G'day

Firstly, a Saito 91 (nice engine) in a Kadet Senior (nice plane) is total overkill. I flew mine on a Saito 56 and Saito 65 and they were more than it needed so go gently on the power. I did briefly fly an ARF Kadet Senior with a 91 four stroke but I quickly removed it as it was surely going to break the model if I left it there.

Secondly, a trimmed Kadet Senior will just about fly its self. As you say that this model is modified, this may be less true. I assume the dihedral has been reduced and ailerons added?

My suggestions would be -

1. Take off straight into the wind if you can and stand behind the model so you can see exactly what it is doing.
2. Use low power and let it just float off by gradually increasing the power. On full power with the 91, it will be heading up fast after a very short roll and may even try to go into a loop.
3. If it shows a tendency to roll to the left or right on take off, reduce power and bring it back down and re-trim to reduce the roll.
4. It will probably need some down trim to fly level at about 1/3 throttle. Most Kadet Seniors do and it even says this in the instructions.
5. Make sure you can stop the engine with the radio. You will have a large prop and even at idle it is going to want to float on forever and not land. You may have to stop the engine to get it to land. It may actually be easier to land this way. Kadets are surprisingly good gliders. I stayed up for 6 minutes 20 seconds after a 30 second engine run at a fun fly.
6. DON'T do any violent maneuvers at high power. The wings may not take it.

And have fun.

Mike in Oz