I was trying to break in my new engine its a thunder tiger .28 out of a mgt and it ran well for about 10 seconds and then began to rev up high on its own then slow down again then I got scared and turned it off . I know it was not overheated because its cold out and I only ran for a few seconds so is this normal for breakin and if not does anyone know whats wrong?

What I mean is it was surging on its own like when u overheat it