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My Datsun 240k runs very rough at low RPM, and if I push the accelerator to the floor quickly, the engine splutters and dies. If I slowly rev the engine to about 3200 RPM, and then floor it, it revs great.

What could be the reason that the car is unable to rev freely at low RPM, but can rev freely once the RPM has slowly reached around 3200?

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    Welcome to Motor Vehicle Maintenance & Repair! It sounds like your accelerator pump in your carburetor(s) may be plugged or malfunctioning. It probably needs cleaned, or the jet diaphragm may be damaged (I'm assuming it has one of these). (PS: Way cool car, btw.)
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jan 4, 2020 at 13:56

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I experienced similar issues with my old pickup for some time. Here is everything I checked to diagnose the problem --

  1. Vacuum Leak : Check for air leaks around the intake manifold and carb seat. Any air that isn't passing through the carb is unregulated and will cause sputtering/stalling. If you have an aftermarket carb, ensure that vacuum caps have been placed on any ports not in use. Check your PCV valve. Given that your problem is affected by your RPM, the changes in vacuum while revving make this a likely culprit- vacuum decreases as RPMs rise. There are plenty of ways to find a vacuum leak, but the only one I have had any luck with is spraying carb cleaner (in small amounts) in suspect areas. If your RPMs increase while spraying, your engine must be aspirating the cleaner through a leaky area.

  2. Spark Issues : Ensure you have solid connections on all spark plug lines. This is what did me in - I did not have a good connection on the line that connects the ignition coil to the distributor cap. It was arcing across, providing me with spark at irregular intervals while damaging the distributor cap/line.

  3. Timing Issues : An engine can perform admirably at high rpm, but horribly at low rpm if the timing is set to do so. Drag Racing vehicles do not idle well, the vehicles are timed to only perform at 5k+ RPM. That being said, you can advance/retard your vehicles spark by rotating your distributor cap. A timing light will come in handy to make sure you've got it right. If your timing is severely off, you may have to do some heavier engine work to clock the chain.

I'm hoping this helps you to diagnose and repair your engine. Plenty of gremlins can cause those symptoms, but this ought to cover the big 3.

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