vapor locking on 73 with 71 carbs PERCOLATION!

[hls3073z]

I've had this car since '76, it was a daily driver a long time etc. then sat around so now I'm into a restore.

From day 1 in 1976  its had vapor locking issues, I live in the Austin area and 100deg days are quite common.  The symptom is you drive somewhere on a hot day, leave it parked for 15 minutes, come back and try to take off.  It jumps and bucks a  but after 5 minutes of driving things smooth out.  Stop at traffic light for a while and it  might start bucking again at takeoff.  I do have an AC, gotta have that.

I have done a lot to help this problem.

removed mech fuel pump (you could hear fuel bubbling in there) electric pump was there but not wired up properly, so I fixed that.

rerouted the fuel lines

check carbs for proper float level

put an extra shield under carbs above exhaust manifold

replaced standard air filter with separate oval round filters.

Finally the thing that helped the most is that I switched to a metal flex fan.  It pushes A LOT of air at low RPM.  Although loud it makes the most difference.  

So now, I've got the engine out and looking things over to see what else might be done to keep those carbs and fuel cooler.  Any advice appreciated.

On my list is

install one of those ZX injector  fans that mount over the spark plugs with ducting over to the carb area.

go with electric fan(s) for the radiator.

what else?

Edited July 15, 2018 by hls3073z

[jonathanrussell]

As you have indicated, it is a combination of things. One thing not on your list that I find helps is to make it so that the metal fuel rail doesn't make metal to metal contact with the engine (or anything that mounts to the engine). Rig rubber grommets at the mounting points (engine side and bolt head side). Have also thought about eliminating fuel rail in favor of all rubber fuel lines. Only thing that stops me is loss of the original look. 

You could ceramic coat the exhaust manifold too. I haven't tested this but am doing it on a car I am putting back together so it is mostly theory.

 

[siteunseen]

Mine has the ceramic coated header from MSA. Greatly reduced the under hood temps. I just found a point and shoot thermometer at work and am anxious to get some readings now that we are in the dog days of summer now. Might try to do that tomorrow and post the results?

[hls3073z]

I'll look into the fuel rail connect points.  I have it reversed since it doesn't have to go up front to a fuel pump.  The other thing I'm considering is to make that exhaust manifold shield more effective by making another sheet just like it, layering them together and spraying foam insulation in between making sandwich kind of construction.  I have sprayed that insulation around one of the fuel bowls.  With engine out it will be a lot easier to put some things in place.

 

I wonder how they handled this with turbo installations?  That had to get super hot on that side.

[Zed Head]

Have you seen this Nissan document? https://www.classiczcars.com/files/file/60-fuel-system-modification-plus-73-74/

 

Nissan turbos used fuel injection.  But some still have heat soak problems with the injectors.

[hls3073z]

got the doc.  great.  tks.  I wish I had this doc 40  years ago, not kidding.  Most of my Z had all the epa plumbing removed when it had the carbs replaced by the 71 versions before I bought the car.

One  big suggestion is the performance hood.  I'd hate to cut up my hood but if I could get another I would try that.  I'm trying to remember, did the 280Z FI versions have vents on top the hoods, the ZXs did?

The 81ZX I drove to death never had a vapor locking problem, even when using the AC.  It was a great boulevard cruiser, cold ac, soft ride, etc.

[Zed Head]

The vented hood didn't show up until 1977, on the 280Z.

One thing that I haven't seen tried is insulating the carburetor itself.  There is lots of surface area, especially the domes, and it will absorb both radiant and convective heat.  It's aluminum so will conduct that heat rapidly to the surrounding metal, down to the float bowl.  If it was mine, because I'm not really concerned with appearance, I'd wrap some sort of insulation around that dome for starters.  If it helps, consider insulating the bowl also.  I've used that metalized bubble material on my EFI system and it works well.  It's plastic (Mylar [PET], I think) but withstands the heat well except for the really hot spots, where it has crinkled up.  Some zip ties and scissors and you're set for a quick trial run.

[heyitsrama]

Does the car have a fuel return line? I thought allowing the gas to flow removes the opportunity for it to get heat soaked. It seems that people also run those insulated asbestos lines over the fuel rail. That should also help prevent it from soaking heat.

 

[hls3073z]

Yes, it does have a return line.  You are correct that it should remove the opportunity  for the fuel to get heat soaked in the lines, but if you stop and park the fuel stops circulating so that fuel gets soaked too.   I even had the problem with idling  at a long red light on hot summer day.  I did see something in that doc about changing the fuel flow circulation in the fuel lines.  Once you get rid of the mech fuel pump there is no reason to keep those lines going up front but I just never decided to cut them off and reroute near the firewall.  Too much plumbing like work.  I did run some pipe wrap anti freeze insulation over most of the lines.

 I think that  most of the problem is in those carb bowls that hang right over the  exhaust manifold.  I suspect that the fuel is boiling in there or in those hoses that go to the jets of the SU's.  Since the engine is now sitting out in the open I can get creative with some shielding.

[heyitsrama]

Yes, it does have a return line.  You are correct that it should remove the opportunity  for the fuel to get heat soaked in the lines, but if you stop and park the fuel stops circulating so that fuel gets soaked too.   I even had the problem with idling  at a long red light on hot summer day.  I did see something in that doc about changing the fuel flow circulation in the fuel lines.  Once you get rid of the mech fuel pump there is no reason to keep those lines going up front but I just never decided to cut them off and reroute near the firewall.  Too much plumbing like work.  I did run some pipe wrap anti freeze insulation over most of the lines.

 I think that  most of the problem is in those carb bowls that hang right over the  exhaust manifold.  I suspect that the fuel is boiling in there or in those hoses that go to the jets of the SU's.  Since the engine is now sitting out in the open I can get creative with some shielding.

 

 

Here’s a heat shield I saw the other day, I liked the layer approach. I just noticed the use of that reflective layer under, I wonder how much that helps reflect radiant heat.

 

[bartsscooterservice]

Even with 100 degrees, it shouldn't happen. I think this has more to do with a ( proper tune/some parts not functioning correctly/quality of the gasoline ), then so called vapor lock.

I've driven my z in 40+ C days ( 105 F ), without any issues. I do only use ethanol free fuel...

Vapor Lock is air in the system, there's an underlying problem you need to find.

 

 

[hls3073z]

5 hours ago, heyitsrama said:

 

 

Here’s a heat shield I saw the other day, I liked the layer approach. I just noticed the use of that reflective layer under, I wonder how much that helps reflect radiant heat.

 

I was thinking of doing the same thing except instead of that metalic foil  sandwich in between I was planning to spray that expanding foam insulation in there.  It would require some screw spacers before spraying in the foam but then afterwards the screws could be removed.  At the moment I have a glob of that foam around one of the carb bowls.

BTW, this problem existed even when I used good old leaded fuel, so ethanol is not the problem.  I do hate ethanol, don't get me started.