[don.whiteside512]

Problem: engine starts fine, idles fine till very warm, then it surges and kind of sounds like it's starving for fuel.
I'll post a video later. The engine at idle will run perfectly, then rhythmically loses RPMs....the the idle will eventually lower till it dies.....This is NOT FUN when sitting at a light and you have to hold the emergency brake on, and sit with the clutch depressed and one foot on the gas pedal trying to keep it at a fast idle.

Car: 71 240Z. Original engine (block yes, head E88)
Recently purchased from owner in Albuquerque.
Mallory Electronic ignition
Pertonix coil.
Stage 2 Cam.
6 into 2 into 1 headers.
New battery

Long story short: Car engine was rebuilt in 2017, owner died of cancer 2 years later, he never really drove the car much after the rebuild (less than 200 miles.)
Engine sas original round top SUs. I bought it and drove it to Austin Texas and it did have this problem when I came down in altitude. I thought it was the carbs being set leaner for the higher altitude.
ALtitude: 5282 ft New Mexico vs. 800 ft Texas
I did recently retune the carbs per the Factory Service Manual. Still won't idle warm. Does run a lot better, when it it's off idle.

I have been going through lots of checklist items to track down the problem. The idle is weird....like a clogged fuel filter or something, only it looks new. I don't have a fuel pressure gauge, but I'm thinking about installing one.

Vacuum hoses look good and have tight connections.
Carb body screws are all tight.
Carb to manifold spacer nuts are tight.
Intake manifold nuts are tight.
Running no ethanol enhanced regular unleaded fuel.

Is there a chance the intake manifold has a crack? I've sprayed carb cleaner all around every vacuum connection on the manifold and all the hoses and carbs...virtually everywhere while it's idling and can't find a place where it makes any difference.

Where do I turn next????? I'm stumped.

Any help is appreciated.

 

[don.whiteside512]

Additional information:
Mechanical fuel pump.
There are some gas fumes inside the car at times. Seems to be from the rear where the vent/fill tubes are. Not a lot, but some. Also have engine fumes inside the car when driving.
Left front fender fresh air vent tube missing. I have ordered a new set and will install. That may be one source of engine fumes.
It looks like the car was "wrecked" at some point. Mild collision damage repair evident. Left front fender doesn't line up with headlight bucket and at driver door. Some radiator core support damage evident at circular holes on driver side, and inner front inner fender well. Looks hammered (poorly) back to "close enough for government work" specs.

There's plenty of work to do, but she's a keeper!

 

[Dave Datsun Tech.]

 

[jacobs6]

Problem: engine starts fine, idles fine till very warm, then it surges and kind of sounds like it's starving for fuel.
I'll post a video later. The engine at idle will run perfectly, then rhythmically loses RPMs....the the idle will eventually lower till it dies.....This is NOT FUN when sitting at a light and you have to hold the emergency brake on, and sit with the clutch depressed and one foot on the gas pedal trying to keep it at a fast idle.

Car: 71 240Z. Original engine (block yes, head E88)
Recently purchased from owner in Albuquerque.
Mallory Electronic ignition
Pertonix coil.
Stage 2 Cam.
6 into 2 into 1 headers.
New battery

Long story short: Car engine was rebuilt in 2017, owner died of cancer 2 years later, he never really drove the car much after the rebuild (less than 200 miles.)
Engine sas original round top SUs. I bought it and drove it to Austin Texas and it did have this problem when I came down in altitude. I thought it was the carbs being set leaner for the higher altitude.
ALtitude: 5282 ft New Mexico vs. 800 ft Texas
I did recently retune the carbs per the Factory Service Manual. Still won't idle warm. Does run a lot better, when it it's off idle.

I have been going through lots of checklist items to track down the problem. The idle is weird....like a clogged fuel filter or something, only it looks new. I don't have a fuel pressure gauge, but I'm thinking about installing one.

Vacuum hoses look good and have tight connections.
Carb body screws are all tight.
Carb to manifold spacer nuts are tight.
Intake manifold nuts are tight.
Running no ethanol enhanced regular unleaded fuel.

Is there a chance the intake manifold has a crack? I've sprayed carb cleaner all around every vacuum connection on the manifold and all the hoses and carbs...virtually everywhere while it's idling and can't find a place where it makes any difference.

Where do I turn next????? I'm stumped.

Any help is appreciated.

Is it worse whet outside temperature is high. On a cool day does it run near normal?

 

[don.whiteside512]

You Tube Cold Idle video

Hot Engine Idle--beginning to stumble at idle

 

[don.whiteside512]

Is it worse whet outside temperature is high. On a cool day does it run near normal?

I can't say. I have only had the car a few weeks. I believe the problem is not outside temperature related.

 

[don.whiteside512]

View attachment 112746

OK! I will try it NOW and let you know. Great suggestion! And, if that is the case, what is the solution to this "stuck" issue?

 

[don.whiteside512]

OK. I operated the choke and observed the jet. Then I took the choke cable loose from each carb and moved the choke mechanisms separately. Neither one seemed to stick in the up or down position and seemed to move relatively easily. So, now we have eliminated that as a possible problem.

