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620 stock carb rebuild, does it help?


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Hi Ratsuns,

         I bought a 74 620 2 weeks ago and having been fixing all the electrical. So now I am at the point where I replace new tires and start taking her for a joy ride. Everything been going fine until about 30 miles or so when the engine started to idle rough. It seems to die out at stop signs unless I feather the gas pedal. I have been reading a lot of post from here and will check my screws that hold the carb to the manifold to make sure that the nut did not loosen causing air leaks like the other member of this site. I was also thinking about a fresh carb rebuild and wanted to hear from someone that did this to see if it helps or should I just buy a weber and call it a day. Thank you in advance for your help.

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Probably needs some adjusting. Lots of things happen when the engine warms up. Rebuilding the carb should only be considered if lots of other things have been checked out first otherwise the rebuild costs money, time and it won't fix the problem.


When warmed up...


Check set the vale lash

Check confirm the ignition timing

Adjust the idle mixture and set the idle speed.

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You should probably do a complete tune up on the engine.  Start by checking all the vacuum hoses.  Check everything visually.  Check compression, when warm.  Adjust the valves.  Check compression again.  

When was the last time the spark plugs were replaced?  What about plug wires, distributor cap and rotor, the points?   Learn how to set point gap, or set gap with a dwell meter.  Set the timing.


After all that is done, then work on the carb.

Datsun carburetors are very sophisticated.   They are more complicated than a lot of four barrel carbs.   You need to check the side play on the throttle shaft.

Most carb problems are caused by water. or dirt in the gas.   Do not completely dissassemble the carb unless you are sure it needs it.  Go ahead, and remove the carb, and check the screws that hold the throttle body of the carb to the main body, and the air horn to the main body.  Confirm you get a good squirt of gas out the accelerator pump jet into the primary venturi when you operate the throttle.

When you get the carb back together, and after you know the ignition, and compression is good, and the engine is warm, then adjust the carb.  Do not set the idle too high.  Use a tachometer to set the idle speed around 700 RPM.  The idle must be below that when you set the timing.  Use a vacuum gauge to check the manifold vacuum, when adjusting the carb.  Adjust the mixture screw for highest vacuum, and if the idle speed gos above 700 or so RPM, back out the idle speed screw to lower it, and readjust the mixture some more.  Yes, the idle speed and mixture interact with each other.

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Hi Ratsuns,

      Below is an update of what I have done so far.

1. Check the idle timing, I had to move the distributor a bit, but it was set good at 12% btdc

2. Check and replaced all worn vacuum hoses.

3. I was able to adjust all valve except for the second lobe. For some reason turning the nut would not do anything. Did not tighten or loosen. What can be the problem?

4. Checked compression and all four pistons was around 160.

5. Replace fuel filter, the old one was pretty dirty inside.

6. Made sure the the base of the carb and intake was tight with no leaks

My truck still seems to idle rough, shaking and seems to be missing. :confused:

Will replace the spark plug and wire tomorrow. The cap and rotor looks new. I do not know how to set point gap or set gap with dwell meter. Please advise.

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Point gap is a mechanical measurement, dwell is an electrical measurement.  But they are basically the same thing.


Just a little bit of electrical theory, sort of.  An ignition coil, or any coil of wire, acts like an electrical flywheel.  It wants the same current to flow all the time.  The coil resists changes in the current flow.  It does this by making a magnetic field in the coil.  When you connect a coil to a electrical voltage, it hinders current flow as the magnetic field is building. On the other hand, when you disconnect the voltage from the coil, the magnetic field collapses, and tries to keep the current flowing.  This creates a high voltage kick, and the secondary of the ignition coil uses this kick to fire the spark plugs.


The points have to be closed for a period of time, for the magnetic field to build to full strength.  If the points do not close long enough, the magnetic field does not have enough time to build, and the spark is weak.  The points have not "dwelled" long enough, or the point gap is too big.  On the other hand, if the dwell is too long, or the point gap is too small, the points just spark, instead of the gap opening enough to force the energy elsewhere, out to the spark plug.


