You have to determine whether there is really something that has gone wrong with the engine or if it is something to do with the installation. Put the engine back on your test stand. If again it runs perfectly, then it has something to do with the installation. I would suggest that if it does run well on the test stand, one by one use the components of your installation (fuel tubing, tank, battery pack and switch etc.). If it still runs perfectly, then the airflow into the carb and the relative pressure at the inlet vs the pump may be to blame. With the MVVS 160 with its front mounted carb you can be ramming air down the carb throat making the engine go lean. If this is the case, then a richening up of the needles may help or protect the carb inlet from the ram effect.