Engine stumble when cold

When I ride my Scrambler from my house after it has been sitting (i.e., cold engine), the engine does a stumble (a significant drop in revs, then, if the bike is not going too slowly, it recovers) at approximately the same point along the route.

I live near the top of a steep hill with a gravel road and I am usually do 15-20 mph on that road. At the bottom of the hill, the road becomes tarmac at a junction and I slow to make sure the junction is clear before entering it. If the stumble happens while slowing, the engine will stall and stop the bike. If it happens after I get onto the tarmac, it recovers. In either case, there are no further issues after that.

Any idea what is up?