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84 720 idle problems


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Hey everyone, I just picked up a 1984 720 on Monday and it runs pretty well, however, after idling for a minute or two it starts to stall out. I've done some research and looked at some other threads, here's a list of things I've tried: 


-new fuel filter 

-verified fuel pump works 

-replaced damaged fuel lines 

-drained old fuel and filled with new fuel

-adjusted throttle cable 

-replaced almost all of the small vaccum lines, I believe all lines that run to the carb 

-removed the pin and spring in idle cut solenoid 


The truck has 201,000 miles on it and sat since 2019 I believe. The previous owner would start it and let it run every few weeks or so but was not actually driven. I checked the spark plugs and ordered new plugs for each side and new plug wires from ngk. I plan on taking off the carb tomorrow and getting carb cleaner from a parts store on Thursday when I'm in town. Could it be that the carb is just sticky from the old gas? This is the first carboureted vehicle I've really had any kind of experience with so I don't know much about them. I can get any pictures or videos needed for reference tomorrow, thank you for any and all help!





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This is a warmed up engine?


I think the idle mixture is set at the factory and the screw sealed with a plug. If you don't have emission testing you can drill a suitably small hole in the soft plug, insert a wood screw and pull the plug out of the carburetor base. This will give you access to the mixture screw.




This done on a running hot engine with idle below 800, turn the screw in or out to find the strongest fastest idle. There tends to be almost a full turn where it peaks so set roughly in the middle. Idle speed should have increased, so turn the idle speed screw back down. Adjust mixture screw to find the fastest smooth idle speed and again turn the idle speed screw down. Repeat as many times as needed till you cannot improve the idle and it's at about 750-800.

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Yes to the above.




Not this...



The mixture and idle settings may not fix the problem but it does eliminate them, and it doesn't cost anything to do.


One other thing is to pull one intake and one exhaust side plug wire, put an old spark plug in the end, lay on a grounded surface and turn the engine with the starter and check that you have spark. Both coils must be working for proper idle. Again cost nothing and eliminates a cause.


Timing should be 5 degrees.


Check set valve lash. 0.012" on all when engine is HOT.

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I don't think it's been fully warmed up yet, I remember the highest it ever got was 160 degrees. I'm not sure if it's a mechanical or electric choke either. It sounds to idle around 2,000 and after about a minute go down to normal idle speed. And even then it idles fine until I press the accelerator. So I assume the choke works. I also noticed that when I pull the throttle cable from the engine bay, the butterfly valve doesn't open much at all. The farthest it opened was about as much as in the 2nd picture you attached.

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Oh I thought there was a jet for idle and a bigger jet for higher rpm. Well I got the manifold off and the carb taken off of the manifold and when I pull the throttle cable, only one valve opens on the bottom. I noticed part of what looks like the linkage for the other valve wasn't connected, but after I reconnected it it still didn't move.

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Idle is just a fuel feed to a very small port on the side of the carburetor throat where the closed throttle plate is. Only a very small amount of air can squeeze past and this forms a venturi that sucks the fuel out. There is an adjustment screw that allow more or less fuel through. As the throttle is opened more and more air get past the throttle plate and as the fuel supply is limited it can't supply what is needed to run the engine much above 1,000 RPMs. The primary barrel has a venturi in it and enough air is moving that it can just barely generate enough vacuum to suck more fuel in. This fuel is limited by the primary jet to roughly match the maximum air that can be drawn through the primary and generate a 14.7 to 1 air fuel ratio. The secondary is added in under certain conditions for additional power and it is larger with a larger jet. Jet sizes can vary depending on altitude or to run a leaneer mixture for emissions purposes.  


The idle circuit starts in the fuel bowl. There is an idle cut solenoid that opens electrically when the ignition is on and allows fuel down into the idle circuit. When the ignition is off the fuel supply is cut off to prevent the engine dieseling or running on. From there to the idle adjustment screw that fine tunes the idle mixture.  

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13 hours ago, DNI_g35 said:

Oh I thought there was a jet for idle and a bigger jet for higher rpm. Well I got the manifold off and the carb taken off of the manifold and when I pull the throttle cable, only one valve opens on the bottom. I noticed part of what looks like the linkage for the other valve wasn't connected, but after I reconnected it it still didn't move.

2 things I read, first if you mention the second picture datzenmike posted, that is the choke plate not the throttle plate. If closed it's trying to pull extra full for warm up it should be open to run . 


Got any pictures you can post of the carburetor?


As for the statement about the actual throttle butterfly one opening and one not,  the second could be vacuum activated... not 100% on that.... datzenmike Mike would know better

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when COLD the upper plate is closed. this is the Choke

when it warms up the plate will be open.


the throttle plate is mechanical on the main one the 2nd only works when you get high vaccume.


to me you didnt need to take of the carb off yet as you said it seems to work after a little bit so maybe just needed a slight adajustment on cable or choke plate adjustment.

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I did a little research last night and I'm starting to kinda understand how the stuff works, I just bought some carb cleaner and marvel mystery oil from the parts store. I'll take some better pictures of the carb later but some of the seals seem shot. Might need a rebuild kit, but I'll get y'all's opinions first. I tried adjusting the accelerator cable but it didn't really do anything. I'm not sure if I was just adjusting it wrong or if there was a problem in the carburetor itself. Like I said, I really don't know much about carbouretors since this is the first carboureted vehicle I've ever really gotten to play around with. Would it help y'all if I took some videos of any parts moving from any specific angles? My grandpa knows more about that kind of stuff and he's coming Saturday to come look at it and help with the drum brakes.

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1 hour ago, DNI_g35 said:

Would it help y'all if I took some videos of any parts moving from any specific angles?

I would say, post some pictures of the carb 1st so that it can be identified, paying special attention if you can find any logo's that are part of the casting, like the one below the accelerator pump, printing, or other labels, along with the electrical connector.

The picture I uploaded is an original Hitachi DFP-384 for the 85/86 looks like.  


From there, others might ask for other pics or detailed info

Screenshot 2022-09-15 at 5.09.47 PM.png

Edited by 720_Jeff
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Sorry for the late response, some unexpected things came up. I looked at the owners manual the other day and it turns out it's a 1983 and not a 1984. How do I attach files? I see you have to insert via a link. I tried uploading through Google drive but it did not work.

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Try an imgur account, it's free and unlike this Google Drive the viewer doesn't have to join to view any pictures you post.


It's probably an adjustment so when you get it back on the engine follow my first post on how to adjust the idle and idle mixture screws.

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