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73 620 runs rough and dies. Help!


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I am at my wits end. I purchased a 73 620 several weeks ago. It appears to have been the recipient of an L18 from a later truck because the plate says it’s an L16 but the block is stamped L18 and I seem to have to ask for 74 parts to get the right thing.


After fixing the fuel lines the truck would start but then die immediately. After rebuilding the carb, it runs (very rough) as long as you keep using the accelerator. Let off the pedal and it promptly dies.


The points looked worn so I replaced them, adjusting to .020. The plugs/wires are new and firing correctly and strong. Timing is roughly on target. Compression is 150 +/-5 across the cylinders…which brings me back to the carb? Or vacuum? I followed a ratsun carb rebuild thread step by step and had no problems so I don’t think I messed that up. Vacuum, I have no idea. This may not even be the problem, but the hoses were not all there when I bought the truck and diagrams in “how to keep your Datsun alive” don’t seem to show everything so I’m a little lost. Even Google has failed me this time. I have made zero progress in the last week despite working on it nearly every day using the maintenance checks in how to keep your Datsun alive.


At this point, if there is someone in the San Diego area who is willing to come by and help figure this out, I will buy the food and booze. I’ll even be the designated driver. This truck needs a lot of love, but I don’t want to start on the body or a disc brake conversion or whatever until I have it running. Please help!

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Set to TDC and lift the dizzy cap. The rotor should be pointing to the #1 plug wire directly above it. Going counter clock wise it's then 3, 4, 2.


While at TDC  check that there is some slight movement in the rockers on #1 Int and Ex, #2 In. #3 Ex ...now turn motor one turn and check #2 Ex, #3 Int and #4 Int. and Ex. As long as there is some looseness this means the valves are closing and they can be properly set later when running. 


Assume timing is close.


Tighten the bolts holding the carb to the manifold.





Does the L18 carb have an idle cut solenoid? It's the hex shaped thing (above) with the red wire. If you have it turn ignition on off on off on off... you should here a clicking sound. If not find out why or get 12 volts on that wire from a switched ignition source. This will allow fuel to get to the idle circuit.

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First, thanks for taking the time to help.


Ok, at TDC dizzy rotor points right at #1. Rest of firing order is correct. Went through each cycle and all the valves have slight movement when the cam is not pushing them down.


Tightened the nuts holding the carb to the manifold and checked the solenoid. It clicks normally when operating the key. No change, truck starts and stays running as long as I have my foot on the pedal.

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The screw with the coil spring around it is the idle mixture. The one to the left partly hidden is the idle speed screw. Turn the idle speed screw in slightly and see if it will keep going.

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I played with these yesterday. The Idle speed screw is at 2.5 turns out. I have tried it from 1.5 to 3 turns out. 2.5 seems to be the best place for it. The mix screw was at 8 turns out before I messed with it. It's currently at 2, but I haven't seen a difference.

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I went back and retraced my steps and kept coming back to the carb. So I tore it down again and rebuilt it but no dice. I have no idea what I did wrong, but the problem is solved. I bought a new Weber carb and the engine ran beautiful as soon as I got it on there. So I guess that was the problem.

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If you consider replacing the carb as fixing the problem then... yes.

After rebuilding the carb twice and it still wasn't working, either there was something fatally wrong with it (unlikely) or I just really suck at rebuilding carbs (probable). Either way, replacing it with a new weber 32/36 made all the difference. Thanks for your help, you prompted me to go back through everything which led me back to the carb.

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Bet the idle circut was plugged


not sure, it would run very roughly even with my foot on the gas. worth every penny to be done fighting with it.


when you do with 32/36, don't overtighten the adapter plate...


it arrived and I did the swap on Friday. Runs nice and smooth now.

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