Smell your dipstick, if it smells like gas you may have a leak in the fuel pump which is putting gas in the crankcase. The crankcase vapors get rich enough to change the mixture through the PCV or the breather to the aircleaner. The added rpm's pull the vapors from the crankcase leaning the mixture back down and the rpm's drop back.
If the diaphragm in the pump leaks, it will pump gas into the crankcase. The scenario I postulated is possible, but not probable. Everything would have to be just right. It would explain why the rpm's drop after they climb. It's a little difficult to come up with a reason for that happening.
As mentioned, block the PCV off. Remove the vent breather to the carb as well. I would check, also, to see if the knock is coming from the fuel pump. You can use a lo-o-o-ng screwdriver as a sounding rod.
Good luck, and don't forget to let us know what the problem turns out to be.
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Rich
The real democratic American idea is, not that every man shall be on a level with every other man, but that every man shall have liberty to be what God made him, without hindrance.
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