Gravity.
the tilting of the bike causes the floats in the carburator to rise wich cuts off the fuel intake
Is the vehicles transmission downshifting to a lower gear as you go uphill? If not, that would be your first place to look. After that I would suspect that your engine timing is not autmatically advancing as it should be.
downshift
Going uphill takes more power than going on the flat, so unless you can add more power, you have to trade some speed for power to be able to go uphill.
check the air intake
The parameters for engine cc are the amount of power or torque required for the purpose to befulfilled. Taking example of the motorcycle engines - as the amount of cc in the engine increases, the capacity of the engine to carry the passengers at a faster speed and the ease of going uphill increases.
Any engine will lose RPM going up a hill, its called gravity and the engine has to overcome it. The heavier the vehicle the more RPM you lose. Thats also why transmissions have all the extra gears. Lower gear equals more pulling power and less speed, higher gear is less pulling power and more speed.
When alot of engine power is needed, the A/C compressor will be shut off to reduce engine load.
Automatic transmissions lose acceleration driving uphill because of the torque converter. The engine turns one side of the torque converter and the other side is splined into the input set of the planetary gears. The torque converter uses fluid to turn the input shaft in the transmission. When the transmission is locked into gear and going uphill the torque converter has to work against power losses from the tires on the road and when the engine is working harder the torque converter cuts or "shears" the fluid and it causes the transmission to overheat. Always make sure you have enough fluid in the transmission
A bus going uphill must overcome the force of gravity pulling it back down, which requires more energy and makes it move more slowly. Going downhill, the bus benefits from gravity assisting its motion, allowing it to travel faster.
Your engine produces torque(turning power) and revs, and these are related. You can't get more than a certain amount of torque out of the engine at a given number of revs. If you hit a slope in the wrong gear/at the wrong speed, then forcing the car forward will require more torque than the engine can deliver, and it will stall. It's very much like trying to ride a bike uphill in the wrong gear.
Because it doesn't have enough power to pull itself. All vehicles slow down uphill if you don't feed them more fuel.