I noticed this with my own Vis last sunday practice. I talked to numerous people about this issue including Garrett from Blast and Joel from Paintball Gateway. So I replaced both of the 006 orings on the bottom side of the engine casing with 007's. Doing this fixed the problem. Be sure to cover the oring grooves with lube before inserting the new orings, and then make sure the outside of the orings has lube on it as well.
Keep in mind that when you push the engine back into the gun, it could potentially scrape the orings. Inserting the engine with the holes on its sides and then turning it inward can help. Dan from Yourpbfriend has a video of him doing it. Click on the link below and skip to 13:05.