Quote Originally Posted by jiott View Post
Also, I would never recommend running the engine at idle for a few minutes before shutdown. Idling will cool the engine down and quickly cause carbon deposits to form on the plugs. You want to shut the engine off when still nice and hot. I use the method suggested by.....can't remember: When done taxiing immediately rev it up to about 2500 rpm, then pull the throttle clear back to idle, as you do this shut off one ignition, count 2 seconds, then shut off the other ignition. It stops smoothly and immediately.
A 914 needs to spool the turbo down and get some cooler oil into it, you absolutely need to idle it a couple minutes before shutdown.

About 900 hrs behind 914's, never shut off the fuel, and always killed both ignitions after two minutes idle. Never had any shutdown dieseling or backfiring, just a smooth shutdown with no gearbox rattling.

I would look at the last thing changed (the plugs)or altered.