my 1986 43F is running fine in the low rpm. but in high rpm it starts stuttering.
I already cleaned the carburettor, so this should not be the problem
And I am not entirely sure if its the motor thats stuttering because the motor runs fine even in high rpm when the transmission is in Neutral.

any ideas?

What RPM are we talking about?
I have found them to be unhappy over 7000rpm due to valve bounce.

Beside that, your number one suspect is adequate fuel supply.