ORIGINAL: flyrockets
Hello
I am setting up a BH 120 Coarsair with a Saito FA200R3. This is my VP30 fuel pump that worked great on a Saito .82/1.20 single and a OS 1.60 single. The pump runs off of the crank case presure. I have a multi cyl glow driver that is powered by a 5000 mAh NiMH 4.8 volt pack. The propeller is a 3 blade 16-8.
OK that's the set up. My question is when setting the high RPM needle valve it reached peak rpm (7500rpm) at about 5 turns out from all the way in. The rpm does not change when I open it further all the way until it comes out; when it stalls.
Since this is my first radial I am not sure if I have an issue or not. It seams to me that if I open the high speed needle valve past max rpm the rmp should drop due to a too rich mixture. Since it is not doing that I am wondering if the engine is fuel (volume) limited.
What do you think? One idea I came up with to test is to bypass the fuel pump and use exhaust pressure to drive the fuel. I have not tested this yet and was wondering if anyone has seen this befor.
Hello flyrockets,
Your fuel pump setup will not work on a radial, as the pressure impulses on this engine type are too weak due to the big crankcase volume and the even suction of the multi cylinder.
My advice is to provide a straigt feed with the tank as near to the carb as possible. Normally You will not need to assist by exhaust pressure, but my 325R5 has a pressure pipe on one
of the exhaust tubes.
And at last, yes, full throttle fuel quantity is an issue on nearly all radials, this is because the even suction causes much slower gas velocity in the carb compared to a single cylinder
with its small intake pipe volume. With the correct carb diameter this is nothing to be worried, just make it rich enough for full rpm and open the needle some extra 1/4 to 1/2 turns.
Jens
Holm und Rippenbruch