Well it is an automatic I am guessing. It means your transmission could be slipping a bit, there is stuff you can buy to try and stop that. I don't know the exact name of it but you can get it an Wal-mart, or any auto store.
Try changing or cleaning out the stepper motor, this controls the idle and is a common fault on the 306.
As the engine speed increase so does the speed of the oil pump. the faster the pump turns the higher the pressure becomes. When changing gear the engine speed drops along with the oil pressure.
Assuming the vehicle doesn't move when you put it in gear, it sounds like the clutch is blown.
Code P0731 means A/T first gear signal fault
The distributor gear is off by 1 tooth, you probably recently have a a timing belt.
linkage is sticking
If it's old enough to have a mechanical speedometer, it'll be in the speedometer itself. If it has an electronic speedometer, there is no speedometer gear - to recalibrate the speedometer to match something like, say, changing tire size would require changing the parameters in the engine ECM.
Yes, if you change the gears in the differential that drive the axles and wheels. The lower the gear ratio, the faster the car will go with the same engine RPM. Example: A car with a 3.55 to 1 ratio differential will go faster than one with a 4.1 to 1 ratio differential at the same engine RPM.
The engine also jumps when changing to Neutral and Drive. Should I have transmission fluid change? A/C was not on.
it means that the gear is not able to push up the car due to low preasure in the gear
Fault with the clutch probably.
The clutch of a go kart is much the same as the clutch on a car. The clutch disengages the engine from the gearbox enabling the gear to be changed. Without a clutch gear changing would be difficult and may damage the gearbox.