Non-return valve

[JASL]

I have been struggling to locate the cause of a slight mis-fire under 3/4 throttle open. I have replaced much if not all the emissions related plumbing, and sealed where air leaks may have been admitted to the engine and the manifolds.
One thing that the manual says can cause the problem is the non-return valve that fits on the Carb pedestal and connects to the Thermic valve on the air filter housing (it's a Mk 2 car).
I have questions for more experienced owners.
1 Has anyone experienced the Non-return valve failing to operate (and thus needing replacement ?)
2 Has anyone located a modern replacement for these (as the parts are now apparently unavailable)
3 Is anyone able to point me to an engineer designed replacement that I could build at home over the winter ?

Many thanks for any assistance.
Regards
Alan

[wilf]

Just block it up and see if it improves. The thermic valve isn't needed in our climate, certainly not at this time of year. In fact mine caused no end of problems until I tracked it down - supplying very hot air to the strommies isn't a good idea.