[Dick Boneske]

Yes, it could be that the power valve spring is too strong for the vacuum your motor produces. Try the weaker spring.

But, first, does all flow of gas into the throat stop when you shut the engine off? If there is any evidence of gas after shutoff, the needle & seat is not closing completely--due to too much fuel pressure or the seat is loose in the airhorn theads.

Also, does gas drip off the venturi nozzles when idling--in the center carb and/or each of the end carbs?

You may have a problem with the end throttle plates not being sealed tightly. You can check this by putting your hand over each airhorn and observing whether idle changes. If is does, your throttle plates on that carb are leaking air.