Tuning rocket mass heaters

I have designed a rocket based on Tom B and E&E's book, but I wonder if anyone is tuning a rocket to the barrel, similar to how we do with a car's carburetor. A 2-inch clearance from the top of the riser to the inside of the barrel just doesn't seem sufficient."

Yes, that has tripped up many people. 2" is basically the minimum that has worked. More is almost always better.

When I joined this forum, the gap was a frequent discussion and lots of people offered to solve the  question with calculations and advice.
This subject really confused me and it seemed like a critical measurement but as it tuned out there is a minimum sized gap but as Jordan points out anything above the minimum is fine.
However if you are using the barrel top for cooking or boiling, then the gap becomes more important.
The hight of the riser may be more critical but again it seems a minimal hight is more important to achieve than a max hight.