L20b vapor locking

[cruznude]

I have a 79 kc and I've always since I have had this truck had the problem of the gas getting to hot and vapor locking.  This really sucks man cause not only is it embarrassing its always just a plain FN hassle.  I heard a while back somewhere that a electronic fuel pump would stop that problem.  But after reading another thread and the mention of the return line to cool the gas and prevent vapor locking, could my return line get clogged and be causing this situation?  Where should I start?

[datzenmike]

Your L20B should have a return line on it. There is a small restriction in the end so that pressure will build on the carb side else the fuel would simply push back into the tank.

 

The return line is above the pressure line from the pump. A hose goes in this end and down over to the return line on the pass side frame. The return line is slightly larger diameter than the fuel line.

[cruznude]

Okay, but is there a chance that it can block up?  I've had this problem forever and once the engine cools down, it starts right up again. To eliminate the gas from getting too hot and me living in the desert, I was wondering if I ran all my lines steel braided shouldn't this eliminate the gas from over heating?  Unless my carb is so damm hot that it boiling when its going through?  I don't know. Do you have anymore ideas?

[datzenmike]

Pull the return line off and hold over a pail... crank the motor. Does gas squirt out? If it does, it would appear to be working.

[Pacific coast Datsun]

Id like to chime in if i may.

My 74 620 vapor locked a few weeks ago. I noticed that the rubber fuel line looked swelled up & the fuel filter was full. My manual pump doesnt have the plastic spacer..could that be part of the problem ?

I dont have a fuel return line either could that too be part of the problem ?

Im running new rubber fuel line from the tank to the carb & both the return & overflow ports on the tank are capped off. I may vent the port on the top of the tank to see if that helps. I live in AZ where our summer temps are in the 100,s every day.

[Farmer Joe]

i have the same issue.

 

ive come to the point where just the single feed line doesnt work.

 

i dont have the stock return hooked up.

 

im going to be setting up a full circle system like the stock one. gonna use a high pressure pump, and put a pressure rgulator up by the carb.

[Z-train]

Hi pressure pump on factory carbs & ESPECIALLY Webers is not a good idea.Get a good low pressure pump and use a return line.

PCD-that spacer also changes pump arm geometry.

79 factory A/C trucks came with a rear mounted electric pump.

[cruznude]

Thanks Datzen Mike but I can't crank motor I have no distributor or front end on truck yet.  Sorry to here that your having the same prob Pacific.Yea that venting sounds like the way to go but then Farmer mentioned about the return line and you know what?  Now that I think about it,thats where the whole problem starts cause the gas seems to be so damm hot at the fuel pump that it starts boiling at that point.  I also have that spacer there and it doesn't help.  I remember thinking about adding a heat sheild or a stylish looking piece of aluminum to the side of fuel pump to draw the heat off.  I can fabricate it at the shop and see what happens but not really gonna know until it warms up a bit.

Edited May 12, 2013 by datzenmike

[cruznude]

Thanks Z-train, yea I had an after market A/C that didn't work that well so I ripped it out.  I did have 2 fuel tanks on my KC which my uncle added and he had an electric fuel pump added to it.  maybe I'll bypass the manual and go electric.  I remember I had to do the day with my 520 35 years ago in So.Cal.  It had no return either.  This seems to be all of our problems.

[Z-train]

Don't waste your time with the shield.The heat issue is coming fom the "carb side" of the engine.There are factory shield for the carb.Between those & a return line you should be good.I have both and i have no issues and i'm in Tucson.

Another route that's more involved:Notice that early 720's had hood vents.I picked up some ZX vents and they will be going on my 810.the 620 will be done after that.

[datzenmike]

'74 and earlier had L18s, and they didn't have return nines on them. This was added to the '75 L20D because of this exact problem. Actually it's a good idea on any car because the fuel is constantly filtered and crap has a hard time collecting in the tank.

 

The under hood temps easily get above the boiling point of gas so a constant circulation of cool fuel from the tank or simply having a small outlet for the vapor bubbles to go sure helps.

 

A high pressure pump regulated to proper pressure is unnecessarily complicated. Millions of these were made and worked just fine with the stock pumps. If you must go electric get a 2 1/2 to 3 1/2 psi pump. Done

[Farmer Joe]

i know the OP is talking about the L20, but i still get the issue with the KA. 

