Lumpy idle. Petrol in air box
 

Gd eve. V164 twin stromberg. Lumpy idle and plugs give indication of running too rich. Valve clearances good, timing spot on. I’ve tried adjusting the trim screw and whilst I can see a slight improvement on my colour tune it is still rich. Dropped off air box cover and found fresh petrol in air box which I think is originating from the front carb. My first though is leaking needle valve or an issue with float height and the slight vacuum in the air box is drawing fuel from carb somehow. It would account for the rich running at least. Does this sound about right or anyone else an idea what to check that might result in raw fuel in air box ? I’m asking before I whip the carb off tomorrow. Cheers.

Ok I may have found the issue. Took suspect carb off and found float height way out - far too small so would lead to high petrol level in bowl and issues i mentioned. Everything else looked fine. Adjusted as should be. Stripped second carb down too but was fine, which also supports thought was front carb «*leaking*» into air box. After a cup of coffee will refit and see if issue sorted.

Yes that's the number one cause of petrol and spitting back in the airbox

Float height settings on stromburg carbs
The setting is easy to do too but don't forget to check the operation of the needle valve while you have it apart