You are getting a spark from somewhere, which is igniting the explosive mixture of hydrogen and oxygen in the jar. It may be some sort of backfire from one of the cylinders, possibly because the mixture of gasoline and air, along with the hydrogen and oxygen, is igniting and the flame is propagating bast an open intake valve. Diesels do not have a mixture of fuel and air in their intakes, apart from your hydrogen and oxygen which is probably below the lower explosive limit.
Try a flame trap in the line from the bottle to the manifold. You could probably make one from copper screen. Better, in my opinion, take out the whole system and throw it away, because it is probably useless in the first place.
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