720 Carburetor Tutorial with Pictures

1980-1986 Datsun 720 forums. All 720-specific topics and discussion can be found here.
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pistolkeith
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Car: 1985 Nissan King Cab 720

Re: 720 Carburetor Tutorial with Pictures

Postby pistolkeith » Mon Jul 02, 2012 10:47 pm

hi Fastboatman, you have to be one of the most valuable people on this forum. the time and effort you put in to your posts and DIY's is unreal. so firstly, thank you so so much for that!

My carb is the original and seems to match what you have here. However it has run a little weird since owning it. Basically when i button off in gear and clutch in it revs out for half a second or so then dies back down to idle. almost like a slow connection or some sort. ideas? secondly, what i have identified as the automatic choke mechanism is broken. The little plastic(?) link that goes to the rod going into the choke housing on the back right hand side of the carb is broken. Is it the kind of thing you can just super glue back together and it'll be sweet or is it a "pull her apart and replace it" kind of job. i have a spare complete carb for parts if need be however i am not the most confident as to take it all apart and fix it up myself. it hasn't had any issues with choke and idle but then again it has spent 99.9 % of its life in southern california where temperatures below 70degrees is classed as dangerously cold. any light you can shed on it would be amazing :)

thanks again!


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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Tue Jul 03, 2012 10:25 am

pistolkeith wrote:when i button off in gear and clutch in it revs out for half a second or so then dies back down to idle. almost like a slow connection or some sort. ideas?
I think that your throttle linkage is gummed up or sticking. With the engine off, have someone get in the truck and press the accelerator. When they take their foot off the gas, watch to see if the throttle linkage goes back slow of fast. It may just be sticky or have a weak spring.


pistolkeith wrote: secondly, what i have identified as the automatic choke mechanism is broken. The little plastic(?) link that goes to the rod going into the choke housing on the back right hand side of the carb is broken. Is it the kind of thing you can just super glue back together and it'll be sweet or is it a "pull her apart and replace it" kind of job.
I would like to see a photo of the part. If it is plastic, you can brobably repair it with glue. Just not sure at this point until I know more.

Thanks for the kind words.

Midnighttouring
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Car: 1986 Nissan 720 4x4

Re: 720 Carburetor Tutorial with Pictures

Postby Midnighttouring » Sat Jul 28, 2012 12:56 am

Being new to a Carb this is great. Many Thanks!

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cws1983
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Car: 1984 Nissan 720 4x4 KC

Re: 720 Carburetor Tutorial with Pictures

Postby cws1983 » Thu Sep 20, 2012 7:06 am

Hi all new to the forum. Dear Mr.Fastboatman29212 I own a 84 720 KC 4x4 and I am gearing up to rebuild the carb because its been sitting for 6 years. But the i'm have a problem with the pictures of your carb rebuild loading on my computer for some reason. Your vacuum tutorial loaded fine so I don't know what it is. But is there any way I could get you to email me the pictures that you posted on forum?

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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Thu Sep 20, 2012 11:47 am

cws1983 wrote:But is there any way I could get you to email me the pictures that you posted on forum?
Send me a PM with your email address and I will send the pix ASAP.
FBM29212

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cws1983
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Re: 720 Carburetor Tutorial with Pictures

Postby cws1983 » Thu Sep 20, 2012 1:44 pm

Thanks my email address is cws1983@att.net.

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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Thu Sep 20, 2012 2:21 pm

I found the pics. Watch for emails. There will be 11 of them.

Natertot
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Re: 720 Carburetor Tutorial with Pictures

Postby Natertot » Sun Oct 07, 2012 6:13 pm

This thread has been a ton of help but I am still stumped. I have a 1986 720 (carb not TBI) that was running when I parked it in my garage 6 years ago. It wouldn't idle w/o revving the gas but it ran none the less. When I decided to get it on the road again it wouldn't start. i drained the gas tank, put fresh gas in, replaced the fuel filter, checked the fuel pump (which is working) but no luck after all of this. I have replaced all of the vacuum lines, or at least the smaller diameter ones, the larger diameter are next. I finally found that if I hold the choke flap closed and disconnect the hose to the choke push off, it will start and idle pretty well (runs rich but runs). I then replaced the choke relay thinking maybe that went bad but no change. There is power getting to the choke heater as well. Based on the pics on this thread nothing seems to be visually missing or broken. Any ideas? My mechanical skills are limited but can do some minor DIY.

