls. think again how regulator needle(common principle for all) works...
Yep.... that's easy: the regulator keeps a constant pressure relative to what it regards as the atmospheric pressure, before the orifice of the needles. If it doesn't manage to keep that pressure constant relative to the pressure in the atmospheric chamber of the regulator, it is strained beyond its capacity and that would be very unusual.

It is purely the pressure difference over the needle (regulator pressure minus venturi pressure (yeah, Bernouilli effect and all that...)) that determines the flow of fuel.
If the throttle setting changes, the venturi pressure changes, but the regulator will not notice anything about that. It will only see a change in fuel mass flow, and supply new fuel with exactly that same pressure. As the air mass flow and the venturi pressure do have a relation, but not a linear one, you need a highspeed needle (full flow limiting orifice) and a low speed needle (fuel metering orifice, mostly a tapered needle, sometimes a real variable orifice like on OS or Perry carbs) to keep the fuel mass flow and the air mass flow in the proper ratio to each other.

If the propwash somehow gets into the atmospheric chamber of the fuel pressure regulator, the fuel pressure before the needle will start to change.
For example, you are running the engine on idle (low propwash) and all is adjusted OK. Now you give it full throttle, and the propwash increases pressure in the atmosperic chamber of the regulator to above atmospheric level.
That means, the fuel pressure also rises, with increasing RPM, and the mixture richens up: Power and thus RPM drop because of that, so propwash decreases => fuel pressure decreases => mixture leans out => power and RPM increase again, etc etc, and you have an engine that will not respond to the needles the way you want it to: at full RPM you need a lean setting to compensate for the higher fuel pressure, but that setting will not allow the engine to reach full RPM as long as the propwash, and thus the full fuel pressure, is not yet there. So effectively, the engine dies when you open the throttle. If you open the main needle more, the engine will pick up on the throttle and as soon as RPM picks up, immediately start to run rich.

Find a different location for your pressure regulator, OR connect a hose to the atmospheric opening on the regulator and route that hose to somewhere inside the fuselage or another spot where airspeed or propwash do not have influence, and your problem should be solved.

Brgds, Bert
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Last edited by Brutus1967; Nov 10, 2012 at 05:47 AM.
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