I went out and looked at my G Tubes and I don't have that bubble ...so it may be a process issue with air in your injector and that is where the the bubble in the plastic ends up (maybe...!). Since air bubble rise...is it always on the up side of the mold? If for some reason you are injecting air late in the process....
More likely...Are you clamping at that particular spot...and closing off the vent? Clamping too hard? The purpose of the vents is to provide a place for the displaced air to go when you inject the plastic. If that vent is getting closed off the air will be trapped in the bait. I shot a couple last night and I clamp at the front of mold (sprue) and light about 1/4 of the way up from the end of the tail. Just enough not to get flash. I am not clamping at the junction of the body and the tail.
I don't clamp that mold very hard because I have had issues with tube molds in general not filling the skirt completely if I clamp to hard or don't have my plastic/mold up to temp.
There were some 4" G Tubes on sale a week or so ago that the owner had modified the vent at that position "per instructions from Jason".
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I went back and read that post and he was having the same issue. He concluded by saying:
Subsequent conversation experimenting revealed that simply double clamping the molds and using a slight hold was all that was necessary to get great baits
http://www.caneycreekmolds.net/index.php?topic=3368.0
