[willy]

Have you had the engine cover off when you tilt the engine up?
If not see if you are dumping fuel out of the carbs when you do.
Could be a carb rebuild in your future, your floats may be sticking and or clog in the low speed jets and fuel is backing up.
The only other thingt I can think of that tilting the engine would effect is a loose wire in the harness or ground or battery cables which flex when you tilt up and may be forced into making a connection under that pressure and vibrating loose when vibrating when running.