What I am about to post ought to spark some controversy. Don't shoot the messenger!
In conversation with someone in the carb business for 30 plus years told me the following:
"When the fuel is atomized in the intake the vapor is sucked into the cylinders in a sequence as the valves open. Because it occurs at different times for each cylinder it does not matter where the carb is located, the fuel/air will be distributed evenly.
There are a lot of variables to consider when evaluating fuel distribution in a log/plenum manifold fed by a carburetor. Size of the plenum and it's ratio to the displacement of the cylinder(s) it feeds. Air speed which affects the 'size' of fuel droplets being pulled from the carb. At lower speed the droplets will be bigger leading to a richer mixture - at higher speeds the mixture will lean as the droplet size decreases. Temperature of the plenum can cause the fuel to drop out of suspension perhaps even pooling in the floor of the manifold (in the coldest spots) - especially at lower speeds.
Also, the 'atomization' of fuel and air into a 'charge' doesn't actually happen until the compression stroke in the cylinder. The stoichiometry of the charge depends on how much fuel managed to get included in what was drawn from the plenum. If distribution is poor enough one could have certain cylinders running rich while others are pinging from detonation.
Open a gas can on a hot day and watch the vapors play in the air like the tongues of many snakes. The environment in an intake plenum is similar.
This is why multiple side draft carbs were eventually fitted to inline engines - especially for racing. Many configurations eliminated the plenum entirely with a carb dedicated to single cylinder or pair of cylinders (depending on head configuration).
Ultimately, carbs were replaced by fuel injection which meters fuel directly to the intake valve leading to the most even fuel distribution possible.
Hilborn Tech Paper Classic Inlines regards,
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