Surging is usually an indication of air leaks.
Your backplate pressure tap may be the cause - Did you just install one?

Suggest you start process of elimination by taking the Perry pump out of picture.
Plumb the clunk straight to the carb, and vent the tank to atmosphere. Plug the backplate tap.
Run the engine level, on the ground. If that eliminates the surging, you know where to look.

If not shoot some WD-40 on joints where leaks could occur, while engine is running.
This trick is good to momentarily plugs hairline leaks below carburetors, and around backplates and heads.
If the running engine changes behavior, you have spotted your leak.

Good luck! Let us know what you found.