Vapor lock questions for the hotter climate guys

[jalexquijano]

I guess i will just then first try with good fuel hoses and clamps! Also i forget to mention i did lower the fuel mixture to 3 turns each carb approximately. Ill try the vehicle tonight and if the problema still continues then remove the fuel rail and try with a good fuel hose.  its weird since i wrapped the fuel rail with isolation cloth!

Edited October 24, 2016 by jalexquijano

[sweatybetty]

that cloth is a double edged sword. it can also keep the fuel hot

[zKars]

18 hours ago, Zed Head said:

The image attached is how Nissan explained it, in their 73-74 Fuel system Modification Plus manual.

 

Many ways of saying the same thing. Too much fuel in the bowl, getting the static level above the bridge height, causes raw fuel to be siphoned into the carb venturi. How the bowl gets too full can only be from pressure that forces the needle valve open. The argument that evap empties the bowl some, opening the needle valve, which allows fuel in to over fill is silly. The float would rise and stop it from exceeding normal height, unless, again, the inlet pressure was above the needle seat hold pressure. 

Simple test to put the old external fuel level indicator on the drain plug, get it hot, turn it off, and watch the level in the bowl. Also a pressure gauge on the line after/pump pre-carb would show you if it does get "high" enough to force the needle seat.

If none of that is going on, then it's something BLOCKING flow into the bowls preventing the car from starting (ie too lean, way way too lean, like none!). This could only happen if evolved (boiled gaseous gasoline) in the line sits in a high spot blocking liquid fuel flow. Again, float level indicator would show an EMPTY bowl not refilling during cranking during attempted restart. Line pressure gauge would read very low or no pressure.

Finding the cause of your problem is a matter of careful and methodical testing. 

 

[Zed Head]

My mental summary ends up at "hot carb boils or vaporizes fuel in bowls, boiling fuel leaves via jets, unmetered".  The basic principle of the carb's float bowl is a placid calm pool of fuel pushed in a controlled fashion through precisely sized orifices.  Bubbling, vaporizing, recondensing fuel screws everything up.

But the basic idea of all of these things and Nissan's obvious work, and their modifications, and writing up a whole book about it, and attempts to explain the concepts is "heat is bad for the carbs, avoid heat".  That's the message, I'd say.  Remove and avoid heat transfer to the carbs.  Heat is bad.  Heat is the enemy.  See a path for heat to get to the carbs - remove it.  Is more testing necessary?  Heat, heat, heat, heat...

Just joking around.  Seriously though - the key is to remove the things that transfer heat to the carbs. 

[jalexquijano]

Okay! so what will be your recommendation to solve this? Just replace the metal fuel rail with a normal fuel hose and clamps?

[Zed Head]

On 10/15/2016 at 7:40 PM, Jeff G 78 said:

My endurance racer 260Z with round tops was a disaster until I rerouted the fuel line and used all rubber.  We tried an electric pump, return line, no return line, heat shielding everywhere, and even running without the hood.  None of the changes helped under racing conditions.  I even swapped to a different set of carbs mid-race with no improvement.  Finally in one race, I pitted and installed a rubber hose, bypassing the steel rail.  The car ran flawlessly for the next 9 hours of the race.  We fought back from 35th place to 9th place.  I am convinced that the only real fix is eliminating the heat transmitted through the fuel rail mounts.  The heat shielding, electric pump, and hood venting likely help, but only the rubber fuel supply eliminated the issue.  We still run the shielding and the electric pump along with the back of the hood propped up a few inches just to be safe.  My biggest surprise was the lack of improvement with the hood removed.

Yes, that would be a place to start.  If it works you can modify it for safety and/or appearance.  Jeff G has already done the experiment for you, described above, 10 days ago.

[jonathanrussell]

On 10/16/2016 at 10:09 AM, Jeff G 78 said:

I thought about doing that, but the isolators would have to be able to handle very high temps.  The bolts would also have to be isolated, otherwise the heat would still transfer.  I'm sure it could be done, but I decided to just remove the steel rail and keep it all rubber.

Jeff,

I read through the past 3 pages and read where you suggest ditching the steel rail for rubber. Heat transfer from the mounts has always been my vapor lock cause theory and I have thought about going all rubber but have never tried.....mostly because I have never had vapor lock except on a 240z my dad had when I was a kid. How are you handling the fuel return line? In the original rail, fuel keeps moving to the return, at least to the extent that the engine isn't drinking it and the needle valve shuts, theoretically keeping the fuel cool if things slow down. Maybe in a race car / track car fuel never slows down so the return isn't needed? Any chance you can share a photo of how you have it plumbed? Sorry if you already covered and I missed.

 

 

[Jeff G 78]

Jonathan, I do not run a return line. Before I ditched the rail, I actually found that the return was part of the problem. When I clamped the return hose, I could prolong the onset of the vapor lock condition.

I don't have any pics handy but I might be able to take a few this weekend


Sent from my iPhone using Classic Zcar Club mobile

Edited October 26, 2016 by Jeff G 78

[jonathanrussell]

Thanks Jeff. Since you did not run a return line, no pics necessary, at least for me. Easy enough to visualize without the return line. Thank you.

[Jeff G 78]

No problem.  I saw zero difference in how it ran with and without a return other than the slight change in when vapor lock occurred.

[sweatybetty]

4 hours ago, jonathanrussell said:

 fuel keeps moving to the return, at least to the extent that the engine isn't drinking it and the needle valve shuts, theoretically keeping the fuel cool if things slow down. Maybe in a race car / track car fuel never slows down so the return isn't needed? Any chance you can share a photo of how you have it plumbed? Sorry if you already covered and I missed.

 

 

but, the fuel is only going to be as cool as the ambient air temperature. if you are heating it in the engine bay and returning it to the tank, pretty soon the fuel in the tank will be as hot as the fuel in the engine bay/ambient air

[Mark Maras]

11 hours ago, sweatybetty said:

but, the fuel is only going to be as cool as the ambient air temperature. if you are heating it in the engine bay and returning it to the tank, pretty soon the fuel in the tank will be as hot as the fuel in the engine bay/ambient air

I've considered this possibility too. Makes sense, but, the early set-up with 4 screw Hitachis and two steel fuel lines (one return) didn't have the problem. I'm still curious if the big difference is in the two vs. the three steel fuel line set-up.