You notice a strange sound from the engine and lots of oil on your leg and all over the bike.
This is how I have solved that problem.
Drill a 1mm hole throu the cap like this
and sequre it round a bolt with a wire like this.
Mead & Tompkinson used a rubber O ring made for Amal carb flanges.
Do not tighten the cap too hard, the tread will brake on the cap!!!