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Idle problem?


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First of all I would like introduce myself my name is Josh. I am new to this forum along with Datsun/Nissan trucks. I am an avid minitrucker and own a few minitrucks. I have used this forum for about a week or so doing a lot of reading and trying to become more familiar with the Nissan 720 /z24 engine. This forum has been very helpful with some minor issues that I have had with my 1983 Nissan 720. The problem that I have encountered now, I am having a hard time trying to search for a specific answer. My Nissan has been garaged for a number of years from the previous owner. It has 53,000 orig. miles. Nothing has been tampered with or messed with. The truck is entirely stock. By the way I consider myself a beginner/novice when it comes to engine problems and solutions.

------Here is my issue- My carburetor has been acting up recently. Upon start up the engine will rev until I press the gas pedal to kick down the carb. As soon as I do that the idle drops in rpms and stalls. I have driven it by feathering the gas while working the brake and clutch to raise the rpms. It drives perfect at highway speeds 55- 65mph. As soon as the truck is put in neutral the rpms slow to the point that the truck stalls. I restart the truck with no problem after the stall. It just does not seem to have a smooth idle. I have not touched the idle mixture screw because this problem has just came about and I figured that if I were to make an "adjustment" that I would be constantly trying to get the fuel mixture correct. Could the problem be the EGR, or the anti dieseling sensor, or the throttle valve switch, or any other part that could cause this with my stock Hitachi carb. I'm at a loss for solutions for this problem. My Haynes manual does not answer these questions. Any help would be greatly appreciated. Later Josh


p.s. sorry for the long post

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if the wire fell off the anti deseling selinoid it could do this or the selinoid the fuel jet is plugged up with rust/dirt. Thats why you got to pump the gas. which your bypassing by shooting gas from the top, then once you get rolling faster your using the main jet.

When you choke is ON you got enough vaccum to pull gas from the main jet or 2nd barrel side. but when you hit the gas it kicks down going to the idle jet which has the selinoid.



maybe get a new fuel filter and cut open the old one. esp if gas tank is in wheel well where dirt can get in the filler tube.(common on 521 trucks) I dont know the later trucks so Im guessing


also if the mixture is out of adjustment it can cause the same symtoms also.the brass screw with the tension spring at the base of carb. turn this IN counting how many turns till it bottoms out. then turn 1/2 turn out till it runs. But really it should be out about 1 1/2 turns. then once running turn it in/out till it runs the smoothest.



ck for leaks as the later motors have so much emmission hoses that could crack and cause a vacuum leak. ck intake and carb bolts

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Thanks for the response. Yes the carb is stock. I will check everything that you listed tomorrow morning. I'll respond with my findings.

Are there any other things that I should look into? Thanks again -Josh

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First, have someone turn the ignition switch on, off, on, off, on, off without starting. Listen near the back of the carb and you should hear a light clicking sound. This is the idle cut solenoid working. When the ignition is ON the solenoid is energized and held open to allow gas into the idle circuit. Without it, no gas, no idle.



Make sure the choke plate (air filter off) is open when engine is warmed up as in the picture below:





If OK, try turning the idle speed screw up. Your carb will look generally like the one below. Shown down near the bottom are two screws with springs on them. The one on the left is the idle speed screw. With engine warmed up turn in to get and maintain an idle.




The second screw just to the right, is the mixture screw. Turn in until idle quality changes and engine stutters, back out until it stutters. Set screw about half way between, idle may rev up. Go back to the dle speed screw and turn in or out to raise or lower idle so it runs smooth. Go back to the mixture screw and turn in and out, select the middle position where it idles smoothest. If idle speed increases, go back and adjust the idle speed screw again. Repeat until it idles smoothly at about 700 RPMs give or take, and any additional mixture screw adjustments produce no change in idle quality.


This should give you a good stable idle or at least an idle from which the timing can be checked.

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Mike, Thank you for the response and the detailed pictures I will check it in the am.

I have used a prior response from you about hazard light switch not allowing my blinkers to work. They work now after doing what you said so, thanks for the help with that. Later Josh

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That will work to power the choke an warm it up.


