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New 720 Owner in Need of Help with High Idle


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#1 mphilleo

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Posted 12 October 2013 - 12:59 PM

Hey guys/gals, I decided to register on this site because I've found a handful of helpful threads while doing some initial troubleshooting on my 720. For those who are curious, it's an '84 with the Z24 and automatic transmission. When I bought it, it ran fairly well, though it had a tendency to hesitate from a stop if you didn't feather the pedal. Regardless, it still got me home all right, so I was enthused. I did with this truck what I usually do with any vehicle I buy used. Do a little going over and tuning up/deferred maintenance. I'm not savvy regarding the correct lingo, so please be patient with me as I describe my issue. :)

 

When I bought it one of the thicker hoses coming from the air filter housing was plugged off by a screw and although there were a lot of OEM-looking rubber plugs on some of the carburetor hose barbs, there were still missing hoses that I couldn't identify the routing for and replace. It also looks like the two metal pipes that lead from some part on the exhaust to the air filter housing had been cut. It was also missing the pre-heater hose from the exhaust to the air filter housing. As I said when I drove it home, it was fine with the exception of needing the gas feathered when starting from a stop or it felt like it might die.
 
Anyway, I tried to use some carb cleaner and a toothbrush to clean the choke valve up top and I think it got misaligned or off track somehow, because that's when my high idle issues started (like 2k RPMs at idle). So I tried to fix the choke valve and must have corrected it somewhat because the idle went down to about 1200 RPMs. I then backed the throttle screw out a few turns or more, but it's still probably 800-900RPM. It seems like it won't go any lower no matter how far I continue to back that screw out. So as a result, even at idle, with the foot off the gas, I'm still cruising at 15-20 MPH. I'm suspecting a return spring might be stuck somewhere or possibly the throttle cable, since the gas pedal feels "soft" (doesn't have a lot of resistance). 
 
Just last night I replaced the plugs, all wires, distributor cap and rotor (routed correctly and double checked) and buttoned everything back up just as I got it before, although I did replace the pre-heater hose. It was back to its previous high of about 2k RPMs and I'm absolutely puzzled. I tried turning the idle screw counterclockwise to absolutely zero effect - in fact, I backed it out so much it came out and I had to put it back in! At this time the screw that comes in contact with the dashpot has been backed off so that it is close but does not make contact. I'm not sure how to check the mechanism that controls the fast idle speed in park or neutral because the Haynes manual that came with the 720 is about as useful as a sack full of butt cheeks. I hate to be "that guy" who pops on right away with an issue, but I really need any help you can provide. I'm reaching the end of my rope. Thanks in advance, everyone!

 



#2 mphilleo

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Posted 12 October 2013 - 04:20 PM

After some more tinkering and investigating, I've narrowed down my issue to the fast idle cam. It seems that the adjusting screw rests too high on it after it warms up. Hopefully, there's no broken linkage, etc to be found. Can anyone speak out of experience whether the cam itself is integrated with or separate from the white piece of plastic that pivots on the metal point it's connected to (if that makes sense)? It appears that that piece of round plastic is free-rotating on the metal point and not connected to my fast idle cam. I'm trying to determine if it's broken or not. : /



#3 datzenmike

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Posted 12 October 2013 - 05:32 PM

Once warmed up the fast idle cam should drop out of the way and the idle will lower. Spray around the cam and linkage with carb cleaner or WD-40 and work the linkages by hand through their range of motion. Hold the throttle part open in order to do this.


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#4 mphilleo

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Posted 13 October 2013 - 11:24 AM



Once warmed up the fast idle cam should drop out of the way and the idle will lower. Spray around the cam and linkage with carb cleaner or WD-40 and work the linkages by hand through their range of motion. Hold the throttle part open in order to do this.

datzenmike, thanks for explaining this to me! I followed your instructions and saw that as the truck warmed up and the choke opened, the fast idle cam moved out of the way as you said it would. The idle in park and reverse was set to about the 650-700 RPM area that was advised by the EPA sticker under the hood. The only thing I can really complain about is that the idle in park in neutral once warmed up, though lower than when it's cold, is still kind of high, maybe closer to 1000-1200 RPM...at least as it sounds to my ears.

 

Of course, it might be related to vacuum hosing being missing from these items, though I'm not sure where they go from or where they go to.

