It should close tightly with only minimal pressure from your finger. If not, either the valve is defective or it has some crud caught in it. I've also seen them leak between the valve and the cover (although pressing on the lever probably wouldn't stop that).
If it hasn't been done in the past 10 years or so, I would change all of the soft fuel lines under the hood. They can start shedding little bits into the fuel that will cause exactly the problem you describe.
Personally, I would throw the Grose Jets as hard as I could, then install some original type needle valves.
Also worth checking the fuel pressure. Someone else on this forum recently had exactly the same problem (although with HS4 carbs as I recall), and discovered that his fuel pressure had mysteriously gone up to 10 psi!