The 720 set up for the choke is to have a relay powered by the alt output. If the engine should stall in the driveway while still cold and you are in getting a coffee the choke won't keep warming up and open. When you come out the choke is still on and it's easier to re-start.

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Ok today I checked the idle solenoid and it clicked. So I then started the truck and brought it to temp. I then checked to see if the butterfly is opened which it was. I checked to see if the anti dieseling solenoid was connected which it was. I then kicked down the carb and it began to loose RPMs. I then adjusted my idle screw. It took me a while and some driving till i got it where i like it. It still is a bit high in RPMs but i'm going to try to adjust/fine tune it tomorrow. Thanks everyone for your help it was greatly apperciated.

Also Mike my carb only has one adjusting screw from what I saw. Maybe I was not looking in the right place but I only saw one. The screw was nearest the firewall on the left side of the carb that i adjusted.(Looking at the carb from the passenger side)

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Those pictures are just for example. I didn't know what year your 720 was so if there is no idle mixture screw then it's an '83 and up? My '84 has it. The mixture is pre set 'at the factory' and sealed with an aluminum plug. To prevent owners changing the setting... like we are idiots. Behind the plug is the adjustment screw.




... anyway just drill a small hole into it and pry the plug out or tighten a screw into it and it will back out. Easy. Then perform the adjustment.



Two things that need doing:


Check and set the valve lash with the motor hot. (and off)


Check the ignition timing. Motor running idle low at around 700 RPM. If you can't get the idle this low or you aren't sure, disconnect and plug the rubber vacuum advance hose the before setting the timing. Ignition timing will seriously affect the idle speed and quality. I would have suggested doing this first but you said it wouldn't idle properly so we fix that first. Once the timing is set, re-adjust the idle speed if needed and re-adjust the idle mixture if needed as the timing change will change these previous settings. With idle mixture, try for the fastest smooth idle and turn down with the idle speed screw and repeat till you cannot improve on it.

Edited by datzenmike
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One other thing that will affect the idle. Both plugs must be firing. Pull each coil wire off and stick an old plug or screw driver in the end and lay on a grounded surface and crank the motor. You should see a spark. If the exhaust side isn't working check the fuse on the far left side of the fuse box.

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  • 1 year later...

Hello y'all!



I have a 1986 720 with a carbureted Z24 (electric fuel pump) and nearly 240k miles. My truck has been running very well and has got up to 31mpg on my last Vegas trip!


However, it has developed the exact same problem described in the original post, seemingly overnight. It starts up quickly as usual....runs smooth as I kick it down a couple levels, and then it will sputter and die on the lowest idle level....when it gets warmed up. It starts up again easily, but will not stay running if I take my foot off the accelerator.


I've taken the air cleaner off and found/fixed a few vacuum leaks. I've also examined all linkages and everything seems to be intact and moving freely. The electric choke works as it should and the truck drives down the street without hesitation. It just dies if I lift my foot off the accelerator.


I'm tempted to adjust the idle screw to compensate, but would rather try and fix whatever issue may have developed. One thing I noticed is that when the ignition is turned on there is a repetitive tapping sound coming from the rear of the carb.


The carburetor is original, and the truck is from California. Any insights would be appreciated! I would also appreciate any info I can get on what exactly the computer under the seat does in this Z24. Some online say they have solved this type of problem by unplugging it. But, that doesn't make sense to me.


Thanks! _Luke

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  • 9 years later...

Hi I have an 1983 720 japan model well I have  intermittent problems sometimes it will diesel when turning off. When I start the truck it revs up to 2k then idles down. Then my throttle will sometimes hold open after I let off the gas and shift. I have cleaned all linkages and lubricated all of them. Most of the time it will work fine idles around 750. 

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Whats a 'Japan model'? Right hand drive????



A cold engine has a fast idle function. It will rev to around 2K when cold and over an 8-10 minute period slowly reduce back to proper warmed up idle. This is normal and is to warm the engine faster and make it easier to drive during warm up.



I think your throttle cable may be sticking or the throttle plate is. Have someone step on the gas and slowly let off on a warmed up engine while you watch the moving parts. It should return all the way to idle. A possible cause is the throttle cable too tight.

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