JAP Engine throttle pickup

Had another run of the 4/2 this afternoon, it's running driving the countershaft and alternator that I have rigged up.
Seems to tickover lovely (could listen to it all day!) but when I operate the governor lever to let it govern the engine speed it opens the throttle wide which upsets the running mixture which appears to go rich causing hesitation before steadying out at the set speed, when it also runs well, see video.
It's the initial throttle opening that's the problem.
I've experimented with weaker and richer settings of mixture screw which will slightly change the effect but it's still present.
The only load I have on it so far is the driving of the unloaded alternator.
Anyone point me in the right direction, please!

Alan
https://www.youtube.com/shorts/apecqrXDKpc?feature=share