ORIGINAL: w8ye
The carb on the Saito FG-20 and FG-14 engines has a Saito pump carb with no separate regulator
The pump has a second diaphragm on the liquid side of the pumping diaphragm that is called a surge diaphragm. the back side of this has a vent hole to the atmosphere.
This is supposed to eliminate the need for a regulator? The end result is a weak pump. You need to have the tank right against the carb like on a glow engine.
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I agree with youthis second pump diaphragm seems a poor substitute for an actual regulator. I don't know the actual fluid/air schematic of this Saito, but it seem the only LEGITIMATE reason to to vent this second diaphragm to ambient is to compare the venturi vacuum to it, and to have the fuelmetered in response to that information. I can't be sure, but it seems hard to do this from the pump side. It almost seems more like Saito was trying to damp pressure surges, and leave the tank positioning to the user. Perhap rationalizing that model users are used to critical tank positioning.
I can tell you for a fact that ARFs do not always provide for proper tank positioning, even though a factory designed plane should. Case in point is the Hangar 9 Fokker Dvii, where you have a GIGANTIC tank, way too long, positioned by the factory well below the nominal carb position. It took me a long time to finally sort out all the fuel flow issues I had with this plane. I can tell you that just raising the tank much higher didn't do the job....the tank is just too big for the engine no matter how high it's positioned. Intuitively, one would think that tank size would be secondary to position, but it's not. I've never found a good engineering explanation for this, but it's known fact anyway.
Too bad about Saito's new gas modelsneeding critical tank positioning just like glo engines. My opinion is they've missed one of the main reasons to go to gas....less tuning woes. Some may argue about that, but once I cowl in a gasser, I never seem to have to remove it at the field for additional tuning like I do glo engines. Scale enthusiasts (read that cowled engines) hate thishassle as it takesaway from flight time. It also can lead to power loss and crashes.
Thanks for the pics of the Saito setup. At this time, I'm awaiting a WT-456-1 carb for my conversion. Should arrive tomorrow. I wonder if a different intake adapter will be necessary?The current one is for a WT-669,the carbI decided not to try. Funny, all the local lawnmower shops need 10 days to get the carb, but a guy on Ebay had itin ONEDAY and shipped it out to me. Terrific service...but why can't the shops match it? Everyonesays to "buy local", but no one wants to invest in inventory. Even the WAREHOUSES are watching their inventory. That's like asking a coal mine to not have too much coal on hand. I'd better shut up now......