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My above car runs very well, but will not rev above 4,000 rpm. Can you advise what you think the problem may be?

Hi there,
I have looked up a similar question and they said look into the crank pulley and make sure the harmonic balancer is not loose or falling apart, so may be a good place to start. Or it sounds like it could be a failsafe mechanism.
I recommend getting it looked at and not driving it as it could have a serious issue, and you don't want to make it any worse.