You might want to check the condition of the notches on the brass stop plate which limits the amount of advance and throttle opening while in F-N-R. The vertical brass lever which engages the notches can get worn to or bent below the shift lever which won’t allow it to engage properly. Your stop plate should have three positions on it. One for starting while in neutral or warming up the motor, one for part throttle while in reverse, and the last one is for full throttle operation. It sounds like the third one isn’t catching and the mag plate is advancing too far.
