My Speedfight has no power, can barely go over 30?!?

I have a Peugeot Speedfight 2 100 Prost, 01 plate. Basically, this has been a problem I have had before. I fixed it roughly 2 weeks ago by cleaning the air filter and it seemed to run fine, but when i tested it out it was kinda cold during the night.

Today riding to college it was running perfectly fine, if not better! When I arrived at college I noticed I had about 1 or 2 bars of petrol (out of 16 bars) so decided to go to get some petrol after college.

As I was riding to the petrol station I noticed it had hardly any power and I assumed this was because I didnt have much petrol in the tank, so I filled it full of petrol and the problem still occured. Then checked my oil level and it needed some oil so i filled that up and the problem still exists.

Do you think it could just be the heat causing this problem? Because it was quite a hot day today?

Basically this is the problem. When I turn the throttle about the 1/4 of the way it runs kinda normal, but if I turn it even more I get absolutely no power. It struggles to accelerate past 20/30 but once I get up to around 35 it accelerates up to 50/60 pretty normally.

I have made sure that the fuel tap tubes are correctly fitted so there not loose, and made sure that there is no bends in them. If I'm right, the fuel line is the top tube, and the vaccum is the bottom tube of the fuel tap?

Do you think Its really the air filter causing this problem, because I've only just recently cleaned it so it should be that dirty!!