vapor locking on 73 with 71 carbs PERCOLATION!

[hls3073z]

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.

[Zed Head]

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.

[heyitsrama]

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...


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[87mj]

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. 

 

[bartsscooterservice]

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.

[87mj]

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. 

 

[hls3073z]

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.

[hls3073z]

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.

[bartsscooterservice]

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

[87mj]

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.

[Burl]

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 July 15, 2018 by Burl

[hls3073z]

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.