720 Carburetor Tutorial with Pictures

1980-1986 Datsun 720 forums. All 720-specific topics and discussion can be found here.
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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Fri Oct 24, 2014 11:43 am

First verify that your carb fuel bowl shows gas in the bowl. You should be able to see it through the front glass. If not, then you might have a fuel delivery problem such as a bad or weak fuel pump.
I have a question. When it cuts off, does it sputter a little and then die or does it simply shut off. How hard is it to get it started again? If the fuel bowl in the carb is not filling properly, then it may just need the float needle adjusted. These are just some of my ideas but others might have thoughts too.
Let me know what you find out.


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Charlie69
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Re: 720 Carburetor Tutorial with Pictures

Postby Charlie69 » Fri Oct 31, 2014 10:10 pm

One word "WEBER"

busterone
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Re: 720 Carburetor Tutorial with Pictures

Postby busterone » Tue Feb 17, 2015 4:42 pm

I recently purchased an 84 720 and so far I am very pleased with it. It is a project truck referenced in my first thread here- new-here-recently-purchased-a-720-t597310.html. I have a hopefully simple question about the Hitachi carb. Before I removed all the vacuum lines and plugged up the ports, the fast idle was operating as it should, and idled down after warm up. The engine was rough and erratic though, due to vacuum issues. Since I plugged all the ports except the vacuum advance and brake booster, the fast idle does not operate at all. The engine runs fantastic this way, however I have noticed that on cold start, it isn't idled up as it did previously. The idle is consistently the same speed on cold start as it is after warm up. What exactly controls the fast idle operation? Is it related to the now non-existent emissions that I have disabled by removing all vacuum ?

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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Tue Feb 17, 2015 4:52 pm

The DIY section has a carb rebuild section that I made a while back. Study the section about the fast idle cam. That might help you find the problem. You can also see the same info on my website at the following LINK = http://720nissantrucks.weebly.com/1986- ... build.html

busterone
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Re: 720 Carburetor Tutorial with Pictures

Postby busterone » Tue Feb 17, 2015 7:38 pm

I have studied the rebuild tutorial and the link you gave me. That shows me how to disassemble and reassemble the carb, but it doesn't tell me the principle on how the fast idle cam is activated. What I am asking is it related to vacuum or not. Everything that was connected via vacuum lines whatsoever is now out of the loop. -EGR, Canister, vacuum switch, etc, the whole works.

As I stated, it was working correctly before I removed the lines and plugged the vacuum ports, so my obvious assumption is that it is related. :)

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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Tue Feb 17, 2015 7:59 pm

I believe the position of the fast idle cam is reulated by the choke. Vacuum regulation would not make sense. Perhaps the automatic choke has stopped working. It is also possible that the linkage bar from the choke to the fast idle cam is broken, or perhaps a return spring is broken. I doubt very much that vacuum has anything to do with it. Sorry you are having trouble.

busterone
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Re: 720 Carburetor Tutorial with Pictures

Postby busterone » Tue Feb 17, 2015 9:02 pm

After I posted, I went back through the tut and pics and came to the same conclusion, I just haven't had a chance to post back. Without having the actual carb in front of me, it appears that it is related to the choke, or by pressing the throttle once before starting to cause it to move to the fast idle position. It isn't really much of a problem for me, more of a curiosity. I am sure the choke is working since the engine starts so easily when cold. We have had days consistently in the high 20's here in NC, and it has started immediately each time. If the issue was opposite, and the fast idle was activating but never releasing, then I would consider it an issue that needed repair, but in my case, it never activates. The engine starts and runs fine at the normal idle, so for now, I am going to let it be. Thanks for the help and all the info you have previously posted. There is some good stuff there.

busterone
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Re: 720 Carburetor Tutorial with Pictures

Postby busterone » Sun Mar 08, 2015 11:24 am

I'm back. :) I repaired the fast idle issue. Now I have a question for all the Hitachi experts.
In the process of capping vacuum lines, etc, I never noticed that this carb has no dashpot. I have no idea how I missed that, but I did. The bracket is there, so someone must have removed it in the past. My limited understanding is that the dashpot has something to do with deceleration. What sort of running issues can I expect to see with it missing or should I just go ahead and spend some money on buying one?