 

[steve240Z73]

Check your carburetor oil. If it's low, you can get 3-in-1 20wt oil from Ace Hardware or Amazon.

Also, you might be running lean due to an air leak. You could use an unlit propane torch around the intake to see if it changes the engine speed.

 

[jacobs6]

Problem: engine starts fine, idles fine till very warm, then it surges and kind of sounds like it's starving for fuel.
I'll post a video later. The engine at idle will run perfectly, then rhythmically loses RPMs....the the idle will eventually lower till it dies.....This is NOT FUN when sitting at a light and you have to hold the emergency brake on, and sit with the clutch depressed and one foot on the gas pedal trying to keep it at a fast idle.

Car: 71 240Z. Original engine (block yes, head E88)
Recently purchased from owner in Albuquerque.
Mallory Electronic ignition
Pertonix coil.
Stage 2 Cam.
6 into 2 into 1 headers.
New battery

Long story short: Car engine was rebuilt in 2017, owner died of cancer 2 years later, he never really drove the car much after the rebuild (less than 200 miles.)
Engine sas original round top SUs. I bought it and drove it to Austin Texas and it did have this problem when I came down in altitude. I thought it was the carbs being set leaner for the higher altitude.
ALtitude: 5282 ft New Mexico vs. 800 ft Texas
I did recently retune the carbs per the Factory Service Manual. Still won't idle warm. Does run a lot better, when it it's off idle.

I have been going through lots of checklist items to track down the problem. The idle is weird....like a clogged fuel filter or something, only it looks new. I don't have a fuel pressure gauge, but I'm thinking about installing one.

Vacuum hoses look good and have tight connections.
Carb body screws are all tight.
Carb to manifold spacer nuts are tight.
Intake manifold nuts are tight.
Running no ethanol enhanced regular unleaded fuel.

Is there a chance the intake manifold has a crack? I've sprayed carb cleaner all around every vacuum connection on the manifold and all the hoses and carbs...virtually everywhere while it's idling and can't find a place where it makes any difference.

Where do I turn next????? I'm stumped.

Any help is appreciated.

It appears that the problem is heat related to the carburetors from the exhaust headers. I am very familiar with this problem. Would prefer to discuss by telephone. If you wish call me at 610-268-0565

 

[don.whiteside512]

It appears that the problem is heat related to the carburetors from the exhaust headers. I am very familiar with this problem. Would prefer to discuss by telephone. If you wish call me at 610-268-0565

Jacobs6---Tried to message you last night at that number to arrange a phone call at your convenience. So, since that didn't work, I'll ask here: What time works for you? And, thank you so very kindly in advance! That is really nice of you to take time to help a stranger!

 

[don.whiteside512]

OK, so now I have observed some more things about this situation. It bugged me that this car has in place a mechanical fuel pump AND as I knew but had not located until a day or so ago, a small electric fuel pump (noisy little bugger) back by the fuel tank attached to the differential support brace.
So, I am going to replace the fuel filter on that electric pump to see if the problem is a 52 year old gas tank that sat for several years which may have caused some problems with fuel gumming or sediment or debris. I am also going to remove the mechanical fuel pump and insert a blocking plate in that area. The will leave me a nice area to attach a fuel pressure gauge in the future if I want to....
Additionally, after researching vapor lock some, it would appear the addition of headers may have a tendency to overcome the ability of the small original heat shield that is in place for these carburetors.
And to add insult to injury, fuel blends have changed over the years, and the newer blends may (although I don't have the scientific research behind this assumption) tend to vaporize at a lower temperature. But, there is a fuel filter that provides for a supplemental return line off of it and I'll install one of those to make sure there is a place for vapor to go back to the tank.
So, there's a lot to unpack here, but I think it's fun learning about all this stuff and hopefully this post might help someone else to figure their own car problem out by reading this....hahahahahaha----Probably not.

 

[steve240Z73]

I think you're on the right track. You could use something like this on the fuel rail: https://www.amazon.com/gp/product/B013DPBB56
Also, if you have ethanol free gas in your area, that might help. Pure-gas.org - ethanol-free gasoline in the U.S. and Canada

 

[Zooter71]

OK. I may have found the problem. I did order a replacement fuel filter for the electric fuel pump. And I did pull the fuel lines off the mechanical fuel pump and discharged the fuel from it. Then I checked the suction on the mechanical....what I found was virtually no suction. So, easy decision: Remove mechanical fuel pump, order and install a block off plate, replace electric fuel pump filter and make sure it doesn't leak since I found that the other was leaking, and it seems like the problem of idling is 80 percent corrected. I think I need to go back now and tune the carburetors since I tried tuning them with fuel delivery issues. A factory manual re-tune and this problem should be solved. Thanks to all who provided advice and listened to a newbie rant and flail around helplessly!!!! It's been fun though! Learned a lot....like, in the process I learned that I need a new fuel sender and wires. The 52 year old wires and sender were at best rickety. So I ordered new and will install over this coming weekend. The car is coming along nicely folks.