If that is a new motor, perhaps you should try the non-synthetic oils first for mabay a couple of gallons and then go to the synthetics. This will seat the rings quicker and give you more power and more consistent running engine. If you try adjusting the fuel rich and lean both ways and it still hesitates on take off, make sure all of your seals around your carb is good and nothing has loosened up. If not the case, check the inlet nipples that attach to the fuel lines to make sure they havent lossened up as well. If you do all that and it still is hesitating on throttling up, call BME and tell them you may need to send the engine in to them! BETTER SAFE THAN SORRY...... I am not sure were else to take it,,,, I have spent over 10hours straight trying to get an engine to run right and then I found it had a faulty ignition when I sent it in for warranty..... Oh, one other thing ... Try changing the spark plug for new one and then see if it still stumbles.....
Hope I have helped.