Ok this can be several things with the new engine. Sometimes when an engine comes from the factory it is usually set up to be in a break in type of adjustment. Where the engine will start, but will not go very far or seems to have a hard time getting up to speed because the Tuning needles are tuned really rich with fuel. A suggestion is to adjust the HSN(high speed needle) by 1/8th turns to lean it just a bit, but not to much just to see if the car will get rolling. If you dont think it an engine problem, make sure your drivetrain is altogether and that your gears are meshed properly. Check Dogbones to see if they are on properly. Good luck!