VAPOR LOCK: SUDDEN RPM DROP FROM 950 to 500 ON 240z - Nissan : Datsun ZCar forum :Nissan Z Forum: 240Z to 370Z
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post #1 of 33 (permalink) Old 02-06-2017, 04:43 PM Thread Starter
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VAPOR LOCK: SUDDEN RPM DROP FROM 950 to 500 ON 240z

My 1972 240z with 2 SU CARBS is still presenting sudden rpm drop when left at idle for more than 20 mins. I was advised that replacing the fuel rail with rubber hoses will solve this issue.


In conclusion, I still have not swapped the original Steel fuel rail that i have wrapped with heat reflective cloth. However, i did replaced a 170F Original Nissan Thermostat with 160F Stanton Thermostat to make the car run cooler. Left the car at idle after driving for 20 mins. Result: RPM started to drop randomly from 950 to 500 RPM tending to stall but never stalls. My father says that since i dont have the stock cam but a Schneider 274F Cam, the idle will never remain as stable on any traffic. Ill give you a Little more of details of what my car has:
L26 crank which makes my motor 79 mm stroke instead of the stock 73.7 mm
stock bore is 83 mm for the L24 and the L26 motors
my block has been bored out 1 mm to compensate for wear
that makes my motor a 2.627 liter motor, My E88 head from 1972 will give you about 8.7 to 1 compression.
Schneider 274F Camshaft, Pertronix electronic ignitor (no points), Pertronix 1.5 ohm flamethrower coil,
http://schneidercams.com/274FL6.aspx
Airtex 8012S 4.0 to 4.5 psi electronic fuel pump. Its really pusing 4 to 4.5 psi.


Just give me your opinión on the random rpm drop at idle. I also notice that as the rpm drops the ahmeter drops from 45. I have a 60 amp alternator.
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post #2 of 33 (permalink) Old 02-06-2017, 08:07 PM
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Ditch the corn gas. It's boiling at 155 degrees F. Run real gas. Puregas.org
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post #3 of 33 (permalink) Old 02-06-2017, 11:27 PM
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Ditch the corn gas. It's boiling at 155 degrees F. Run real gas. Puregas.org
OP is in Central America, IIRC Panama. Not sure what kind of gas is available there.

Scott
Tucson, Arizona

9/74 260Z VIN RLS30 060061
Restored!
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Isn't it "warm" in Panama? Sitting at idle for 20 minutes is going to allow a lot of heat to build up around the rail in the relatively stagnant engine compartment, as compared to a moving vehicle which will push and "draft" additional air thru the engine area. With the fancy-dan cam any irregularity in idle may be accentuated.
How does it run moving down the road?

The fan clutch is set up to kick off and on based on air temp coming thru the radiator for a factory thermostat. Is the coolant temp holding at 160* at idle with the new 'stat? That is to say, is the fan doing the job operating at it's factory on-off engagement cycle or are you still running up near 170*?

zippityzda

Last edited by dumbestone; 02-07-2017 at 02:00 AM. Reason: add fan clutch
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post #5 of 33 (permalink) Old 02-07-2017, 09:16 AM
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I'm not inclined to think you have a vapor lock problem here. Vapor lock is most evident when you shut the car off while it's hot and then it won't restart. While the car is running, the pump is constantly recirculating fuel through the rail; the excess fuel flows back to the tank through the return line; it's not just sitting still there getting hot. I'm more inclined to think that your spark plugs are beginning to foul up when you run at idle for so long, and that you are burning them clean as soon as you start running at normal loads and speeds again. It's not at all uncommon to need a different plug when you go to a different cam. I suggest you try a hotter plug. You can check for this problem pretty easily....idle the car until the problem appears, then shut the car down and pull the plugs and have a look.
If you don't understand the difference between "hot plugs" and spark gap and voltage, I suggest you do some research. A hotter plug has a different insulator design and stays hot longer, which prevents fouling. It's not a reference to gap size or voltage.

