RE: K&B .40 DELTA
sounds like the pipe/engine/prop are seriously mis-matched.
If its running at 18-20K rpm, that is likely what the pipe is tuned for at the moment. Pipes work to boost RPM, but they also work as an engine governor, as they prefer to operate at the rpm they are set for.
Add some prop to that sucker. Just for starters, put a regular 9x8 on it.
And if its a big venturi, its going to need either crankcase tank pressure or some REAL good pipe pressure (try a check valve in the pressure line)