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Non stable Idling and sudden engine shut off on my 240z

Old 03-29-2015, 07:03 AM
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Non stable Idling and sudden engine shut off on my 240z

Recently installed a Schneider 274F cam, with new valve springs, lashpads and rocker arms. I also sent my round top Hitachi SU carbs for a complete remanufacture to Bruce Palmer at ZTherapy. He did an incredible job. I am really satisfied.


My mechanic here in Panama (33 degrees Celsius) carried out the installation and tuning of the cam kit, SU carbs and replacement and tuning timing chain to 10 degrees. However, i am still not pleased with the idling as it is not perfect. I have returned the car to him 2 times as the idle is not mantained in 1000 RPM, it fluctuates to 900 or could rise to 1,050 to 1100.


If i am stuck under traffic or leave the engine on for 7 to 10 minutes the car starts decreasing its idle from 1000 RPM to 950 or 900 RPM. So i decided to swap the ATF oil with Castrol 10w 30 engine oil which seems to be a thicker oil. RPM still drops and sometimes engines shuts off if i step on the brakes harshly. Sprayed Carb cleaner to see if there were any leaks and could not find any. I am just going nuts with this erratic idle and sudden engine shut off.


Really need help!!!!
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Old 03-29-2015, 11:37 AM
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Originally Posted by jalexquijano View Post
If i am stuck under traffic or leave the engine on for 7 to 10 minutes the car starts decreasing its idle from 1000 RPM to 950 or 900 RPM. So i decided to swap the ATF oil with Castrol 10w 30 engine oil which seems to be a thicker oil. RPM still drops and sometimes engines shuts off if i step on the brakes harshly. Sprayed Carb cleaner to see if there were any leaks and could not find any. I am just going nuts with this erratic idle and sudden engine shut off.


Really need help!!!!
While you checked for vacuum leaks, it might be your brake booster leaking internally. I base this on engine stopping when you hit the brakes harshly. A simple test would be to disable the brake booster by disconnecting it at the vacuum source (seal off the source of course) and see if the problem goes away.
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Old 03-29-2015, 12:20 PM
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You could have a weak fuel pump. next time it shuts down like that drop the fuel bowl on the carbs to see if there is gas in them. My car did that and I noticed the linkage was sticking and the front carb was running out of gas.
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Old 03-29-2015, 07:40 PM
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Originally Posted by theramz View Post
You could have a weak fuel pump. next time it shuts down like that drop the fuel bowl on the carbs to see if there is gas in them. My car did that and I noticed the linkage was sticking and the front carb was running out of gas.
Well i managed to fix the problem by pulling each of the NGK BP6ES spark plugs, cleaning them with a wire brush and electronic contact cleaner, spraying contact cleaner to each of the terminals of the distributor cap and pertronix ignitor. I also filled up a Little bit more of Castrol 10w 30 engine oil in each of the carbs. Retuned each carb and balance them raising the fast idle screw to 1,500 RPM.



I set the idle mixture ***** in each carb to 3.5 turns and the idle RPM to 1,000 RPM.



At the moment no backfire or engine shut down. I will keep testing tomorrow and keep you posted! I need to sort this idle issue first before i install the AC system.



Question: Are these NGK BP6ES spark plugs pregapped from Factory? or will i need to get a feeler gauge? If so what is the gap?
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Old 03-29-2015, 09:33 PM
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As you're finding out, you have to check everything. I have changed plugs for many years and although they come gapped I found one out of 200 that the gap was closed to about .020.
That adjustment on the carbs varies with altitude. I'm at 4500 elevation and I am about 2 3/4 turns. You really need to have the exhaust analyzed. BTW high lift cams are supposed to be idled higher. 1000 rpm is probably about right.
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Old 03-30-2015, 02:19 PM
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Originally Posted by theramz View Post
As you're finding out, you have to check everything. I have changed plugs for many years and although they come gapped I found one out of 200 that the gap was closed to about .020.
That adjustment on the carbs varies with altitude. I'm at 4500 elevation and I am about 2 3/4 turns. You really need to have the exhaust analyzed. BTW high lift cams are supposed to be idled higher. 1000 rpm is probably about right.
I am at sea level. I will have the exhaust checked anyway as i believe the backfire popping was not coming from the carb intake but from the manifold to exhaust intake.

What will be the perfect gap measurement for each of the 6 NGK BP6ES spark plugs?

Please confirm!
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