This may be of help, l fitted a pair of 389 monos to a C rapide, new from amal, spares co manifolds, flowed and polished. l chose the following settings; slide 3 and a half, main 280, 106 needle, 25 pilot, mixture screws one to one and a quarter turns from snugged. Ignition set at 36 btdc, 8 to 1 omega pistons. Machine starts easy, good pick up, no spitting back, typical fuel consumption 55- 65mpg. Owner reports huge improvement over 276 carbs,especially with regards to even idle speed and performance in the higher rev range. It must be pointed out that l upped the c/r from 6.8:1 and flowed the heads and installed new guides with seals, valves lapped, new springs, so that no doubt helped matters along with the carbs, just my experience, hope this helps, good luck.
ET: Engine (Twin) Slide size 389 monobloc?
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