Messing with the thrust line will have adverse effects on flight performance. May I suggest you remove the washers and go back to your original setup? If what your friend says is true, you should be able to duplicate the results by raising or lowering the nose of the model while it is idling. Personally, I would expect that this trick has nothing to do with the glow plug, but moreso with the air intake in the rear of the engine. Perhaps you are mounted too close to the firewall, or the fuel line is pinched somewhere? If the engine is lean enough at idle, it's not taking in enough fuel to pool in the plug.