73 240Z very orange
Dual Weber DGV's, header and 2.5 exhaust
Electronic dizzy with G.M. HEI module
Tokico Struts/Springs, Full Poly U bushings
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post #6 of 33 (permalink) Old 02-07-2017, 10:29 AM Thread Starter
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I'm not inclined to think you have a vapor lock problem here. Vapor lock is most evident when you shut the car off while it's hot and then it won't restart. While the car is running, the pump is constantly recirculating fuel through the rail; the excess fuel flows back to the tank through the return line; it's not just sitting still there getting hot. I'm more inclined to think that your spark plugs are beginning to foul up when you run at idle for so long, and that you are burning them clean as soon as you start running at normal loads and speeds again. It's not at all uncommon to need a different plug when you go to a different cam. I suggest you try a hotter plug. You can check for this problem pretty easily....idle the car until the problem appears, then shut the car down and pull the plugs and have a look.
If you don't understand the difference between "hot plugs" and spark gap and voltage, I suggest you do some research. A hotter plug has a different insulator design and stays hot longer, which prevents fouling. It's not a reference to gap size or voltage.
OK. I think that with your help and other members i will solve my problem. To give you some more information:


1. Sparkplugs i am using are NGK BP6ES, should i purchase NGK BP5ES as my camshaft is a Schneider 274F and not stock camshaft?


2. I installed a couple of Stainless Steel heat shield extensions to cover the float bowls section on both front and rear su carbs.


3. I have the NGK stock spark cables (blue ones). Should i upgrade to magnecor?


The symptom you are mentioning is what is happening: Car sitting under the sun, weather here in Panama at 33° C, more tan 20 mins in traffic. Result: Spark plugs get black. Race the car again and they get cleaned up!


Do you really think i should start with replacing the metal fuel rail which looks better than those rubber hoses lying around the cam cover? Please guide me, i am getting there!
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post #7 of 33 (permalink) Old 02-07-2017, 10:34 AM Thread Starter
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Originally Posted by dumbestone View Post
Isn't it "warm" in Panama? Sitting at idle for 20 minutes is going to allow a lot of heat to build up around the rail in the relatively stagnant engine compartment, as compared to a moving vehicle which will push and "draft" additional air thru the engine area. With the fancy-dan cam any irregularity in idle may be accentuated.
How does it run moving down the road?

The fan clutch is set up to kick off and on based on air temp coming thru the radiator for a factory thermostat. Is the coolant temp holding at 160* at idle with the new 'stat? That is to say, is the fan doing the job operating at it's factory on-off engagement cycle or are you still running up near 170*?
It runs fine down the road! i replacer the OEM 170° F nissan thermostat with a Stanton 160°F thermostat to keep the car running at lowe temp i also plugged and removed the heat control manifold. We dont need that extra heated coolant flowing through the rear of carburetors.
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post #8 of 33 (permalink) Old 02-07-2017, 08:42 PM
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Spark plugs getting black with extended idle but burning clean with normal running is indeed an indication that you need a hotter plug; I recommend the BP5 like you said. You ask about replacing the plug wires as well, which is confusing the heat range of the plug with the voltage...don't confuse the two! Try the hotter plug.

73 240Z very orange
Dual Weber DGV's, header and 2.5 exhaust
Electronic dizzy with G.M. HEI module
Tokico Struts/Springs, Full Poly U bushings
Daily driver, all work done by me
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post #9 of 33 (permalink) Old 02-08-2017, 05:22 PM Thread Starter
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Spark plugs getting black with extended idle but burning clean with normal running is indeed an indication that you need a hotter plug; I recommend the BP5 like you said. You ask about replacing the plug wires as well, which is confusing the heat range of the plug with the voltage...don't confuse the two! Try the hotter plug.
Ok. Just replaced the spark plugs with NGK BP5ES and ditch the BP6ES as i found Spark plug 3, 4 and 5 sooty and considering i had left the car idle for 20 mins at the garaje. After replacing all the plugs and racing the car for 15 mins came back and pull all the plugs out. All were fine but plug 4 looked a Little bit sooty. Both carbs are adjusted to 3 turns clockwise.
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post #10 of 33 (permalink) Old 02-09-2017, 12:20 PM
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Both carbs are adjusted to 3 turns clockwise.
The specific number of turns your carbs are adjusted is relevant only because it tells us you don't understand how carbs are adjusted.

Have you balanced the flow between the carbs?

Did you adjust each carb to get the mixture right?

Carb adjustments can vary all over the place. Some of the things that affect the adjustments are:
The carb's condition
Air pressure (mostly the altitude where you are)
Fuel
Condition of your motor/intake
And so on.

There are a lot of videos and posts that describe the carb adjusting procedure. Take a look.

Scott
Tucson, Arizona

9/74 260Z VIN RLS30 060061
Restored!
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