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agouge888

1974 620 been sitting...trying to get it running

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Hey...I’m new here. Just bought a stock 620 18L and I’m looking forward to my first real drive(Driving it on the trailer doesn’t count:) It starts fine most of the time and idles pretty good for around 30 seconds, it then will high idle for a minute or two then just die. It doesn’t sputter....it just dies It’s as if u cut the key off. It doesn’t restart until I pump it 3 or 4 times then it will finally start, but idles kinda rough and dies this time more like it’s not getting fuel.  I looked at the stock carb and the butterfly is about 1/8 from being closed. When I get it started and it’s running rough...I can put my finger on the butterfly and close it till it’s about a 1/16 gap from closed and the idle smoothes out, but idle goes up to a high idle. It will still die randomly as if u cut the key off though. Shoukd I try a carb rebuild, or go with a Weber ? 

 

any help is appreciated.

 

thanks

 

Andy

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How old is the gasoline in the truck?  How long has the truck been sitting?  How dirty is the gas tank?   Are there any vacuum leaks in the intake system?

Before dealing with the carburetor, is the compression on the engine good?  Are the valves adjusted correctly?

Is the ignition system in good shape?  How old are the spark plugs?

Make sure the compression is good, and the ignition is good before you even think about the carburetor issues.

 

The truck is 45 years old, getting the gas tank clean is a very good start no matter what carb you end up running on the engine.   Fresh gasoline is also a good thing.

Stock OEM Datsun carbs are very sophisticated.  They are difficult to rebuild correctly.  Additionally, if the throttle shaft is worn, that need to be corrected.   But if you get the stock carb working, that is the best, in my opinion.

If you can move the butterfly with your finger, that is not the throttle butterfly, that is the choke butterfly.  It is connected to the throttle by some linkage.

 

A carburetor had many different circuits for fuel to flow, depending on the operating conditions of the engine.  There is an idle circuit, a low power cruise circuit, a wide open circuit at low RPM, an additional different circuit that comes into play at higher RPM, a circuit that is only in play when the engine is cold.  That is only some of them.   Learn and understand how all the different circuits on the carb work together, and when they are in play in engine operation.   

 

I took some automotive community college classes in the 1970's.  One term (12 weeks) of the auto program was just on carburetion.

 

I would suggest you clean the gas tank, and get fresh gasoline in the clean tank.  Get a new fuel filter.  Clean the float bowl on the carburetor.  Be very careful, there are small parts on and in the carb that are very easy to lose.

Next, I would check to make sure the idle circuit on the carb is not obstructed. 

That should keep you busy for a while.

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My opinion here mostly same as above(Daniel)

 

To me its running with the choke on bypassing the idle circuit. I would see if idle circuit is dirty or the power to the idle cut off valve is is bad.

also make sure the 2nd barrel is fully closed. so it dosnt make it look like a vaccumleak.

 

take the old fuel filter and cut it open if really bad with rust or dist then this will more likely confirm  the above theory I have.

 

Shoot lots of carb cleaner in there and rev the motor up.

 

learn how to adjust points ignition and get a timming light. YOUTUBE Datsun!!!!!

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Listen near the back of the carburetor and have a friend turn the ignition switch on (not start) then off, on, off, on, off. What you should hear is a faint clicking sound. If you don't the idle cut solenoid is not working. This is an electric shut off for fuel to the idle circuit mentioned above. No click, no fuel for idle... no run at idle.

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The above applies when the engine is warmed up and the choke is OFF. The butterfly you mention is the choke plate.

 

During cold engine start, a choke is used to make the fuel mixture richer so enough gas will evaporate for it to run. Once the engine and intake warm up this extra gas is not needed. An EFI engine sprays fuel in a fine mist that evaporates easier and is not as sensitive to engine temperature. The choke uses an electric heater that warm and uncoil a mechanical spring  which turns and opens the choke plate. While the engine is cold and the choke on, a fast idle cam holds the throttle open to speed up the warming of the engine. While the choke is on no setting of the idle is possible or sensible. If the choke fails to open fully after 5-10 minutes the 12v supply to the heater may not be working.  If the choke is set too rich it will literally 'choke' the engine and it won't run.

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Lots of good info on the stock carb. but no one really answered the weber question. The weber is a lot simpler than the stock carb. Lots of diagrams and info available for them too. I would recommend getting one. especially if your not familiar with carbs. took me a while to comprehend all the circuits and functions, allot easier to learn on a weber 32/36.

