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hls3073z

vapor locking on 73 with 71 carbs PERCOLATION!

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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 by hls3073z

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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.

 

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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?

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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.

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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.

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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.

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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.

 

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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.

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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.

 

Image1531207585.108298.thumb.jpg.803dd26dd084ec0b574b14926c682c11.jpgImage1531207595.205207.thumb.jpg.3a373a753397696ba6e31704d0172fd8.jpg

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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.

 

 

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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.

 

Image1531207585.108298.thumb.jpg.803dd26dd084ec0b574b14926c682c11.jpgImage1531207595.205207.thumb.jpg.3a373a753397696ba6e31704d0172fd8.jpg

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. 

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6 hours ago, bartsscooterservice said:

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.

 

 

There must be something fundamentally different about your fuel/carb setup.  If you read the doc listed at the end they suggest that in the rare situation where the customer drives in 100deg weather you may need an aux fan and then they go on about how to set that up.  Perhaps the return line fuel flow limiter was changed.  In any case I think the basic problem is that heat exposed to that small hose that connects from the bottom of the bowl over to the carb jet is one source of the problem.  See photo attached.  I've also attached what I have done to the rear carb and my first pass heat shield.  I did all this with the engine in the car so I was somewhat limited.

HosetoJet.jpg

carbBoalInsul.jpg

CarbShield.jpg

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9 hours ago, bartsscooterservice said:

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.

Nissan thought there was a problem.  They wrote a whole book about solutions in August 1975.

image.png

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Uhhhhh so, those evap lines look like they are being pinched, if that’s the case the vapor won’t be able to escape. An electrical fan is starting to make more sense, at least you can throw a temperature timer on it...


Sent from my iPhone using Classic Zcar Club mobile

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I had the same problem back in the late '70s with my '73.  I still had the flat top carbs though.  I was never able to figure it out. Then I sold the car and got a nasty phone call from the buyer a couple days later. 

 I know this sounds too simple but have you tried making sure your fuel lines are tight and then switch to alcohol free gas?  The ID of my fuel lines going from my rail to my carbs are very difficult to remove even with the clamps off. 

 

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Yes, seeing those pictures, the state those parts are in ( if that is yours or just an example pic ?! ). Make sure everything is in good condition first.  Like 87mj says about the fuel lines make sure the fight tight, and the hoses are black and new, ethanol free fuel helps to.

I work with carbs all the time for my work ( bike carbs, but the act the same as SU's.. ), the problems ethanol gas ( although it's still only max 10% here ) can give are huge.

Everything in the fuel system from tank to carb, needs to be clean, free flowing, and air tight, or else you will have problems.

Do you have the spacer ( isolator ) between the fuel pump ? Mine was missing to with the previous owner, I just noticed it when rebuilding the engine.

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I know this sounds like a solution coming from a simpleton (which it is), but I wonder what would happen if you installed a larger capacity clear fuel filter in various places so you could actually see what was going on.  For example, right before the engine, then between each carb and the fuel rail.  Maybe even on the return line.

I swear when it happened to me, my mechanical fuel pump was not sucking gas from the tank.  I concluded the fuel pump was bad so I replaced it but there was no difference. I recall putting the fuel hose in a glass jar of gas and it still didn't suck gas (when the sputtering occurred). When the engine was cold, it ran great.  Keep in mind this is coming from the mouth of what was a dumb 16 year old kid but it still haunts me to this day. 

 

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Well at least you didn't use your mouth to try to suck fuel into the hose.  That has been done by men older than 16.  

It got dramatically better when I bypassed the mechanical fuel pump.  Actually, that and the flex fan made it acceptable but not perfect.  I can see the fuel in the fuel filter and it doesn't feel warm so I think the fuel is cool enough.  Right now I"m going to focus on that tiny fuel hose that connects between the bowl and jet at the bottom of the SU carb.  Thats a small amount of  fuel  in that hose sitting right over that hot exhaust manifold.  It must be boiling in there.   There is some clear motorcyle fuel line that might work instead and it may be possible to wrap insulation over that.  Only problem is that it has to be flexible  for the choke to work.

My goal is to eliminate the need of that flex fan.  It is very LOUD.  Not only that but at low rpm it steals just enough HP that it is noticeable on takeoff.