Point gap on new points is around .020, or 20 thousands of an inch.  The Dwell specification is 49 to 55 degrees.  To measure the dwell  you need a "Dwell-Tachometer"  This device measures low RPM, and by moving a switch, the point dwell.  Measuring the dwell is a more accurate way of setting the point gap, especially on used points.  Used points build a peak of point material on one contact, and make a valley in the other contact, and that makes it harder to physically measure the gap with a feeler gauge.  You also have to make sure the feeler gauge you use in the points is absolutely clean, any oil or grease will insulate the points, and make them quit working.

Nissan recommends checking the point gap, or dwell every 3,000 miles, and replacing points every 12,000 miles.


A far better system is to get a matchbox distributor and coil from a later model Datsun.  The Matchbox distributor is a electronic distributor, and you do not need to adjust the points every 3,000 miles.


You have to set the point gap, or dwell before setting the ignition timing.  Changing the dwell a little has a big effect on the timing.

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 I was able to adjust all valve except for the second lobe. For some reason turning the nut would not do anything. Did not tighten or loosen. What can be the problem?



loosen the 17mm jam nut

then turn the 14mm post . It moves up and down. to get the valve lash you want.   if turn it loose(left) Post should move up unless the threadsw are fucked. Which I find hard to believe, then the valave lash will tighten.


if you dial the post down all the way and you cant get a feeler gauge undet the rocker arm then you valve will be sunk and most likely its the exahaust valve

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Thanks Daniel, I would have to do more research on setting gap point. Do you know where I can buy a matchbox dis. and coil from a later Datsun that would work on my truck? Website and part numbers?


Thanks Hainz, I dial the post down all the way and can not get the gauge under the rocker arm. My valve is stuck, it an intake valve. What can I do?

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Thanks Daniel, I would have to do more research on setting gap point. Do you know where I can buy a matchbox dis. and coil from a later Datsun that would work on my truck? Website and part numbers?


Thanks Hainz, I dial the post down all the way and can not get the gauge under the rocker arm. My valve is stuck, it an intake valve. What can I do?


Valve isn't stuck, it has sunk up into the head from a worn face or seat, or both... it happens. The adjustment has just run out is all. The post can't be lowered any more. What you can try is loosening the lock nut and removing the pivot post. Without the lock nut replace the pivot post and see if you can get it to turn down lower without bottoming out. If you can, thin the lock nut on a grinder. This will allow the post to turn in deeper and maybe get the clearance you need. This is not a fix for a sunken valve. Only a valve job can fix that. But it might keep you going for a while. 

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Well when the adjustment bottoms out the valve will begin riding more and more on the cam and can't close fully. Several thingscan happen If an exhaust it is cooled during the time it is closed and sheds heat into the valve seat. It will run hot and the hot gasses will push through at some time burning a small opening. The compression will drop. The valve can even crack or split.




An intake runs cooler and will just leak compression. When the cylinder fires it will backfire back through the carb.

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If you could do a valve job yourself, you would know it.

An L-16, with the valves sinking means you need to replace the valve seats.  This requires machine cutting of the old seats out, replacement with new seats, and machining the new seats.

You can remove the head, take that to a cylinder head shop.  You have to block the cam chain before removing the cylinder head.  There is a spring loaded cam chain tensioner in the front cam chain cover, and if you do not block it, it comes out, and you do not know it until you try to get the cam chain back on the top sprocket, and the sprocket on the cam.

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SUNK not suck. I write this so fast and dont check what I write



EZer to just replace the whole head with a free one if soemone has one (like me)


soem shops do a good job soem dont as the seat needs to be machine so you can use the same lash pad

Hope it dont pop out but I would think aluminum heads are common now and be ez for them to do. and hope it dont pop back out .

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I gave my extras away alreadly. No place to store all the stuff.


California should have alot. look in classifieds and Parts wanted.

for price a a L16 head should be close to 50$ to FREE. this will get ir running down the road.

others think its worth gold but its really made out of aluminum.

otherheads might be worth alitlle more but if off a running L motor and just stock I wouldnt pay over 100 unless a SSS head or something cool.


What motor you have? L16 L18 or L20.


a 74 is a L18  I assume. but coaul have been changed out by now

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Hi All,

      Thanks your your help. I will check the engine tomorrow to see if it is really a L18.



     I was able to temporary fix the sunken valve issue today by doing what you suggested. Grind the locker nut to allow more room for adjustment and now I can get the feeler gauge in. So thank you sir, in the meantime, I will be looking to buy a new/used head.

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