You will notice a gain in rpm with a vented tunnel. If there is a valve, when you open it you can actually feel the back rise as the suction is broke. I wouldn't worry with a knife in the tunnel, I rarely had to clear my mega. If you don't have power trim binding the motor down, when you feel that grass is messing up your rpm or plugs up your water pump, just let all the way off and push hard down on the tiller handle and the motor comes out the water and slings anything off. Then get back on the throttle. This can be done without coming off step while running across shallow flats
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