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How are you judging that it is running rich, black smoke, sooty plug?The mixture you will get with a relatively steady flow through the carb will be different to with an intermittent flow.
If the intake valve spring is too strong you will not get use of the whole intake stroke as the valve will only open when the suction is great enough to overcome the spring, if this only occurs near the end if the stroke you will not get enough charge to fire every time.
Your mixer throat may be a bit large for a 20.4 mm bore hit and miss engine.You could try reducing the mixer bore from 5 mm to perhaps 4mm. The reduced flow area will raise the velocity of the air passing over your fuel jet which will help draw fuel into the air stream.
thanks to share these results of experimentation as carburation in these little engines is never simple !in addition the compression ratio is not that commonly tested !