I had a case of surging on a 914 which to this day I am unsure why it occurred. Problem came on gradually getting worse and worse over a number of flying hours. I was called in when the problem got so bad they couldn't continue flying it.
Engine would surge from 3000Rpm to 5500RPM. The amount of surging would worsen the more the throttle was opened. It was as if the throttle was being opened and closed in about a 3 second cycle.
We tried all sorts of thing including locking the wastegate, removing the turbo pipe. They all seemed to improve it, but not cure it.
Eventually we discovered the springs in the carbs were catching on the inside of the pistons causing the pistons to lift irregularly. New springs fitted and the problem disappeared. To this day I cant explain it, but it may be worth a try.
