RE: First experience with Bubble Jett tanks
Are you overfilling the tank on the ground? Or is this happening in flight too?
I always pinch the fuel line, disconnect the filler, then aim the fuel line away to ensure no residual pressure from overfilling - it is easy to do this, and it can last a few seconds to cause nightmares during startup, exactly like you describe, at worst you can tear the bladder. Haven't burst one yet, but I have watched a few cc's of fuel squirt out after disconnecting. It takes the engine a bit to burn off that fuel, so connecting it at that stage results in pressure feed and flooding, hydraulic lock etc. Make sure there is no pressure before connecting to the carb.
I wouldn't hesitate to lean the low end to a good operating setting right off the bat. There is far less heat being generated, and you do not want the engine to cool further. It's the top end to keep a bit on the rich side.
MJD