I have a similar phenomenon when I start the motor rotates normally and calmly.
After about 500 the motor begins to spin faster, without which I give gas. I then cleaned the carburetor from the outside, and all newly oiled.
The small role between the carburetor tends to wear out after many use it slowly and then bit pinched it, so the carburetor for the rear cylinder receives more fuel mixture. It is important to know because the rear is the # 1 cylinder and thus the working cylinder is the only makes everything turn.
Good luck with your search
Andy