Hi
Yes it is possible to adjust thrust using the washer method, you need to check to see if there has been any washers added. Don't know why you would need left thrust, none of my planes have that, some have a little right, some have none. I would pull any washers that are there out and fly it, then you can decide for yourself what it needs. Do a little searching and you will find out what manouvers to fly to see if you need to add thrust. Generally if the plane climbs a lot under full power it may need some down thrust and if you fly straight up with full power and it pulls to the left then you may need to add some right thrust.
Hope this helps
Andrew