They are normally heavy especially when compared to more recent CV carbs in which you only open a butterfly as opposed to the K6 with slides.
Assuming that all the linkages are free moving there is a bit that sometimes gets missed in that the throttle pull cable has to be adjusted with enough slack so that it will not conflict with the closing cable.
It seems odd to set it up with slack in it but without you'll just get one binding against the other and this will make it very stiff to operate.
As a guide to check it with motor not running, open fully and let go it should flick back closed easily. Then try it at both extremes of steering lock it should still do the same both ways, if not you don't have enough slack.