A flat slide carb will always run cleaner low down than a round slide. The reason being is that when a round slide carb is at low revs and low throttle openings the air coming in eddies around the round slide cut out rather than efficiently going straight down the carb/manifold causing poor atomisation of fuel resulting in a round slide carb needing to be set over rich to avoid leaning out when the throttle is cracked wide open from say just under 1/4 throttle. At higher revs in lower throttle positions the air is being dragged down the manifold a lot faster which reduces the problem.
However, my personal choice in the £80 to £150 price bracket would be the PHBH round slide over the PWK copies because of build quality and adjustability because the PWK runs fixed atomisers which i dont personally like and i believe they also have a fixed sized float needle but as i said, it is just my opinion
If your scoot has been jetted on a dyno by a competent operator and you are leaning out the pilot/ air screw/ slide afterwards, my advice would be not to do it as it has been set like this probably for the reason above.
