High idle is typically a lean condition, so check for leaks. Not running on "ON" is usually a problem with the vacuum operated fuel tap. It sounds like an air leak in the vacuum hose or maybe it's missing altogether.
Check that first.
Then check fuel flow. Disconnect teh hose from the petcock to crabs and insert it into a container and turn the tap to PRI and see how fast it flows. Then turn it to RUN or ON (whatever yours says) and see if it flows. It should not. Then apply a vacuum to the vacuum hose and see how well the tap flows. That all assumes that you have a vacuum tap and that it's clean and the filter is clean.
If that doesn't reveal anything and tap flows correctly, then it's time to look at the carbs and filters.