Duals will also have some cylinder-to-cylinder distribution differences which can't be fixed with jetting adjustments.
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The errors will appear and vanish with changes in engine speed and throttle opening. There are probably some differences between manifolds by model and brand.
Net result: you have to jet for the leanest (most knock-prone) condition in the worst cylinder, not for best power or best mileage.
A 3 X 1 gets much closer to matching the 3 ports.
