Nice thread. I have one addition to the discussion. The combustion chamber design. A flathead motor will not compete with a OHV in efficiency. Fuel droplets do not all center conveniently around the spark plug for a good flame propagation. Bad combustion chambers will always burn more fuel out the exhaust
Tune is everything on a gas motor. A small adjustment in timing will greatly affect tune. Gas motors can be way out and be "Good". Fat motors cover bad tune/fuel delivery. Lean one down to correct fuel and watch the symptoms pop up. EGT and O2 sensor on compared motors would tell much.
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