There is every chance your carbs need setting up properly as flat spots at takeoff are common unless done so.
With all other things correctly set up (timing, plugs, valves), you balance the carbs at idle after setting the mixture, ensuring you have at least 1 - 2mm of free play at the cable adjusters. You then have to balance the cable pull on the carbs, which is where a flat spot can occur if they pull at different rates.
I won't go much further as there are plenty of info on how to on the net on how this is done, and for a newbie it's best to use a manometer like Rich suggests. It's hard enough showing someone in person how to do this without doing it in words, maybe one day I'll do one with pics and post it on a website I'm proposing to set up!! LOL
Bill....................;-)