Have you synched the carbs? Did you adjust them following the manual instructions?
Try small adjustments to the idle screw. - the brass one at base of carbs.

Also have you checked engine timing?

Use a big socket on the crank bolt to turn engine. Line up cam marks and look at crank damper to see what you chain stretch is. Sometimes you can put pressure on the fan belt and turn the bolt on PS pump.

BTW what car/engine are we talking about?