Strange dying issue....Take a shot..

[steve260z]

Ok..Have a stroker with triple Weber 40s, Datsun distributor, 34 degrees all in, carter low pressure fuel pump, plugs look good, brand new fuel filter, dead headed carbs, old return line vented to atmosphere, Crane ignition module...Think thats most of important parts, just ask for whatever I've neglected to mention.

After 10 minutes of driving, when I come to a stop the car will act like its going to die or will die. This occurs about 50% of the time. If this does not occur I can let the car idle in the driveway and it will start acting like its going to die and will eventually within the next 30 minutes. I can then restart in the car with about a 4 or 5 second crank. Now, if its not in this state of just dying I can turn the car off and restart with a 1 or 2 second start...I think the clues lie here in this 4-5 second crank to start. Now, I've but a voltmeter on the battery as the car dies and the meter reads a consistent 14-14.3 volts while running and dying. When the car dies I can still hear the carter fuel pump running. When it dies the O2 meter will drop down to about 10 AFR from the usually 12 at idle. Now, not sure if this richness is caused by the actual problem or just the result of low bouncing idle at the point of dying which would mess with the vacuum signal the carb sees. Obviously I'm freaking stumped. As mentioned above the 4-5 second crank time on restart is interesting. I've been thinking maybe the fuel is getting too hot, pump is crapping out (only 5K miles on it), all kind of esoteric different Weber ideas and many other ideas...

 

How about your ideas?

Thanks,

Steve

Edited May 15, 2017 by steve260z

[30 ounce]

Sounds like vapor lock to me. I wouldn't run the fuel dead headed, run a return line. It will keep the fuel cooler. Also anything you can do to shield the headers from the carbs will help too.

[steve260z]

I was thinking that too but everything I've read on Vapor Lock seems to show the car will not start back up. Mine will start back up...on the 4-5 second crank...Is that true about Vapor Lock? Also, thinking about a heat shield under the carbs. Before I do that I'd like to ID the problem first.

Edited May 15, 2017 by steve260z

[30 ounce]

I had the same issue with my 3.0 liter but I had SU carbs.  It would restart but it wasn't pretty.  I went from dead head fuel system to return line then added heat shield with heat wrap and wrapped all the fuel lines.  Every little thing helped but the return line was the biggest help.

[heyitsrama]

74' should have a return line already on the body. Upper one is for a charcoal canister, the lower one near the feed should be the return.   

[HuD 91gt]

Sounds exactly like vaporlock. Mine would start when having the same issue, but as you said would take many more seconds. I was running SUs, ended up wrapping the fuel lines in foil insulation tape. Got rid of most of the issues. Problem started after adding a non coated header. I wouldnt put another one on without having it coated or wrapping it.

Edited May 15, 2017 by HuD 91gt

[steve260z]

I do have unwrapped headers, the fuel line is rubber unwrapped. Yes, the return line is still in the body.

Another note: I checked the connections on the Crane Ignition unit. 2 or 3 going to the distributor, all good.

[BluDestiny]

Almost sounds like you're just boiling the fuel out. After you drive for 10 or so minutes, have you tried just flooring it? and does it sputter and bog out? 

[steve260z]

Yes, it does. Bogs down...runs like crap if I floor it when its acting up.

[steve260z]

Messing with it now...The Crane XR700 which is mounted on the inside fender below the coil is real hot. Say touch for 2 seconds thats all. Now, it has heat sinks built onto it, so suppose to be hot to some degree. Basically the entire header/intake side is real hot..Just fishing around..

 

 

And....my Amp meter usually has a "tick" in it...ticks to the positive side. As the RPMs dropped lower than usual and car acted like it was going to die the fuel pump was not as noisy and the amp meter did not have the "tick". I reved up back up, tick came back and pump was louder. Need to get the car up on ramps to watch the fuel going into the filter but maybe I have an issue with the relay??? The 260 came with both electric and mechanical pump. Relay maybe?

Edited May 16, 2017 by steve260z

[HuD 91gt]

I was amazed the difference the uncoated headers made vs. the stock manifold.   Everything was hot on that side of the engine as well.   

[steve260z]

Well, here we go. Yes, the car dies. But, the symptom is dying but not the problem. The problem is the idle slowly reduces and the car eventually dies. So, what is causing the slow reduction of idle? Still don't know but I've at least made progress. I increased the idle speed to 1K and it still worked its way back down to 600 and died.

 

For the hell of it I just changed the dead head to a return line system. Maybe the gas in the line is getting too hot. I'll see shortly

[HowlerMonkey]

Didn't the dealerships perform a recall where they installed thick heat insulators over the fuel lines of the 260z?

[steve260z]

When my car was stock, yes it had those.

[steve260z]

Ok. I've laid down a basic return line to see the results. So far so good. The rubber fuel line certainly feels cooler. Was pretty hot. I had the line running in front of the motor, the fan blowing on it with hot air. Its just a quick fix. If this works I"ll do a more professional job. Here's a quick video. 

 

[steve260z]

Well, that didn't solve the problem.

[NewZed]

Nissan addressed a similar problem in 1974.  They defined another phenomenon that they called percolation.  Basically it's hot float bowls causing the fuel to evaporate, making the fuel-air mixture over-rich.  They wrote a thing about it.

 

http://www.classiczcars.com/files/file/60-fuel-system-modification-plus-73-74/

[steve260z]

Thanks, I'll have to download that paper. Percolation is what I'm currently leaning toward as the problem. In addition to fuel pump, fuel pump relay, the crane ignition unit. I just pulled the plugs again and look good. Light brown color. Now, when the car dies the O2 meter goes rich right before it dies. Drops to high 9s or low 10s.

[Tony D]

Not "vapor lock" try 'percolation" the  car comes to stop and the heat in the carb body boils the fuel in the float bowl up and out into the main venturi causing the richness.

 

"Vapor Lock" occurs in the fuel lines and starves the carbs from restarting until liquid fuel is back in and flowing into the carbs.

Heat Shield, Blanket the Headers (don't wrap!), run a return line and try to keep the underhood cooler. My issues were helped using a 160F thermostat as everything underhood ran cooler. 

There  was a car we had on the Vegas ZCON that was vapor locking, I put in a 1 gallon igloo cooler with copper coils in it, and it would take a 7# bag of ice. Completely cured everything on the highway, and in stop and go traffic. Whenever possible now, I will siamese the fuel return and feed lines with the cool AC line going to the cabin, and wrap them  in insulation. Get some heat out going into the carbs, and take it back out going back to the tank. 

[steve260z]

Thanks Tony...For 30 bucks and 10 minutes of work thought I'd try this...Read that coils can start going bad once they heat up but work fine when cold. 

So far so good..No reduction on idle so far. 

Edited July 3, 2017 by steve260z