Tom, the motor speeds up when the hose is crimped because the pump is cavitating. The air entrainment causes the speed, but maintains a certain pressure, apparently enough to not cause the safty mechanism's to kick in.

The fire-fighting rig we set up for the cottage uses a similar system, the pressure controls the rpm's of the engine/pump combo (an old Ford 300 straight 6 hooked to a surplus irrigation pump). At 3,000 rpm the governor merely stops accelerating provided there is still pressure above 50 psi in the manifold, if not the engine cuts off totally.

Best of luck.

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