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On 7/10/2018 at 2:31 AM, bartsscooterservice said:

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.

 

 

Bart, I see that you are in the Netherlands.  There is a good chance that the design of the car in Europe was different enough to avoid this problem.  Are you sure that it ever gets to 100 deg in the Netherlands? 😁  It is that way almost every day here.  Nissan definitely knew that there was a problem that was the reason that they added the electric fuel pump and published that document.  Its also probably the reason that they decided to design  in that separate cooling fan for  the fuel injected Zs.  They didn't want the problem to happen again.

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4 hours ago, hls3073z said:

Bart, I see that you are in the Netherlands.  There is a good chance that the design of the car in Europe was different enough to avoid this problem.  Are you sure that it ever gets to 100 deg in the Netherlands? 😁  It is that way almost every day here.  Nissan definitely knew that there was a problem that was the reason that they added the electric fuel pump and published that document.  Its also probably the reason that they decided to design  in that separate cooling fan for  the fuel injected Zs.  They didn't want the problem to happen again.

No, only in summer. But I have a US spec 240Z..

I think it has more to to with the state of the parts/ethanol free fuel though

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Just a thought but what if it isn't vapor lock at all.  Maybe the choke needs adjusted.  Maybe it doesn't fully disengage?  Someone obviously messed with it when they swapped the carbs.

I have a very early series 2 car with the old style center console.  A PO swapped the transmission with the later style which moved the shift lever so my console no longer fit.  My fix was to install a later series 2 console with a series 2 choke.  Right after I did that, it sputtered on me.  It eventually worked itself out but I was freaking out anyway. I thought I was reliving 1979 all over again.

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Also in Austin TX and experienced a similar problem up until last month.  I had been filling up the gas tank every time I got gas at a Shell station. .  Driving only 25-30 miles a week left a lot of gas in the tank.  It would take me 3 months to fill up again.   With the ethanol blend we have these days  I decided to run the tank down to only a couple of gallons.  Instead of filling up I only put 3-4 gallons in with some ethanol additive and changed to Phillips 66 so the gas didn't get old and break down.  Haven't had the problem since and ran the car yesterday in the heat.  I still have the mechanical fuel pump, with AC but added a radiator shroud.  So far so good 🙂

 

Edited by Burl
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On 7/10/2018 at 2:31 AM, bartsscooterservice said:

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.

 

 

Bart, I see that you are in the Netherlands.  There is a good chance that the design of the car in Europe was different enough to avoid this problem.  Are you sure that it ever gets to 100 deg in the Netherlands? 😁  It is that way almost every day here.  Nissan definitely knew that there was a problem that was the reason that they added the electric fuel pump and published that document.  Its also probably the reason that they decided to design  in that separate cooling fan for  the fuel injected Zs.  They didn't want the problem to happen again.

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1 hour ago, Burl said:

Also in Austin TX and experienced a similar problem up until last month.  I had been filling up the gas tank every time I got gas at a Shell station. .  Driving only 25-30 miles a week left a lot of gas in the tank.  It would take me 3 months to fill up again.   With the ethanol blend we have these days  I decided to run the tank down to only a couple of gallons.  Instead of filling up I only put 3-4 gallons in with some ethanol additive and changed to Phillips 66 so the gas didn't get old and break down.  Haven't had the problem since and ran the car yesterday in the heat.  I still have the mechanical fuel pump, with AC but added a radiator shroud.  So far so good 🙂

 

In all fairness its been 3+ years since I drove the car.  I'm going on experience to what it did even before ethanol came out.    As I mentioned before way back in the late 70's I switched to a flex fan and that helped plus I had no more water pump failures.  But on extra hot days like lately it  would stumble on take off from traffic lights.  As others mentioned Nissan wrote an extensive document about the  problem so its well known.

It could be that that when my 73 was converted to 71-72 round top carbs that something was left out that makes the percolation worse.  Coolant no longer flows thru that chamber, I think that would be beneficial in reducing it but who knows.  For the moment I've got the engine out and I'm going to repaint the engine bay so this percolation problem will have to wait.

Regarding the ethanol.  If you go to the Walmart Murphy gas at 1431/Parmer, also at 620/183  they are selling ethanol free gas now.  I have been buying it for my boat and all my lawn mowers etc.  What part of town do you live?   

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