maiden voyage, couple of issues

having got this "new" 1960 series 2 125 insured, and my test done, i had to take it out, if only to work out how to use the gears.
somewhat scarily did 2 wheelies
whilst revving too much in a gear change, but i'm getting there. its a 185 cyclinder in a 125 engine.
i found out that it really didnt want to go up a hill in 4th (having been ok on the flat), and having changed down, it felt somewhat underpowered even in lower gears as if it had a minor blockage in the fuel.
this lasted a few mins (on flat) and then it seemed ok again after going down a hill. i also found a significantly different engine tone,in neutral, between me sitting with one foot on the ground and fully sitting down.
anything to worry about or is this all part of the fun?
jaybee
somewhat scarily did 2 wheelies

i found out that it really didnt want to go up a hill in 4th (having been ok on the flat), and having changed down, it felt somewhat underpowered even in lower gears as if it had a minor blockage in the fuel.
this lasted a few mins (on flat) and then it seemed ok again after going down a hill. i also found a significantly different engine tone,in neutral, between me sitting with one foot on the ground and fully sitting down.
anything to worry about or is this all part of the fun?
jaybee