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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Sun Mar 08, 2015 5:03 pm

busterone wrote:My limited understanding is that the dashpot has something to do with deceleration. What sort of running issues can I expect to see with it missing or should I just go ahead and spend some money on buying one?
I believe you are correct about the dashpot function. In my words, I'd say it prevents the throttle from going to absolute zero too quickly. I would not spend money on a new one.

busterone
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Re: 720 Carburetor Tutorial with Pictures

Postby busterone » Sun Mar 08, 2015 6:32 pm

Thanks, I suspected as much. If I find one cheap on ebay or somewhere local, I may get it, but the truck seems to be running pretty good as it is now. It just passed inspection yesterday and I will have tags on it Monday. I plan to drive it around for a few days on short trips to test it out. If I see an issue, then I may start looking for one.

bscott25
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Re: 720 Carburetor Tutorial with Pictures

Postby bscott25 » Thu Apr 16, 2015 2:20 pm

I just bought an 84 that has just started blowing black smoke and idling a little shaky. Is something stuck in the carb that I can fix without a tear down?

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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Thu Apr 16, 2015 5:12 pm

bscott25 wrote:I just bought an 84 that has just started blowing black smoke and idling a little shaky. Is something stuck in the carb that I can fix without a tear down?
First thing you should do is check the compression of each cylinder and look at the spark plugs to see if they have a lot of oil on them. Your engine might have bad compression. Black smoke is not usually caused by a carburetor problem.

irstaxhelp2
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Re: 720 Carburetor Tutorial with Pictures

Postby irstaxhelp2 » Wed May 20, 2015 4:23 pm

Same truck with similar problem. Just hesitation when accelerating. More so when cold (choke issue)

Any ideas?

If I put a weber on what about smoggin it?


Thx for the help and I love my truck!
John

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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Wed May 20, 2015 5:32 pm

Back to the basics. Check engine compression before you mess with the carb. You could check for a vacuum leak first. That's quick and easy to do. Don't waste money and time trying to diagnose a carb problem until you are sure the engine compression is good. A Weber carb will make it run great . . . . . only if the compression is good. Each state has different inspection rules so I can't say how that will affect you in your state.

irstaxhelp2
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Re: 720 Carburetor Tutorial with Pictures

Postby irstaxhelp2 » Thu May 21, 2015 1:31 pm

Thank you I will do that

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Re: 720 Carburetor Tutorial with Pictures

Postby dsfairplaine » Tue Jun 23, 2015 4:06 pm

Ayo! Just got me a 85 Nissan king cab, plastic arm coming from choke pull back was broken. The truck would lope at idle dying more often than living. Died at every stop light. The choke flap was limp in there as well. Rebuilt carb with your impressive tutorial here. Great pic's and write-up. I am faced with a similar problem after rebuild. NO IDLE! From what I gather after inspection is that the fast idle cam basically floats in there. I have to physically put it into place to use it and any movement with the throttle throws it into non op setting. Also the linkage arm attached to it is as limp as the cam. The thermo coil in the choke housing seems limp as well. The Butterfly will be almost closed when cold. However the flap opens as it warms indicating the spring is working. There seems to be a lot of play up there where the fast idle cam linkage arm, the choke pull back arm, and the butterfly arm all connect. How does the fast idle cam return to "functioning" position?

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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Tue Jun 23, 2015 4:11 pm

There is a small spring on the fast idle cam. With all those issues, I'd suggest buying another carb for parts or switching to a Weber carb.

dsfairplaine
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Re: 720 Carburetor Tutorial with Pictures

Postby dsfairplaine » Tue Jun 23, 2015 4:22 pm

Thanks for such a fast response. I'm a cali boy and the webs wont smog here... I recall a section where a guy said he filed the little nub that prevents choke adjustment to tighten the thermo coil up. Would you recommend that or would that even help with the fast idle cam function?

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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Tue Jun 23, 2015 5:22 pm

When activated, the fast idle cam will hold the lower throttle plate open a little more than usual. Once the engine warms up just a wee bit, then the fast idle cam becomes inactive. So it really only functions for a minute or less. If the choke is not working properly, then the thermo coil is likely broken. Is there any way to shange it to manual choke?

fortcollins
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Re: 720 Carburetor Tutorial with Pictures

Postby fortcollins » Wed Jul 15, 2015 12:20 pm

I have a 85 Nissan 720 Z24 4x4 pickup.
It is gutless

FYI
In trying to get more power I read in the Nissan shop manual. It says that the timing is varied by vacuum to the distributor advance.
I ran a vacuum line directly from the base of the carb to the distributor vacuum advance. It runs much much better.

