In general, the result was the following ... I once again completely checked the left carburettor. There is no leakage through the seals or the needle valve. It's all in the accelerator pump system, which is in the carburettors No. 1 and 3. In the left (No. 1) carburetor, a spring has broken, which supports the iron ball that overlapped the channel with gasoline. Therefore, gasoline from the carburettor № 2 without stop gets into the float chamber of carburetor № 1, whence it is discharged. If the channel between the carburetors is blocked, the leaks cease at any angle.
The only thing is that it hardly can be repaired, as the bottom of the float chamber can not be disassembled.
Now it is necessary to warm up the motor, keeping the motorcycle straight, to close the gasoline cock during the parking.