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1984 720 z24 Stuck at Max RPM


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Just recently got this truck and I had the carb rebuilt. Picked the truck up and was driving just fine no high rpm yet. Drove up the mountain and on the way down I noticed it was nearing max RPM in neutral. If i put it into gear it would still rev high but not max, but i got it home. 


Doing some research on these forums i found one similar issue a guy had with return spring. His was a little different he had a throttle body on an 86. i checked the carb and its not stuck wide open. The gate is closed and im still getting the same issue. Springs and returns seem to be functioning okay. 


MY haynes manual is not here yet.


Is it possible that this is not related to my carb? I didnt want to do too much until i get my haynes manual. 


Curious if anyone has any ideas. or perhaps a link to a thread that i could learn some information from. or maybe even a guru that lives nearby in yavapai county arizona.


anyways thanks for reading.



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I can't see a vacuum leak (which is just more air getting in, but not through the carburetor) causing it to rev up. If anything the mixture would go lean and run worse with lack of power. Check the secondary is fully closed and not that something fell into and jammed it from closing.

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ok so i looked into the right side on carb and there is something small and black, keeping the gate open 1/8th the way. not sure if its metal im trying to find a good skinny magnet, it looks about ready to fall below. might have to take carb off. i looked around the top and doesnt appear to be anything broken off... 

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I saw that!!!



While a small vacuum leak has little effect when the engine is well revved up, at low speed and idle it can really mess up the mixture. First, just to eliminate it, make sure the carburetor is bolted down snug.


Idle should be around 700-750. Higher than this and the distributor built in mechanical advance will make setting the timing inaccurate. Also you might be high enough to be out of the idle circuit adjustment area so setting the idle mixture will not work. A note here... by '84 the idle mix is set at the factory and the idle mix screw sealed with a plug. A 1/8" drill through the center of the plug and a wood screw turned into the hole will allow you to pull the plug out to gain access to the adjustment screw. If you have emissions testing this might disallow you from passing so just be advised. I'm pretty sure you could rub some modeling clay into the opening to hide this.




To set the idle mixture....

On a warmed engine with timing and valve lash set, reduce the idle speed screw as much as you can so that any improvements in idle mixture adjustments are more noticeable. Turn the idle mix screw in till the engine stumbles or out, depending on which direction improves the idle quality most. The idle will probably smooth  and increase in speed so turn the idle down and continue to adjust the mix screw in and out searching for the strongest smooth fastest running. Turn in 1/8 increments and allow a few seconds for the engine speed to stabilize. You may notice a 3/4 of a turn between in and out where the idle doesn't change.... just set roughly in the middle. Continue this in and out till you cannot improve the idle quality and the engine is idling at 700-750. The idle mixture only adjusts the idle and off idle up to around 1,100 RPM where the primary circuit begins to tip in and overlap it.

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thank you for the great detail included in the post. 


no emissions testing here.


elevation ranges from 3000-7000ft. and im not at 7k very often. mostly around 3000-5000


Id like to just get rid of as much vacuum and uneeded emissions crap as possible. 


need to secure some time to work. 

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Today I had some time to go through the truck. 


I noticed someone had been in there and had changed the vacuum lines around. and some of them were dry rotted with screws sticking out of the ends. I properly capped those and i noted the vacuum line going from the distrubutor to the carb was plugged with a screw. put it back on the carb. started the truck and it idles steady around 700-750! Timing check is next. and motor mounts. also notice an oil leak on the back of the valve cover, as well as one somewhere around the transmission. since i wont be pulling the engine/tranny i guess ill just have to monitor the manual trans fluid. 

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Screws in the ends means someone disconnected it from something else. Very few lines were capped at the factory because they did it right.


The vacuum advance is also connected to the purge on the charcoal canister. Make sure they are not split or leaking.  On a warmed up engine, take the distributor cap off so you can see the rotor. Suck on the carburetor end of the vacuum advance hose and you should see the rotor turn about an inch. Engine has to be hot or the T V V will be open..

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The rotor itself doesn't turn, it's the plate underneath the rotor that turns.


As a rule, I do not like 1980's OEM emissions equipment, but at least they were designed by someone with a job to lose if they got it wrong. So many times I have seen deleted emissions controls lead to a poorly or non running vehicle. Someone with very little knowledge removing parts or hoses is not the best way. Learn the systems and then apply that knowledge to the deletion of any component.

