Not sure I understand you: the throttle cables need more slack, not less; a nice "S" curve so they flex instead of pulling on the carb. Remember this engine mount allows the engine to move/shake, a lot.
You can get very good high-quality bicycle teflon control cable that is very light and flexible.
Some guys have crossed the cables from the kitfox johnson bar: The left cable goes to the right carb and the right cable goes to the left carb. This gives them nice flexibility. Do not tie them to anything that may interfere with slack.
You probably don't have a single big issue, more common is stacking of small issues like carb sync, worn/cracked carb sockets, loose engine mount rubbers, out of balance prop......
You need to address them all to get anywhere; it's pretty hard to sync the carbs when they are shaking so bad fuel is pouring out of the float bowl vent.
