Morris Minor Owners Club

Thanks for those who replied to my earlier query re my duplicated engine breathing arrangement. All now functioning well with new vented oil cap ( fortunately I had a spare rocker cover to suit!) along with ‘can to carb pipe’.
Just noticed that when you remove cap with engine running the revs drop away - i had never noticed this before but assume this is what you’d expect and is just a result of allowing the mixture to become lean. Have I got that theory right ? I’d be interested I. The correct technical explanation as it would seem to be a good alternative way of checking mixture!
Regards
Steve.

Mine does exactly the same, it picks back up if I put my hand over the oil fill hole.