Just thought of something
The g4 carby was never used on the Imperial was it?
Just thinking out loud here, but maybe a g4 would be better than a sliding vane g3 when it comes to fuel (vaporisation?). EDIT - G3 was a butterfly carb, not a sliding vane - im thinking of the 3L, etc.

Maxwestern - you had me worried there for a second

I hadn't noticed that. Maybe whomever worked out the mixture on the can got a bit muddled up in the metric/imperial conversions

Might stem from the fact one UK gallon is actually 4.54 L not 5 L
Good spotting
Regards
Tyler