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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Mon Oct 08, 2012 5:53 am

Your carb probably has blocked passages from fuel that has turned to varnish. The solution to the problem might not be easy. You can disconnect the fuel pump from the tank and run fuel through the pump from a gas can that has maybe a gallon of gas with 1 bottle of Carb cleaner solution mixed in it. That might be enough to clean out something that's clogged. However, for that to work, there has to be at least SOME flow through those passages, otherwise the cleaner can't get to the clog.

The Carb rebuild thread might be helpful too. You can remove the Carb, and spray some Carb cleaner through the tiny passages without taking it all apart.
I'm pretty sure you will get it to run again if you remove it and spray all the passages you can find. Use the little red straw and always wear eye protection.
Good luck.

devondatsun
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Re: 720 Carburetor Tutorial with Pictures

Postby devondatsun » Thu Nov 01, 2012 7:00 pm

Okay so i recently purchased a 83 datsun pickup 4x4 and it needed a new carb so i replaced it and now it is not running i need help whats going on i tried adjusting the throttle and everything but nothing is working it was running before the replacement to

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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Fri Nov 02, 2012 12:51 pm

1. You need to make sure that the fuel pump is getting fuel into the carb.
2. Does the engine sound like it's trying to start? Does it start and then shut off?
3. Pour a few ounces of gas down the throat of the carb and see if it will run for a few seconds.
4. Do you have fuel in the carb bowl?
5. Do you have spark? Make sure your plug wires and coil are hooked up correctly.
6. Post a few pictures of the carb if you can. What kind of carb is it?
We'll try to help if we can.

Natertot
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Re: 720 Carburetor Tutorial with Pictures

Postby Natertot » Sat Nov 03, 2012 4:09 pm

Thanks fastboatman, you were right about the carb being blocked up. Had to have the carb rebuilt, out of my league so had it professionally done, when I got it back it was still running a little rough, replaced cap, rotor, plugs wires, fuel filter (again), air filter, checked timing. Its running 98% better now. Idles perfectly, but about 50% of the time it chokes out a little when first accelerating in 1st and 2nd gears,also has a slight backfire (sometimes). Im thinking the fuel lines may be a little clogged restricting fuel a little but its not happening all the time. The CAT is the original, would it cause this problem if it is going bad? It has no muffler on it right now, (that is next on my list). Want to get it running at 100% since it is my daily driver now. What should I check next?

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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Sat Nov 03, 2012 4:53 pm

I replaced my CAT at 215,000 miles and it was not clogged, so I wasted my money and my truck doesn't run any better. You might check the timing again(3 degrees BTDC). Make sure you have the vacuum advance line disconnected and plugged when you check the timing. The backfire might go away once you replace the exhaust. I do not think the fuel lines are blocked, based on what you've said. You might have a small vacuum leak, recheck all the vacuum lines. Lemme know if you find the problem.

flrboard
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Re: 720 Carburetor Tutorial with Pictures

Postby flrboard » Mon Feb 04, 2013 4:07 pm

I have been carefully looking at the carb and vacuum line pictures and notes. My 1984 720 z24 seems to have the same carb as the 86 version.
On my carb, the pipe marked "carb vent" in the pictures is capped at the carb.
When my truck idles it acts like it is flooding. It has rough idle and sometimes stalls.
It starts after sitting awhile on the first crank. But has to be nursed to start after it has stalled out...just like a flooded car.
There is a relay under the dash on the passenger side that is sometimes referred to in posts as a part that the chilton and haynes techs neglected to describe.
that relay has many jobs and one is related to the fuel pump and it might also control a fuel shut off in the carb.
On my car the relay was bypassed so that the fuel pump is connected directly to the ignition switch.
when the ignition is on the pump is running.
Before I rip out all the fancy emission control related vacuum lines , I would like to know if that flooding would be stopped by
1. un plugging the vent pipe and routing to where ????
2. getting the relay back in the system so that it controls the fuel pump.

Thanks in advance for any light shed on this complicated device!

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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Mon Feb 04, 2013 4:35 pm

First and foremost, you have not determined WHY the truck is hard to start. The fuel pump is supposed to run continuously while the ignition is on.

