it takes a fair size bubble to put the flame out, i was bench running a P-160 the other day and just used my start up / taxi tank connected to the fuel pump, everything was running like a dream then all of a sudden the RPM started dropping out and i noticed it looked like foam in the fuel line it had so much air in it, the pump was drawing enough fuel that it caused an air funnel in the tank and started sucking air, dropped it back to idle and the air stopped and she kept running.