1977 Datsun 620 Pickup Fuel Flow Problem - There Isn't Any!

[JustEnoughTruck]

This seems like such a simple problem, but it's been plaguing me on and off for months.

 

The Hitachi carb on this has a fully-glass float bowl cover (from an earlier model), so it's easy to see what's going on.  If I prime the carb by pouring fuel down the bowl vents it appears in the bowl, and the float (brass) actually does float.  The engine will then start and run perfectly until the bowl is empty, then die.  It's easy to see that there is no fuel spraying out of the float valve at any time.  The float follows the fuel level, and rests on the bottom of the chamber when the fuel runs out.

This fuel system, unusually for a carb setup, has both a fuel feed line from the tank and a return line.

 

Here's what I've done:

 

1) removed the tank and took out the sender to verify that there is no corrosion.  Replaced the rubber lines at the tank.

2) verified that both the feed and return hard lines are fully open.

3) verified that the stock fuel pump delivers large mounts of fuel.

4) changed the fuel filter and replaced the rubber line to the carb.

 

There is a device I initially assumed was a junction block between the fuel feed, fuel return, and line to the carb, which I'm now thinking is some kind of sealed pressure regulator.

 

I'm about to eliminate this "regulator" and the return line and run hose from the pump directly to the carb, but I'd like to learn how this system works before I eliminate it.

 

 

 

My Haynes Book of Lies hasn't been of much help, and I haven't found anything better online.

 

I've attached photos of the "regulator" and the hard lines around the carb.

 

TIA for any help.

 

Richard

Edited March 8, 2020 by JustEnoughTruck
add images

[datzenmike]

 There is no fuel pressure regulator other than what's in the pump itself.

 

All L20Bs have a metal pipe to, and past the carburetor and connects to the return line to the tank. After the carburetor but on the fuel tank end of the pipe is a restriction, a pin hole. Without it all fuel would just gush past the carb and back into the tank. Here's what it looks like...

 

 

Find the hose to the return line and follow back to the metal pipe and pull it off. you should see this above. It's above the fuel pump.

 

 

If there is no restriction, carefully pinch the return line closed with vice grips and try starting the engine. Watch for fuel to fill the carburetor.

[JustEnoughTruck]

Is this small flow of fuel intended to keep it cool and prevent vapor lock?

 

 

[JustEnoughTruck]

I went back and added the images I had promised the first time, mostly to see what worked, since the "Insert image from URL"

button button doesn't seem to do anything.

 

For the record, I right-clicked images I had uploaded to Flickr, picked "Copy Image Location" then clicked into the text of my message, right-clicked and picked Paste.

 

Seemed to work fine.

Thanks very much for your concise, to-the-point response!  It's exactly why forums are often so much better than references.

[datzenmike]

Yes, indeed. The small flow through the pin hole circulates cool fresh fuel from the tank past the carb. Any bubbles from heat that might boil the gas in the lines (vapor lock) are also pushed into the tank. It's for hot weather and high under hood temperatures after shut down (heat soak) and then restarting. An added bonus is that over time all the gas in the tank is also pushed through the filter. The tank stays cleaner.

 

Your pictures show that you do have a return line. The end you want to check is the one near the fuel pump.

[banzai510(hainz)]

Where the fuel lime goes in the carb is a banjo looking fitting with a nut on there.Ita in your photo.

Try tapping in this when you see the float draining down.

to me it could be the needle valve being stuck closed not letting gas in

To check the fuel pump I disconnect the output of the pump and point in a safe direction and a lot of gas will shoot out.

 

taking out the tank wasn’t really needed

[JustEnoughTruck]

Thanks Guys-

 

I did check the end of the hard line that connects (via some rubber line) to what I assume is the return line to the tank, and that's where I found the restriction, as expected.

 

A deeper question is, how can I tell the difference between return line and the supply line?  They both just disappear under the chassis.  I believe I have traced every inch of both of them under the vehicle, and am pretty darn sure I have them hooked up correctly, but how can I tell for sure?

As I said, I have disconnected the output of the pump, and have seen a bunch of gas shoot out when the engine is cranked.

I have been suspicious that the float valve needle was sticking to its seat, because of the shit gas we have now, which was why I replaced it with one from another carb.  Based on blowing through it while pressing and not-pressing the valve needle, it seems to be fine.

Unless you have anything more to add, my next attempt is going to be bypass everything with a piece of rubber line connected directly from the pump to the carb and see how that goes.  I am inferring from what you've said, that the pump has a pressure-bypass valve built in that will prevent the pump from just blowing the float valve off its seat and flooding the carb.

[banzai510(hainz)]

My 510/521 pump will go limp once the carb fills up and the needle valve shuts off. I assume your manual pump is the same

I don’t have a return line if there was it’s gone now and I have a Weber carb now.

as the truck is running tap with a wrench on the carb  banjo fitting may loosen up the inlet valve.

this happen to me carb draining.A guy showed me and started filling up the carb

 

those carbs are 45 yrs old who knows if still good 

Edited March 8, 2020 by banzai510(hainz)

[datzenmike]

Return line is larger diameter than the supply.

[G-Duax]

Hm, not on my '73 620 (or what's left of it, damn women drivers), both are the same size.

[datzenmike]

My '73 FSM doesn't show any return line at all. That's likely the fuel vent line to the flow guide valve located on the drivers side inner fender across from the exhaust manifold. The pipe is aluminum? I don't think the '74 (L18) had the return either but the hotter running '75 and up L20Bs did to help with hot restarts.

 

 

[JustEnoughTruck]

Finally, on a hunch, I installed a line direct from the pump to the carb, and filled it with fuel before connecting.  Floods of fuel into the carb.

This is still an old pump that's rebuildable.  Should I do that, or can you shed any other light on this?