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18 minutes ago, Stoffregen Motorsports said:

The rotor itself doesn't turn, it's the plate underneath the rotor that turns.


As a rule, I do not like 1980's OEM emissions equipment, but at least they were designed by someone with a job to lose if they got it wrong. So many times I have seen deleted emissions controls lead to a poorly or non running vehicle. Someone with very little knowledge removing parts or hoses is not the best way. Learn the systems and then apply that knowledge to the deletion of any component.


Bravo! A properly maintained emission system is less bother than one taken apart under the guise of 'cleaning up the engine' by removing 'pollution crap'. These are rudimentary controls and only adjust idle and part throttle operation. Full throttle remains the same so will performance. If you think that tearing off these well designed controls will magically give you a performance gain you are mistaken. You always fear what you don't know. Learn what these controls do and it's no longer a mistery.


opod.... Spend the money on a real Nissan FSM (factory service manual) The Haynes manual covers all years of the 720 through '82 only, with an add on at the rear for the Z24 engine '83 and up. Way too much to cover in one small book I have the '84 FSM manual and it's 2 1/2 " thick and covers EVERYTHING about the '84. 


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i am not here to try and enhance the power of my truck. it is simply for transporting loads from point a to point b. i do like the idea less wires and lines running around the engine bay, but do not plan on jumping in and deleting things willy nilly. 


The previous owner did a awful job of trying to switching vacuum lines. i am just trying to get stuff back to where it goes so that it runs right. It is running 10x better then when i brought her home. 


So they haynes i have that clearly states in the title ' pick-ups 1980-1997 ' only has a small section in the rear for 85s? i need to look at her. The Factory book looks nice, a bit out of my price range for now. I am interested in at some point purchasing it though. 

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If you know what the emissions systems do and how they interact you can better delete them properly. Actually you probably will be more comfortable WITH them on and working. Some actually increase mileage. One increases the life of your engine by keeping the oil cleaner between changes and I don't mean the filter.

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This is a bit off topic, but while we're talking about performance vs emissions controls, you're right Mike. Simply removing a device won't always give you more power, but there are cases where having a device in place creates a dangerous scenario.


Vacuum advance is a perfect example. If it is connected and set to factory specs, there is little worry, but if the timing is advanced a few degrees as many people do, this can create a problem of too much timing in the higher RPM's. Let's say your cruising at 4000 RPM's on light throttle, and you suddenly stab the throttle. The motion of the breaker plate is not very quick and you have now just introduced more timing than is healthy for this engine. Broken rings can result from high RPM detonation.


So by removing the vacuum advance and modifying the distributor to limit the total advance and the speed at which the advance occurs, you can now run more initial timing to get more of a punch down lower in the RPM range without creating that danger zone in the upper RPM's.


That's one example of an emissions device that when removed, allows for other adjustments that can give you more power. So the removal of the device itself is not going to add power, but it opens the door.

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When Datsuns roamed the earth engines made pollution and the pollution devices cleaned it up. Like a catalytic converter. Or air pump. FF to today's engines. The pollution devices control the engine and will not let it operate outside certain limits that produce emissions.

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I would just leave them alone and simply maintain them. Seen to many people mess with their emissions systems and introduce problems. The Z24 is a good engine, reliable, good torque, good mileage and was designed to be low emissions too. You can't make it run better removing stuff is all. Get the factory service manual (FSM) for the '84. No, removing things properly will have no effect on timing. 






This one is $60 but you should be able to get it for under half that if you look. Just ask yourself what a mechanic charges for an hour's work. First time you look something up it pays for itself. This will fully explain how EVERYTHING in you truck works and how to find it, remove, replace it, how to tune, tighten and measure it. Has all the electrical diagrams, drive train, body, suspension, steering and brakes. It's the bible for the '84 720.

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The only reason i ask is because the previous owner has tinkered with the vacuums. And I would Like to A: return them to factory settings, or B: delete them. The truck is running decent now. But i have some vacuums capped off. I am having trouble reading the diagram under the hood and in my haynes manual. 


I will keep my eyes peeled for that Service manual. on the site you linked it is out of stock i see it on ebay for 60-100$. dont see it on abe books either. Seems to be a bit tricky to find for cheap. 


do you by chance know if the same information is in a different book ? like maybe its titled differently. Now that im looking even the sites where the book is listed for 100$ its saying its out of stock? perhaps you or someone here has an old beatup one and i can purchase it? 

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