quick fuel system question..

[Llittle_Llama]

If you are running a webber carb on the L20, is their a fuel return line? I am about to call summit and return some stuff, and I think I need to return the return line fittings I bought as well. I am running a fuel cell, so whatever the stock tank has does not matter.

[Phlebmaster]

I have an L20b with a Weber 32/36 and I do not use a return line.

[Llittle_Llama]

awesome, I thought that it was disconnected, and I wasn't sure if it NEEDED to be reconnected or not. thanks phleb!

 

 

 

EDIT: one more question, I need a ventr line for the fuel cell right? I need more than just a little filter on the end right? what do you know of that I can use?

[ggzilla]

I've never seen a carburetor that needed a return line. Some of the last Datsuns started to use them -- not for the carb -- but to keep the fuel recirculating so that vapor lock wouldn't occur.

 

The early/mids 70s Datsuns I've worked on vented the fuel tank up to the engine bay. It was a vent line, not a return line.

[HRH]

32/36s don't need a return line, but 38/38s definitely do, as do SUs.

[slodat]

32/36s don't need a return line, but 38/38s definitely do, as do SUs.

 

huh? My SU's don't have a return line, nor did they from the factory.

[Z-train]

Later 620's DID use a return line.The hard line parallels the feed line to the carb.It will have a small hole in the end-like a carb jet.This does bleed off excess pressure and recirculates(cools)the fuel.Cooler fuel & decreasing pump head pressure is always a good thing.

[slodat]

Later 620's DID use a return line.The hard line parallels the feed line to the carb.It will have a small hole in the end-like a carb jet.This does bleed off excess pressure and recirculates(cools)the fuel.Cooler fuel & decreasing pump head pressure is always a good thing.

 

Both my '78 KC and my '79 620 have the return line like you are saying, with a small orifice in it. I'm keeping this setup on my KC with the new Weber 32/36.

[ggzilla]

Yes, the early SUs (at least through 1973) did not use return lines. They dumped overflow right out on the ground!

 

Remember, carbs don't need them. Return lines were used to prevent vapor lock in a cramped, hot, emission controlled Datsun.

[datzenmike]

The L20Bs had a return, 720s also have it and I seem to remember my '76 B-210 having it so maybe all cars after mid 70s. It would reduce emissions from fuel boil over on shut down. The tank vent went to the charcoal canister along with the carb vent to collect fumes after shut down and evaporation on hot days. On the next start up, acrb vacuum would open a purge valve and suck these storred fumes into the intake to burn them.

[Z-train]

 

 

This is catagorically false.

[slodat]

Yes, the early SUs (at least through 1973) did not use return lines. They dumped overflow right out on the ground!

 

That is only the case when the float fails and the bowls fill up. At that point it will vent out the top through the hose to the ground. This is not a normal operating situation.

[HRH]

I don't know, my dual SU setup came with a fuel return setup and I used it seeing as it was there. I've been told they were a little fussy regarding bowl level so a return line was a good idea.

 

The 38/38 DGES I've tried with and without. Without was a problem.

 

Regardless, it depends on how much pressure your fuel pumps puts out and the size of the needle and seat. Never a bad thing to have a return line, makes it easier for the needle and seat to do it's job, depending on where the return is piped into the carb.

[ggzilla]

This is catagorically false.

What, that carbs don't need return lines? Show me one that has a return line, and I'll show you how it can run just fine without it.

 

Or that Datsun fitted them to prevent vapor lock?

 

 

Yes, if you are using the wrong fuel pump (carbs generally want around 3 lbs fuel pressure), i.e. high pressure fuel pump, you can have problems with bowl overflow.

[datzenmike]

It would reduce emissions from fuel boil over on shut down.

 

It would also reduce vapor lock and a carb will run without it

[Skib]

my SU's have a return line

[Llittle_Llama]

wow, didn't realize there was going to be such a vast number of people with so many different (and compelling) arguments. I decided not to run a return line and to go with just the rollover valve with a filter on top for a vent. If I ever get the balls to do the KA swap, I will have half of the system in place and can just build from there.

[hang_510]

Return lines were used to prevent vapor lock in a cramped, hot, emission controlled...

 

i had wooden clothes pins on the rubber fuel lines for vapor lock on the volare :lol:

[datzenmike]

Brian, you had a Volare' ?? Shit I had a new 76 Aspen!

 

I've got the obligatory Hendrix perm.

And the inevitable pinhole burns

All down the front of my favorite satin shirt.

 

... actually they were all over the velour front seat.

[hang_510]

... actually they were all over the velour front seat.

it had corinthian leather :lol: I killed it in a 5MPH corner at 50 :eek:

which led to my third 510 :D

[ggzilla]

Once again, datzenmike has the most accurate answer:

I seem to remember my '76 B-210 having it so maybe all cars after mid 70s. It would reduce emissions from fuel boil over on shut down.

Yes, first put on the 1975 B210 for this purpose, according to Nissan:

For preventing abnormal fuel temperature rise, the fuel return system has been installed...to return a part of the fuel to the fuel tank ... preventing the possibility of fuel heated flowing through the fuel line inside the engine compartment, and elminating "percolation" inside the carburetor.

I had thought they meant vapor lock, but Mike's answer matches Nissan's wording much better: to prevent excessive vaporization for evaporative emission control purposes. It was the same carb as in 1973-1974 where no return line was used.

[Z-train]

Percolation is essentially vapor lock.