At the risk of throwing more fuel on the fire here I'm going to try and point out that there are two timing methods that use the word 'Bubble' in the name and this leads to some confusion. First is the "Bubble Up" method that involves no drip or injector tube and has the person looking for fuel to bubble up from the port on the injection pump. Second is the bubble method described in previous posts in this thread that, if you read it carefully, is the same as the drip method except that that you are dripping air instead of fuel.

Personally I don't have much confidence in the Bubble Up method but have never tested it. The bubble method that uses compressed air instead of fuel pressure to find start of delivery seems to add some convenience if you don't have enough hands to pump the primer and fiddle with the pump and/or motor position while watching a drip tube. There is probably a good argument to be made for the difference between the injection pump moving air instead of fuel but I would be willing to guess that it is not a large difference which leads to the previously stated opinion that the method is accurate enough.

__________________
'82 300D - Light Ivory, 2nd Owner (Back in the wind April 2013!)

'95 E300D - White, grey interior. (Suffering from stuck/broken glow plugs)

Deuteronomy 22:4-
"Thou shalt not see thy brother's ass or his ox fall down by the way, and hide thyself from them: thou shalt surely help him to lift them up again."