I also just thought about this is your tires glued to the rims. If not the wheels will spin inside of the tires which will dramatically reduce acceleration. If there not glued this could be your problem just take some super glue (now some will say super glue is not strong enough) and apply it to the outside of the wheels and the tires and then wait for i don't know 20 minutes or so then try it out. I don't know how much power the engine on the xcellerator puts out but it's a .18 and it should have some good power if it's not the wheels spinning inside the tires then again try leaning out the low speed needle.

I hope we soon find out what we can do to get you accelerating faster soon
But i like this it makes me think (which i dont often do) lol