Thanks! Ok I tried to cover the air shutter more using a hose clamp like this, and it worked! :) I left a little open part.
The "event" did not happen at all. The burner is very quiet.
The flame in the water heater end might not be right though. It starts off light blue but then mostly has a long tongue of yellow flame going all the way in the big tube and around a corner. Just have to see how long it takes to get the water hot next time we use it.
https://www.canadiantire.ca/en/pdp/plumbshop-gear-clamp-0632141p.0632141.html?gclid=EAIaIQobChMI99_b0PyT4AIV-SCtBh2psgzqEAQYASABEgLPMvD_BwE&gclsrc=aw.ds#store=366EDIT--I ordered a new burner, arrives next week.
https://www.amazon.com/Atwood-91638-Main-Burner/dp/B004QMXSXY/ref=sr_1_37?ie=UTF8&qid=1548807628&sr=8-37&keywords=atwood+rv+water+heater+parts