 

the carb and intake are never getting warm.. theyre actually cold. no to a point to freeze. 

 

i can have the vapor lock issue happen, and literally have to take fuel line off the carb, and blow into the vent of the tank to push the vapor out. 

 

then it will flow again. but its not the carb boiling the fuel in all cases. 

 

just more food for thought. 

[datzenmike]

The exhaust is on the other side of the KA motor so VL probably isn't a problem.

 

To be clear, vapor lock is when the fuel boils in the line and the carb. It's under the same pressure as the fuel in the pump so liquid fuel can't be pushed into the carb only vapor. It won't start or run. A return line is like a vent. Vapor bubbles can't be trapped ahead of the pump and are pushed into the tank.

 

If you have to blow into the line then maybe it has a clog in it... or the tank isn't vented?

[cruznude]

Yea no dought z-train, I was also thinking about venting out the hood with louvers or something just to get a better flow of air under the hood.  Check this out, I was at U pull it about a year ago and had noticed that there was a p/u with that aluminum heat sheild tape on the nfuel lines all around the head area.  I kinda laughed cause it looked all FD up!  I've never seen a heat sheild by the fuel lines up by the carb.  They're all around the exhaust area and then they're not that good either.  Wonder if it would still happen w/o the stock carb?

[Farmer Joe]

the tank is a fuel cell with a vent. 

 

theres no leaks in the single line system right now, but when it happens, you can see vapor pouring out of the fuel line of you have it off the carb. 

 

you can hear the fuel pump loose fuel too. no clogs. 

 

the fuel is boiling in the lines under the truck. 

[Z-train]

the fuel is boiling in the lines under the truck.

nope.

[datzenmike]

Gas evaporates at room temp. I doubt it's boiling under the truck unless near an exhaust pipe. Where is the pump? An in line?? Maybe a loose clamp and sucking air in and compressing a big bubble ahead of it.

[Farmer Joe]

dont forget the fuel line is probably 4in off the road surface. its on the passenger side of the frame, nowhere near the exhaust.

 

the fuel's boiling in the lines.. like i mentioned, theres no leaks on the system. i can put 30psi in the system with a compressor, and i have no leaks. 

 

im not trying to argue, but the fact of the matter is what it is.. 

 

ill let yall know how it fairs after setting up the full pressure return system with the regulator. 

[datzenmike]

I believe you believe what you're saying, but gas almost never will boil in the fuel lines under the vehicle.

[Z-train]

I believe you believe what you're saying, but gas almost never will boil in the fuel lines under the vehicle.

Unless the grass underneath the truck is on fire.

[datzenmike]

Well.... that too.

 

Unless the potatoes underneath the truck are on fire.

[cruznude]

Ha! Ha! Thats real funny guys!!! You know that could be a slight possibilitry since I've lowered the truck about 5 inches or so.  I mentioned about the braided lines earlier and it sounds that this is where to install them.  Why hasn't anyone come up with another gas tank?  Maybe something not made of metal?  What if what farmer is saying is right that the gas is boiling in the lines underneath?  Shouldn't we be able to sleeve the lines to protect them?  Oh, by the way I still have the manuel fuel pump.

[Z-train]

O.K.,now this is getting out of hand.There is NO HEAT SOURCE that can cause fuel to boil in the lines underneath the truck short of a fire(nudge-wink).

This is not Datsun specific-this is auto mechanics 101.

 

I have a mechanical pump & a return line with everything sound in-between.No issues last summer with 90 days of triple digit temps in a row.

Get a good pump and a guage.You can dial the FP right to the number by changing the size of the orifice on the return line.You can also make up a fitting that utilizes Holley jets so you can chang the PSI just by changing a jet.

[Farmer Joe]

not getting out of hand at all.

 

i want you to actually feel how warm it gets under my truck.

 

with it being less than 2 inches from draggin ass everywhere, we dont have alot of room for anything other than heat..

 

theres a big difference in my rig and others. its amazing how much heat the undercarriage absorbs on a hot day..

 

in any case, if this cant be considered a factor then i dont know what else could.

[cruznude]

well,not just not sure anymore but will consider all options.  I'll install the distributor and start looking at the threads and trying one by one see what works.