The relay you spoke about DOES NOT control anything in the carb. Don't mess with that.

The vent being capped is not likely the problem, but you can uncap it and see if anything changes. I doubt it will.

The carb has a fuel supply line and when the carb fuel bowl is full, the float needle plugs the carb inlet so no more fuel goes into the carb bowl. Since the fuel pump is still pumping towards the carb, fuel then gets redirected to the RETURN fuel line and goes back to the tank.

If indeed the carb is flooding, then the problem is with the float needle not sealing the inlet. That means you need to open the carb and clean things, or at least run some fuel treatment through the tank/carb.

flrboard
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Re: 720 Carburetor Tutorial with Pictures

Postby flrboard » Mon Feb 04, 2013 5:13 pm

okay. I will go after the float and needle.
I managed to leave out a bit of info. The carb is visibly wet.. with fuel seems to come up past the accelerator pump.
plunger..... after being at idle for a while. It is then that it stalls and is hard to start.
Any other time the car start easily and always drives well.

The accelerator pump was recently replaced by a paid mechanic so I am inclined to trust that much of the carb.


I am happy not to mess with the relay though it does have a brown wire that runs to the fuel pump. the wire is always dead. The previous owner must have run the new wire to the pump.
I saw that on your excellent set of pictures the carb vent ran to that return line.. Since I have too much fuel and a capped vent, I had wondered how the excess fuel in my engine could find the return line.
For now I will use your advice regarding the float and report back.
Thank you for the quick response!

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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Mon Feb 04, 2013 7:37 pm

OK, the vent tube does not return fuel to anywhere. It is simply a way for air to exit the carb bowl. Just forget about it as part of the problem for now.

The picture below shows my carb. The main fuel line comes straight up from below and flows into the carb inlet. You can see on the photo that there is a "T" coming off of the fuel line just below the rubber portion. THAT "T" is the beginning of the return fuel line and takes excess fuel back to the tank.

If the carb bowl is full, then the float needle will close the fuel inlet. The fuel has no choice except to take the path out through the "T" and back to the carb.

One thing you can do is disconnect the fuel line and make sure the return line is not plugged up. It's simple to do. Just disconnect the rubber portion on the carb side. Place your thumb over the rubber portion to block it and turn the fuel pump on. The fuel will have to flow back to the tank. If it builds up high pressure, then you have a blockage.

Image
Last edited by fastboatman29212 on Tue Feb 05, 2013 10:03 pm, edited 1 time in total.

flrboard
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Re: 720 Carburetor Tutorial with Pictures

Postby flrboard » Mon Feb 04, 2013 8:22 pm

thanks I will try that!
I am new to using forums, where do I learn to post photos? Your pictures are so useful I am wishing I could occasionally send a photo rather then stumble along with my words.

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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Tue Feb 05, 2013 5:55 pm

To post photos:
1. Open a photo storage account. They are free. I use Photobucket.com, but other sites work OK too.
2. Upload photos to your Photobucket account. Make sure they are correctly oriented (rotate them if needed).
3. Each photo you upload will have a box either below it or on the same page. It will be labeled "Image Links". Look over the listed links. The proper CODE LINK will always start with the prefix [IMG].
4. Use your computer to COPY the Image Link Code. Make sure it starts with the [IMG] prefix.
5. Go to the forum page you want the photo to appear on and PASTE the IMAGE CODE.
6. After any IMAGE CODE you paste, make sure you hit the RETURN key. That way text you type will have some space away from the photo.

flrboard
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Re: 720 Carburetor Tutorial with Pictures

Postby flrboard » Tue Feb 05, 2013 7:17 pm

again, thank you for your generous sharing.
As soon as it stops raining I will be getting under the hood.

boxhead
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Re: 720 Carburetor Tutorial with Pictures

Postby boxhead » Thu Feb 07, 2013 12:07 pm

Hey all, my 720 '85 idles high for 10-15 minutes, and even when warm will idle high for 3-5 minutes, any ideas?
It does run fine, although I get a hesation from a stop, about once a week.
It has lots zoom, in all gears, ( just changed trans and driveshaft)

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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Thu Feb 07, 2013 2:15 pm

1. Check your vacuum lines and carb for leaks.
2. Check the throttle cable linkage to make sure it operates smoothly
3. Make sure the choke plate isn't sticking.
4. Inspect all the plugs and distributor cap. Then confirm that the timing is properly set.
5. Adjust the idle screw to lower the idle to around 800 RPMs once the engine is warmed up and it idles better.

