I would like to share a hint. When full increase both twist grips ensure ENG 1 and ENG2 came back and do not apply friction holding the throttles agaist the spring. If so, during flight and due top vibrations engines will came back but not in the same way causing a difference N1 power. Result, N1 differences and torque Split.

The MM (Ch 76) will only give you the method to properly rig the FCUs. And that can include the P&W MM in some cases. Neither will give you results for improper rigging except maybe in a troubleshooting chart to "check rigging."

To give you some possibilites of why it happens, perhaps the manual or auto FCU was changed separately or the max fuel stop was adjusted and the throttle linkages were not rechecked/adjusted properly?

Sometimes the "complexity" of these old school FCU systems tend to intimidate younger mechanics with mostly EECU or FADEC experience. However not following the manuals properly is not a knowledge or experience problem regardless the "pains" of working on these legacy systems.

I take it topping checks were passed?

Good question and in a perfect situation I'd be able to answer it, but that's not where I find myself.

Forcing the throttles past the open position with a strong wrist force may cause problems in itself with the rigging. We all naturally add a tiny bit of force at time to make sure the throttle is still up against the stops but when I say tiny I mean a tiny bit of force. No where near trying to go past the stop.
I've got experience in 10 different 412EP’s and have come across rigging problems with the throttles but I still never encountered any surging and never tried to force my way past the 100% position.

Some excellent replies and reminds us of the value of this Forum.

GS Pilot's quote of the Check List prompts my input.

Having a couple of hours on 212/412's....I would note that in reading the Bell Flight Manuals careful consideration of how to interpret some of the language must be taken.

Not that Bell's tech writers and lawyers are any different than other Manufacturers as they all have the same concerns.

When Flight Controls are mentioned does Bell include Throttles as one of them?

I suggest not as they are a separate item after the mention of "Flight Controls-Friction" and are not considered flight controls in the accepted definition meaning cyclic, collective, and pedals.

One concept says Throttles are a secondary flight control for example....how they may be used during Tail Rotor problems to alter torque effect and assist in yaw control of the aircraft.

The mention of friction settings on the Flight Controls is a whole different kettle of fish than the problem under discussion here.

I was taught to check for full travel of the throttles and ensuring there was some cushion felt when at full open....Then to apply sufficient friction to them to ensure they remained at full open unless reduced on purpose by the Pilot.

If there was no "cushion" felt....that could be an indicator of a rigging problem of some kind.