This only happens occasionally upon startup (only, and 80% of the time at cold startup), and I'm still waiting on my wideband o2 sensor to finish tuning... I think it means either too rich or too lean, but I don't remember which.

Too lean!

Well that's no good... Thanks! +

Popping back through the intake can also be due to an intake valve not seating properly, bad cam timing (timing chain jumped a tooth), or just plain wrong ignition timing.

Check the timing first...

Basicaly the cylnder is firing too soon, before the valve closes. Why, thats a whole other novel. Id start with what has already been said

When an engine is cold it needs a richer mixture.That's why carbed engines have a choke.If it were anything mechanical,it would do it all the time.

Thanks. I'll try adding fuel to the cranking enrichment in hopes that it'll go away.

did you check timing yet? thats been the most common cause in my experience

intake backfire.. then went too far the other way and had a huge backfire that blew burnt streeks in the lawn (yeah it blew the grass off, down to dirt)