 

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#5 blackwolf

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Posted 15 October 2013 - 02:48 PM

nissanvacline.jpg

 

this mite help you out rerunning the lines



#6 72O

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Posted 23 October 2013 - 09:16 PM

Look up fastboatman's vacuum hose guide on nico club



#7 DaveZilla

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Posted 23 October 2013 - 11:25 PM

I agree with 720's post... Fastboatman on nicoclub's forum has a really good and easy to read vacuum hose routing diagram posted over there. But what I see in the pics is the open line on the manifold right in front of the carb, that one needs to be plugged for sure, and also any other vacuum lines missing from the manifold or carb, they either need to be plugged or properly routed. With the carb tuned right, and no vacuum leaks, the engine should be able to idle dead smooth in drive at around 700 rpm. If it's idling rough after the vacuum leaks are fixed, then you'll need to bump up the idle mixture. I always set my idle mix by using a vacuum gauge hooked up to the manifold and set it to the highest vacuum reading, or set it with a wideband to 14.7:1 AFR, which will also give you the best vacuum reading as well. Once you get the idle mix right, you can lower your idle speed to factory settings with no worries about stalling out or need to blip the throttle to keep it running etc.



#8 NotAGardener

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Posted 04 November 2015 - 08:52 PM

Chilton's manual has better vacuum diagrams than Hannes I believe.

 

I know this is an old post but. I also just noticed from your picture that your fuel float is looking a little low, I think I kind of make out the meniscus at the bottom bot the fuel float. It should be in the middle, which may be why it's running lean?



#9 NotAGardener

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Posted 04 November 2015 - 10:20 PM

Also, I think there's a good chance, after seeing other photos, that your fuel float window is upside down? Don't know how this changed over the years but I've seen more pictures on this forum of the window the other way around. That means someone must have meddled with the float.



#10 1984720Z24Sunrader

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Posted 15 November 2017 - 04:29 AM

My 84 Nissan 720 Fast Idle issue not resolved yet. Any ideas photos and or suggestions? Thanks. I am still learning the engine and I am not a mechanic I do try to do all my own work, but this 720 I just bought and it's my first one...so lots to learn. Thanks ... 



#11 blownalcohol

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Posted 18 November 2017 - 07:02 AM

Possible vacuum leak? Small amounts of air going by the throttle pates will increase idle speed. Is the timing over advanced? Another thing that comes to mind. My 82 has a lot of wear around the perimeter of the butterflies in the carburetor. Even with the idle screw backed all of the way off it idles at 1000 to 1100 rpm due to the amount of air getting by the butterflies in the closed position.

 

Wow just noticed how old this post is.... maybe it will help someone anyway  :D



#12 datzenmike

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Posted 18 November 2017 - 09:14 AM

I've never seen this. An air leak would lean out the idle mixture and cause it to be unstable. Ive had the vacuum hose to the ATC on the air filter snorkel fall off and it wouldn't idle properly at stops. Easy fix as you could hear the hiss.

 

Only way the idle would increase is if you had an over rich idle or the choke was on. All it needs is more air to burn the extra gas and the idle would go up. But for a properly set idle mixture, I've never seen this to be true.


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#13 blownalcohol

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Posted 20 November 2017 - 07:40 PM

I've never seen this. An air leak would lean out the idle mixture and cause it to be unstable. Ive had the vacuum hose to the ATC on the air filter snorkel fall off and it wouldn't idle properly at stops. Easy fix as you could hear the hiss.

 

Only way the idle would increase is if you had an over rich idle or the choke was on. All it needs is more air to burn the extra gas and the idle would go up. But for a properly set idle mixture, I've never seen this to be true.

I will not argue that a large vacuum leak will do all the things that you indicate but as you also argue it depends on the size of the leak and the condition of the fuel mixture. I have great respect for you and your exceptional knowledge of this manufacturer. I concluded after reading the post that it was safe to assume that this is not a well tuned engine, Perhaps a better more correct term for me to have used would have been un-metered air in place of vacuum leak. Suppose for a moment that the idle mixture screw plug has been knocked out and the idle mixture screw turned out to obtain a smoother idle. Then we see that all of the exhaust emission components have been disabled.  It is not outside the realm of possibility that his engine is running rich at idle. Given a very small external vacuum leak like a cracked hose or a leaking gasket, idle can increase. That said, I was actually thinking of my experience with my own 82 720 4x4. Consider the age and wear in the OEM carburetors. Worn throttle plates and increased air flow through the primary venturi because of them will cause increased idle speed. My 82 will not idle below 1000-1100 RPM, idle screw backed all the way off, throttle cable free and all other variables normal without reducing timing, shutting down the mixture screw and destroying the idle quality. The butterflies are just worn out.



