XRAY - Model racing cars

I don't know why, but the gear mesh on my xt8 never seems to be very good. How I adjust it is loosen the screws under the truck just enough that I can move the motor around on top, get the gear mesh correct, and tighten it down. The only problem is, is before I tighten the screws the mesh can be perfect, but after I tighten everything it somehow messes up the mesh. I even switched clutch bells(both 13t) to see if it helped but it didn't.
Also a funny thing is when I first put it together the mesh was fine, but it was with a different motor. First I had a Werks .28, and now I have an O.S. .21 VG in it. The VG seems to have a longer crank which puts the clutch bell a little farther forward on the spur gear. Could that be my problem?
I have the black aluminum motor mount on my XB8, I could try using that.