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delcena49
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Re: 720 Carburetor Tutorial with Pictures

Postby delcena49 » Thu Feb 28, 2013 1:18 am

Can you tell me how the throttle cable connects to the carburetor? I just recently put a Nissan Z24 engine and transmission in my 1981 Datsun 4X4 720 pickup but I don't know how to connect the throttle cable to the carburetor and the book doesn't explain or show s%@t. Also the bracket for the adjustments of the throttle cable. I have a bracket but it goes on the driver side of the carburetor instead of the passenger side. Like I said the book isn't much help. I need pics of the passenger side of this carburetor with the throttle cable attached if possible. Thank you

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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Thu Feb 28, 2013 5:07 pm

These pics show the bracket that the throttle cable is anchored to.

Image

Image

The throttle cable has a barrel on the end. It fits into the hole shown here (on the throttle cam).

Image

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delcena49
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Re: 720 Carburetor Tutorial with Pictures

Postby delcena49 » Sun Mar 03, 2013 9:40 pm

Thank you for the pics. I see the problem with mine now. It totally has the wrong mounting bracket on it. That would explain why it will not idle at all. Well it will idle at 1500 RPMs then it dives below 1000 RPMs and dies. It dies at every stop light or stop sign. I'm kinda tired of driving with a foot on the clutch and the other foot half on the brake and gas just to keep it running at stops.

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delcena49
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Re: 720 Carburetor Tutorial with Pictures

Postby delcena49 » Fri Apr 05, 2013 11:13 am

You wouldn't by chance have pictures of how to hook up the vacuum hoses would you? The manual confuses me because it is a lousy diagram. Looking at the manual and looking at the vacuum hoses on my truck none of them are in the right place. This is the last step towards getting my truck smogged. I greatly appreciate any help I can get beings I am doing the repairs myself. How do I go about adding pictures here? I would like everyone to see my Datsun 720 4X4.

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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Fri Apr 05, 2013 5:05 pm

delcena49 wrote:You wouldn't by chance have pictures of how to hook up the vacuum hoses would you? The manual confuses me because it is a lousy diagram. Looking at the manual and looking at the vacuum hoses on my truck none of them are in the right place. This is the last step towards getting my truck smogged. I greatly appreciate any help I can get beings I am doing the repairs myself. How do I go about adding pictures here? I would like everyone to see my Datsun 720 4X4.
To post pics, you should open a photobucket account or similar online storage account. It's free and hassle free too.
You can add a photo to your post by copying the IMG code listed next to the picture you want. You just copy and paste that IMG code and the photo will appear. Hit ENTER twice between each IMG code you paste. You will see it if you check the preview before you finalize your post.

We'd like to see your truck too, so get crackin' LOL!

There is a vacuum line DIY that I made in the DIY section.
Link here: post6080291.html

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delcena49
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Re: 720 Carburetor Tutorial with Pictures

Postby delcena49 » Tue Apr 09, 2013 3:21 pm

Can someone send me a picture of the whole picture under the hood from left to right, front to back of the engine with correct vacuum line hook ups and without the air cleaner on it? I have a 1981 Datsun 4X4 720 pickup with a 1984 Nissan Z24 engine in it and none of the vacuum line are in the correct places. I seriously need some help here. I love my truck. It was a birthday present from my baby sister 2 years ago and it needed a transmission. (It had no reverse) so I had the 84 with tranny put in it. The guy was a wanna be mechanic. Turns out he couldn't read but he did a good job putting it in. he just didn't know how to hook up the vacuum hoses. I bought a Haynes manual for it but it doesn't show all the exit tubes coming out of the carb.


BTW, How do I post pictures?

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delcena49
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Re: 720 Carburetor Tutorial with Pictures

Postby delcena49 » Tue Apr 09, 2013 6:30 pm

Image,Image,Image,Image,Image,Image

The first picture is how my truck use to look. The next several pictures are a work in progress. More pics to follow.

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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Tue Apr 09, 2013 6:33 pm

Did you look at the vacuum line post in the DIY section?


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