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1983 720 Fuel Issues

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Hi Gang, love this forum!

So... Few weeks ago some jackass was texting at the wheel and totaled my car. (Thanks, pal.) while I'm looking for a replacement, I decide that I've always wanted and could really use a pickup. I found a sweet little Datsun 720 2wd -1983 which I bought and cheerfully brought home. 2 days later it left me stranded and I used up my first of 4 free tows from AAA. I called the guy I bought it from and he said hit the carburetor with a hammer. We did. It started! So I got some carb cleaner, watched a bunch of YouTube how-tos and read a bunch of stuff here. Proceeded to "cleanse" said carb w/o removing and it was running great. Drove it all over the place until this morning when it stranded me again. I used my second of 4 free tows. Got it to my place where I have tools available, doused it in carb cleaner again and it started up again, only for me to drive it 1/2 mile (downhill, of course) and it stalled and won't start for nothin'.


It looks like somehow the fuel isn't getting into the carb because I can't see anything in that little window. I can barely get it to turn over if I douse the carb in cleaner, but as soon as that burns off, it dies. We pushed it back to my place and there it sits and I am really bummed out.


Now, I don't mind trying to do this myself. In fact, I like to tinker, but I didn't mean for this to be a hobby. I just wanted a useful pickup that would run. And my husband is giving me that look - like there goes his wife buying pickups that don't start because they're cute. And I'm feeling kind of stupid. But here's the thing, I do sincerely believe that this can be a great, mostly reliable truck that is great on gas, easy and fun to drive and really comfortable. So I really want this to work out. And I don't want to resort to a garage because that would be admitting defeat as well as costing more dough than I would like (as in ANY at this point!) I know I can get this sucker running again, I just need a little guidance.


Here's where I'm at:

The truck has a new fuel pump.

The gas tank might be corroded because the fuel gauge gets stuck at 3/4 tank.

It's been recommended that I do a carb rebuild.


I'm thinking that something is stopping the fuel from getting into the carb because it won't go at all, not even a stutter -

I can't see any in that window -

and I tried holding that flap thing on the carb (choke?) open while hubby hit the gas pedal and I didn't see any gas or anything, which I think I should have seen something, because that's where the gas is supposed to mix with the O2, right?

I could hear some kind of motor humming when the key was turned on, but the engine didn't turn over. (That point on the ignition that is past where the lights come on, but back a bit from all the way when you start it). Could that be the fuel pump working?


Here's my question- where do I start in terms of testing and ruling out the issues?

Maybe the fuel filter? That relay thing under the dash that somehow controls the electric fuel pump?


What's the cheapest and easiest place to start?


Many thanks and I appreciate you tolerating my lack of appropriate terminology. I'll work on it.

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Fuel filter is a very good start, easy to do, cheap and if it doesn't work, well it was due anyway. If you have the stock type pump the very bottom unscrews by hand and there is a filter inside. They may still be available but you could maybe just rinse it out for now.


Another problem with the 720 fuel delivery is the pump relay can fail. Forget looking in the repair manuals, it's not often mentioned. It is fixable but lets see if the filter is the cause before moving on to the relay.



Welcome to Ratsun, you have arrived.

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Grrrrrreat!! Thanks!

Ok, I think I can handle that tomorrow. Fuel filter it is.

I found the very useful article about the relay which I may try next.


I haven't been able to get my hands on a Chilton's or Haynes yet - can anyone shortcut me to a diagram of how to find the fuel filter?

I'll keep googling in the meantime...



Scratch that req. I found a fuel diagram here:


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The most important thing about a carburetor is the fuel level in the float bowl. You should see gas about half way on the little window.

This basically how the carburetor works.  Gasoline is held in a bowl, close to the venturi, the hole the air goes down.  There are passages that go from the bottom of the float bowl a little higher than the level of the gas in the float bowl, and they are open in to the air flowing down the venturi.  When the engine is running, it sucks air in to the carburetor, and the moving air sucks a little bit of gas out of the float bowl.

If the gas level is too low, the moving air cannot suck enough gas up into the venturi for the engine to run.


The level of the gas in the float bowl is controlled by a float, and a valve.  Just like the water level in your toilet tank.


You need the fuel pump to supply enough gas to the needle and seat, the valve in the float bowl that controls the level of the gas.


This is kind of a vague description of how your 720 works.  I have a 521, and the fuel pump is mechanical. 

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David it's an electric pump on the Z24. But a good check just the same.


The fuel pump is located on the outside of the frame just forward of the passenger side rear wheel. The fuel filter is between the pump and the tank.


When the key is turned on the fuel relay timer starts the pump. If you do not crank the motor the timer runs out and pump shuts off. (I'm guessing 15 seconds or so???) Cranking the motor over will activate the pump relay and hold it on while cranking. Again if you stop cranking the timer will shut the pump off. If the motor starts, oil pressure and alternator out put are required to keep the fuel pump relay on. In the event of an accident, the loss of alternator output will shut the pump off. Loss of oil pressure while running or failure of proper oil pressure on start up will also shut the motor off to prevent damage

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Here's and update: guy I bought it from says he already replaced the fuel filter. I can hear the electric fuel pump running and feel it vibrate when the key is on. But no gas gets to the carb.


so, is the relay next? Or can it be something in the carb itself? Should I rebuild it?


Here's a pic of the carb- can someone point out which is the fuel line? Thanks!

And I noticed that one of the lines has a bolt it it- like capping it off. I don't think that can be good... It's in the pic.


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Here's and update: guy I bought it from says he already replaced the fuel filter.


Check his work.



If pump running, pull hose off carb and direct into a pail and see if gas coming out.


Yes there is gas.... carb needle valve stuck closed or float stuck in up (closed) position. Brass screen at carb fuel inlet plugged.

No gas... remove hose at fuel pump and see if gas being pumped forward.



Yes pump is pumping fuel.... fuel line plugged

No fuel pumping.... pump no good, filter in bottom of pump plugged or fuel line or in line filter plugged between pump and tank. Tank empty

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Check his work.



If pump running, pull hose off carb and direct into a pail and see if gas coming out.

I mean which is the fuel line out of the carb? There's a metal "noodle" on the front with a hose double clamped to it. The noodle seems to connect to the fuel bowl. Is that hose the fuel line I should pull off to do the test?

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