After running my 240z for 20 to 30 minutes and then parking the car if i leave the engine on at idle the rpm tends to lower intermittently to 500 rpm. If i decide to turn the lights on it will lower a Little bit more until the engines shuts off.

This problem is really annoying as i am worried that once i put an AC kit to the car i would be having the car shut off immediately.

Please advise which direction should i take before installing the AC to the car.

Sounds like your alternator is bad or the belt is loose. Or it could be a bad battery. Put a volt meter on the battery terminals while it's running, mine reads 14 volts at least. You can also see if a screwdriver will stick to the rear round center of the alternator, like a magnet, while it's running. Or something isn't grounded too good.

You may also have a miss firing cylinder that is fouling after a long idle. To verify this, try pulling a spark plug wire when it is idling normally. See if it sound familiar.

I had a cylinder with low compression due to my improper valve cleaning last time I changed the head gasket. That gasket turned out to not do the job, so while changing it out for a NISMO head gasket, I took down the valves for the low compression cylinder and thoroughly cleaned the valves and seats. Upon putting things back together, the engine started up immediately and picked up an extra 200 rpms at idle using the previous carb settings. I had no idea how much power was being lost at idle from that one underperforming cylinder.

That might be something too, valves need adjusting?

f i leave the engine on at idle the rpm tends to lower intermittently to 500 rpm. If i decide to turn the lights on it will lower a Little bit more until the engines shuts off.

What are the RPM at the beginning? Before it drops to 500 then dies?

You forgot to mention that you have a performance cam. You didn't say what kind of engine management it has either, with carbs or stock EFI.

Did you already try raising the idle? Your engine was recently rebuilt, right? Race cams have a rough idle or so I've heard. Maybe it just needs higher idle to keep running.

The 1972 240z has the following specifications:

New Datsun L24 motor with crank and rods from a L26, special oversized 84mm flat top black coated pistons from D.L. Potter Engineering with E88 Head and Far 311 New Race Cam.

2 original SU Hitachi round top carburetors rebuilt with race needle.

I already used a synchronizer to sync both of the Hitachi carburetors. Another thing i did noticed is that the first three spark plugs are turning black!!!

Today I added 10w engine oil as damper oil and topped both carbs, the car is still stalling and lowering to 500 - 600 RPM at idle before shutting down eventually.

Edited April 17, 201411 yr by jalexquijano

What is the idle speed before it starts to stall?

1,100 rpm

Take a look at the front carb.The likely culprits for the first three sooty plugs are; sticking fuel nozzle, misadjusted choke cable, mixture screw adj. or float level.