I have a HPI 15FE engine that I bought used. I disassembled, inspected, cleaned, reassembled, and oiled it, and today I ran a tank of fuel through it on a very rich setting to flush out any dust that might've been left in the engine. During the "break-in" run, the engine VIBRATED LIKE CRAZY. I've never had a nitro engine vibrate so much. I used all stock parts when reassembling it, and I didn't cut or grind any moving parts so they all weigh the same as they originally did. I'm at a loss as to what might be wrong. Is it normal for the 15FE engine to vibrate a lot, or is there something wrong with my engine?

The only thing I can think of that *might* make a difference is this engine is mounted sideways instead of front-to-back. Any chance that would affect the way the vibrations propagate through the chassis?