#14 datzenmike

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Posted 20 November 2017 - 08:51 PM

Fast idle possible causes... (other than the idle speed screw and a tight throttle cable)

 

 

Fast idle cam stuck in an on position even though the choke is fully off.

BCDD set too sensitive and not shutting off at idle

Secondary throttle plate stuck cracked open slightly.(had this once) Possibly a foreign object like a piece of the air filter caught in the butterfly. 

Vacuum secondary linkage stuck or jammed.


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#15 Panterafan1999

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Posted 22 November 2017 - 02:34 AM

I've also had a similar issue with my 85' 2.4 4x4. It idles at 3 grand. It's like I'm always holding the gas down with moderate pressure, even though my foot is off the gas. Any ideas guys?

#16 datzenmike

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Posted 22 November 2017 - 06:11 AM

Post directly above yours has several causes..

 

Add to it...

 

choke not shutting off. warm engine and take top off air filter and look. Choke plate should be vertical and fully open.

throttle cable too tight loosen nut and adjust

Idle speed screw turned in too far. turn screw outward to reduce

 

 

Fast idle possible causes... (other than the idle speed screw and a tight throttle cable)

 

 

Fast idle cam stuck in an on position even though the choke is fully off. inspect, spray with carb cleaner or WD 40, hold throttle half open and work linkage by hand moving through the range of motion to loosen. Plastic fast idle cam should be free to move

BCDD set too sensitive and not shutting off at idle. turn adjustment counter clockwise to reduce sensitivity

Secondary throttle plate stuck cracked open slightly.(had this once) Possibly a foreign object like a piece of the air filter caught in the butterfly. inspect visually with flashlight

Vacuum secondary linkage stuck or jammed. inspect visually, use carb cleaner, hold throttle half open and loosen linkage.


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#17 Sluzarski

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Posted 17 December 2017 - 12:39 PM

I'm having similar issue with my 1984 720 with the z24. When I start it up it idles down low around where it should (at least I think, my tech doesn't work) for the first few seconds, but the high idle kicks in and doesn't want to turn off. It's quite perplexing because for nearly two weeks it was idling nice and low (albeit a little rough) and the high idle would only be on for a couple minutes at most. I don't have a whole lot of experience working on these engines or carbs in general. Any help would be much appreciated!

I will take a look at the choke and fast idle cams and see if they're getting sticky. I would like to adjust the fuel mixture because I'm sure it will idle a lot smoother but thats hard to do when it won't come off the high idle

#18 datzenmike

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Posted 17 December 2017 - 01:10 PM

All chokes (and fast idle) are off on a cold engine because the throttle is closed on the lowest step on the fast idle cam. To start a carbed vehicle, first step on the gas at least once and sometimes more in extreme cold. This releases the spring loaded choke to it's full on position and sets the fast idle cam in position. EFI owners never have to do this as the car electronics sense the cold and alter the start up mixture.

 

 

To check the choke and fast idle function...

 

Hold the throttle part open.

Open and close the choke flap.

 

You'll see a linkage going down to the fast idle cam that lifts up as the choke goes on.

The fast idle cam turns presenting a number of higher and higher steps for the throttle to close against, holding it open.

 

Let the throttle close...

 

During warn up the choke plate begins to open and the linkage moves downward. (open the choke flap)

The fast idle cam would rotate to a lower step but the throttle is closed on it preventing this from happening.

If you step on the gas the cam is released and depending on how warmed up the choke is the idle is reduced.


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#19 Sluzarski

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Posted 17 December 2017 - 04:18 PM

I think i might have gotten it fixed. Fast idle cam was sticky. Shot it with some wd40 and it loosened up. Ended up backing out the idle screw a little bit.

Going to let it sit for a few hours and see how it cold starts again

#20 720inOlyWa

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Posted 17 December 2017 - 06:41 PM

You still have all kinds of vacuum leaks going on there. Mine would not run at all with all of those openings unplugged. I can‘t help much with stock carbs- I don‘t run any, myself.