After 3000 RPM's the HP is flat. I think that the secondary of the carb is not working.
How do I verify that?
Goosing it and looking down the carb I see that the secondary only opens a crack.

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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Thu Jul 16, 2015 6:09 am

To correctly diagnose the cause for lack of power may take a lot of work. I hope it is only caused by a single problem and not several combined problems. Try to get the timing set correctly. Make sure the vacuum lines are not leaking. Add some Seafoam to a half tank of fuel and run the tank almost dry. Check the compression of each cylinder. If you are using oversized tires then performance will suffer. Check the color of the spark plugs. Try to verify that the dizzy vacuum advance is actually functioning. Rebuild the carb if necessary but I'd look into other things first. That's just a list of things to investigate. My truck has 220k miles and runs well enough but is rather gutless on the highway too.

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Re: 720 Carburetor Tutorial with Pictures

Postby fortcollins » Thu Jul 16, 2015 2:05 pm

Vacuum line is good. Vacuum advance is working correctly.
Plugs look good. Both sides firing.
The Cat is not a problem.
I will try some Seafoam but...

Right now I am focusing on the flat performance above 3000 RPM's. The last Datsun I had the higher the RPM the harder it pulled.
Is there a way to verify the secondary is working?
Right now I am looking at going to e-bay and buying a new carb. I have not had good luck rebuilding carbs.

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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Thu Jul 16, 2015 2:11 pm

A visual check is difficult unless you use a video camera or run it on a dyno and visually inspect. My understanding is that the engine must actually be pulling the load of the vehicle weight. Simply looking down the carb at high RPMs while in neutral is not the same as actual driving. Sorry I can't be any more help.

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Re: 720 Carburetor Tutorial with Pictures

Postby Corradokid90 » Wed Aug 19, 2015 11:50 am

Hey I'm new to carbs but I just picked up an 81 Datsun 720 with a z22. Any link to a carb rebuild for that? Or is it similar enough to this? Sorry I'm a noob :confused:

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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Wed Aug 19, 2015 2:01 pm

I am sure the rebuild is similar. I don't know of any thread for rebuilding your specific carb.

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Re: 720 Carburetor Tutorial with Pictures

Postby Corradokid90 » Thu Aug 20, 2015 4:09 am

fastboatman29212 wrote:I am sure the rebuild is similar. I don't know of any thread for rebuilding your specific carb.
Thanks for the reply. I'll give it a whirl. If I have any problems I'm sure I'll be bugging you! :biggrin:

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Re: 720 Carburetor Tutorial with Pictures

Postby Corradokid90 » Mon Aug 24, 2015 5:04 pm

Well I've rebuilt the carb and put everything back together and I've got the same issue. It's idling fine but kinda bogs out when you try to accelerate. I took the air cleaner off and had a friend hit the accelerator. The smaller flap is opening and spraying fuel while the larger is not. I was able to manually open the secondary and it sprayed fuel and accelerated just fine. Trying to figure out if I possibly didn't hook up one of the linkages correctly?

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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Mon Aug 24, 2015 6:26 pm

If you slowly accelerate, like REALLY slow, does it bog then? It might just be the accelerator pump.

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Re: 720 Carburetor Tutorial with Pictures

Postby Corradokid90 » Tue Aug 25, 2015 6:09 am

fastboatman29212 wrote:If you slowly accelerate, like REALLY slow, does it bog then? It might just be the accelerator pump.
Yeah it will. I put a new accelerator pump in with the rebuild kit too. I hate that I know nothing about carbs ha. From what I gathered, when you accelerate more the secondary should be opening. That's functional, and sprays fuel and all. It just won't operate the way it's supposed to. I can pull it back off and post pics ha.

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fastboatman29212
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Re: 720 Carburetor Tutorial with Pictures

Postby fastboatman29212 » Tue Aug 25, 2015 6:17 am

Pics won't tell me much because I won't be able to see how the linkages work. Sounds like you are right. Probably the secondary is not working.


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