I would say that since it's only one side that had a problem, it was an air leak somewhere on that side. I didn't catch who put the carbs on the motor but it could've been something as simple as that that caused your problem. Always do an air leak test before you go starting a new build. If the builder did this before the carbs were put on and it was fine, then theres your problem. You didn't after installing the carbs, before starting.