Chalkie_Stobbart
Hi John,
I enjoy your articles and thank you for sharing your knowledge and experience. PP #63 Part 1 refers:
Quote: “I’m sorry, I have very little help or advice for those with carbureted engines.” Noted, but I doubt that, sir 
(ALL big radials you fly have pressure carburettors AFAIK, but they do have even fuel distribution, yes. I did my apprenticeship starting January 1968 and practical Trade Test on the R2800 1971. PV-1 Ventura.) In 1994 I was the F/O on the SAA DC4 flight to OSH.
This is an OWT: “One old trick is to operate high enough to use full throttle for cruise, but then back the throttle out just barely enough to detect the smallest possible drop in manifold pressure. That may cock the throttle plate just enough to induce a more turbulent flow, or a vortex where the fuel and air mix, and that may give better fuel distribution, allowing a bit more leaning.”
The reason is the MS Carburettors have a thing called the “economizer valve.” Refer to link, Figure B, Page 2.
https://wiki.ad7zj.net/wiki/images/8/85/MSA_Aricraft_Carburetor_Service_Manual_Models_MA3A_MA3PA_MA3SPA_MA4SPA.pdf I tried to post the picture of the tool used to set the plunger for the economiser valve, but failed.
This article was written by me a few years ago on Avcom.co.za https://avcom.co.za/phpBB3/viewtopic.php?f=9&t=157266&p=1602488&hilit=marvel+schrebler#p1602488
The Marvel Schrebler carburetor series commonly used on engines of general aviation aircraft are fitted with what is known as an economizer valve. This valve allows more/less air to be ported to the emulsifying jet. The purpose of the emulsifying jet is to help atomize the metered fuel that is ported into the venturi.
At part throttle more air is ported to the emulsifying jet, than at full throttle. Think of the air bleed to the emulsifying jet having two bleed ports, but at full throttle only one port is open. Why would this be so and how do I know when I have more or less bleed air helping to atomize the fuel?
During climb use full throttle, then when power is less than 75% (usually below 25” MAP and 2500 RPM) start leaning the engine till it runs rough, then richen only enough to return the engine to smooth operation. At your cruise altitude as speed builds up, ensure that you are still running at the point of engine roughness, but still smooth operation. Now slowly reduce throttle (back from full throttle) until the engine starts to run rough. Do not pay too much attention to EGT but this should go down as the fuel is better atomized which results in a faster burn rate and a leaner mixture.
The idea of this exercise is to find out when the economizer valve opens and allows more air into the emulsifying jet. Mark this throttle position and use this as the maximum throttle position during cruise. Jon Johanson reported a 5% fuel saving using this technique during his around-the-world flights.
Why would this be so? At full throttle less air is available to atomize the metered fuel, as a result the fuel will burn slower because the particles are bigger. This is good as slower burning fuel in a fixed ignition advance engine, is the same as retarding your ignition timing which is what you need when you demand full power. Remember, the timing is set for the place it is required to be used for the most of the engine life and this is cruise.
Now during cruise, with the throttle set to the ‘economizer open’ position, the fuel is better atomized, burns faster (same as advancing the ignition timing) which is good as this allows you to run a leaner mixture.
Trust this might be of some use to you. I did have this content printed in Sport Aviation when I responded to an article by Mike Busch where he advocated FWO with FI and Carb engines… FI - Yes. Carb - NO.
Regards, Chalkie.