Moki revelationa
I just finished a CompArf Corsair with a Moki 250 for power, and like many others on this forum have been stymied by the problems with this engine. I have now 11 flights on the aircraft and the engine is running great. Here's what i've learned about my engine.
I built a test rig in the begining and made the initial runs there. And like many others in this forum I found that it would quit, seemingly at "the drop-of-a-hat", would not idle reliably, overheated, and otherwise made me very nervous about using this engine in such an expensive bird.
During the first two test flights I found the engine would quit when I ran it up to WOT on the ground "here we go again". I flew anyway (against my better judgement) keeping the throttle well below full open. I wasn't any more at-ease, though, at the end of this flying session and decided not to fly the bird again until the engine problems were sorted out. At my wits-end and totally frustrated I recalled from the forum comments like: "never run the Moki 250 with the throttle butterfly wide open", "the carb is too big for the engine", "you have to make sure there is plenty of cool air for the carburetor", "the carb needle arm needs to be reset to a new height", "the needle spring needs to be shortened". And here's a good one, you have to have a "turbulator". etc, ad infinitum.
I have the choke on a two-position switch, and while thinking about the problem while tossing-and-turning, unable to get to sleep over this, it occurred to me: why not simply move the choke function from the switch to a rotary knob, that way i could use the choke as an ersatz mixture control. The next day I ran the engine up with this change and, "SHAZAM", I found that i could get a nice burbbley rich mixture anywhere I wanted, wide-open, mid-range, or idle with the choke knob at about 3/4 open.. I could adjust this knob and get the engine to quit rich or lean, "wonderful". Using this method, the engine does not quit any more. EVER. And i can turn off the pump while the engine is running and there's no change in RPM. No turbulator, no pump, no problem.
Conclusion: it seems that indeed, the carburetor is too big for the motor. The total pressure in the venturi is just not low enough to properly activate the metering side of the Walbro. By partially closing the choke you are lowering the total pressure inside the carb throat thus pulling in more fuel. That's why so many found the high needle so ineffective. It seems contrary to common sense to operate an engine with the choke not fully open, but as I said at the opening of my reply, I now have 11 flights on the corsair with plenty of power, a reliable idle, no overheating, and my confidence in the Moki 250 has greatly increased. I'm a happy camper.
Thhis winter I plan to experiment with a different carburetor, as in O.S. 8B. I'm sticking my neck out here, but with a properly sized carburetor the engine may not need a pump at all. Another set of problems eliminated.