On petrol engines, output is controlled (predominantly #) by regulating the flow of air into the engine, i.e., by the throttle valve. If your engine is revving too high, it is ingesting too much air. The leak / fault could be anywhere between the throttle and the cylinder.
as mentioned above, check the throttle is closing, and is correctly sprung closed.
On some SUs, there is an overrun valve incorporated in the throttle. The spring in this can fail, allowing the valve to flow air when it should be closed - this would allow the engine to rev a bit. Either block this shut, for testing purposes, or fit a solid throttle valve.
Make sure the inlet manifold is sealing correctly, and *all* take off pipes are appropriatly connected, or blanked off.
At the risk of reopening an earlier thread's argument, the oil in the carburettor dashpot is not important to you in curing this fault. Neither, for that matter is the grade of oil in the sump.
number_cruncher
# on automatic cars with modern fuel injection systems, the ignition timing is retarded during gear changes to temporarily reduce engine power and give a smooth change. This is, of course, irrelevant for this particular problem.
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