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On 1/7/2019 at 7:48 AM, agouge888 said:

Hey...I’m new here. Just bought a stock 620 18L and I’m looking forward to my first real drive(Driving it on the trailer doesn’t count:) It starts fine most of the time and idles pretty good for around 30 seconds, it then will high idle for a minute or two then just die. It doesn’t sputter....it just dies It’s as if u cut the key off. It doesn’t restart until I pump it 3 or 4 times then it will finally start, but idles kinda rough and dies this time more like it’s not getting fuel.  I looked at the stock carb and the butterfly is about 1/8 from being closed. When I get it started and it’s running rough...I can put my finger on the butterfly and close it till it’s about a 1/16 gap from closed and the idle smoothes out, but idle goes up to a high idle. It will still die randomly as if u cut the key off though. Shoukd I try a carb rebuild, or go with a Weber ? 

 

any help is appreciated.

 

thanks

 

Andy

 

Couldn't find any mention of a Weber question.

 

Course you could spend $300 on one but that's after you can't get the stock one to work, which probably costs nearly nothing. I would hate to replace a carburetor and then find out the problem was a combination of a part plugged fuel filter, the distributor needed new points and the plug wires were really bad.

 

The best carb for the 620 is the stock one. It's already there, gives the best mileage and drivability in all weather and was designed for this engine. It is complex because of this. The Weber is simpler but is not made for the 620, does not drive as well but is a better performer. 

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Just a little update. I started the truck earlier today. Turned the key and it fired right up and ran perfect. I mean I don’t even pump the gas and it starts quicker than a new vehicle. I reved it up some and it’s crisp/ no hesitation or lag.  It seems like timing is spot on. Could points / ignition still be bad if it runs this clean for the first couple minutes it’s started each day?  I even pulled it in the garage.  The truck is all stock w/ 63k original miles. This eve I checked fuel filter, changed it even though it looked very clean. Running new fuel. Pulled the plugs and checked compression for #1 (145psi) #2 (155psi) #3 (155psi) #4 (140psi). This was cold before it had been  running again. Old Plugs were gapped at about .015 , so I put new plugs in while I had them out and gapped them at .032.  Fired it up and it ran smooth again like earlier for about 2 minutes at a medium/ high rpm. It then began to sputter and the rpm”s started to come down like it was dieing...so I pushed the choke butterfly down till it was almost closed and the rpm went up again and it smoothed out and ran clean 30 seconds then died as if I cut the key off.  It would not restart even pumping the pedal.  I then put a shot of starter fluid in the carb and it tried to fire again. So it seems like after it starts to warm up...the fuel is getting cut off and it immediately dies. Also...on another note..... I do have a vacuum leak in the brake booster. With engine off..When the brake pedal is pushed down you can hear a puff of air in the throat of the carb. When u let the pedal back up...you can hear air escaping the brake booster. I unhooked the booster hose from the manifold tube and capped that tube off  that leads to the manifold, hoping that that’s why it was idling high and would die, but it had no effect on the idle and it dying. It still does it.  I’m gonna order a new booster ASAP and M/C and keep working on her. Where’s the best place to purchase booster and m/c? 

 

Thanks and keepem coming. 

 

Andy

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Don't forget that while warming up, the choke is on and the mixture is over rich. As the choke comes off the engine gas to start running on the idle circuit without  help from the choke. If the idle circuit is not working properly it will run poorly or not at all. The choke hides the problem with the idle.  This could be why closing the choke plate helped.

 

Did you turn the ignition on off on off and listen for the clicking sound????

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its the idle circut to me that not getting fuel or the mixture is off at the base of the carb with the screw and tension spring.

 

I had one like this a 521 with no idle. It would rev up fine and run but no idle.

Found 2 carb base bolts loose and then the mixture was off. set mixture and was fine.

Edited by banzai510(hainz)

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12 hours ago, datzenmike said:

Don't forget that while warming up, the choke is on and the mixture is over rich. As the choke comes off the engine gas to start running on the idle circuit without  help from the choke. If the idle circuit is not working properly it will run poorly or not at all. The choke hides the problem with the idle.  This could be why closing the choke plate helped.

Did you turn the ignition on off on off and listen for the clicking sound????

Got home this eve and checked it out further..  i had a friend turn the key and each time I can hear a click from what looks like a long cylinder on the carbs upper back side that I’m guessing is the idle circuit. Is there any way it could still be bad and not working,?

 

thanks,

 

Andy

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Fi clicking it's only presumed to be working.

 

You could unplug the wire and unscrew it. Watch out it has a spring and a pin inside. Catch those and set aside. Put it back together. This removes the solenoid function so that it cannot shut the gas off to the idle circuit. Try it now.

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idle circut still could be full of rust. as its small hole

aslo try messing with the mixture screw at base of carb

Edited by banzai510(hainz)

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Could be an unvented gas tank. Maybe try feeding your gas from a separate container. Or even just temporarily removing the filler cap just to test.

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