'84 2x4 vacuum line confusion and high idling issue

[braiden]

Hi all,

I recently purchased a '84 720 with a high idling problem. I read through the vacuum placement forum and a couple others, but none of them seem to match my engine exactly. I'm hoping to use this thread to gather enough advice to get this thing running like new, so bear with me as i begin this exploration. I've attached an image of the front of the carb below. In the picture, there are two open ports that don't seem to match the advice on the 'vacuum line placement' thread (VLPT). So, here we go...

First, i'm confused by the carb vent line (the port on the left). My carb has one, and the VLPT suggests that it connects to the canister, but as you can see from the second image, my canister doesn't look the same. Because of this, it's not apparent where the carb vent should attach, if at all.



If that image doesn't attach, you can see the carb here: https://picasaweb.google.com/1094029405 ... 9087920034

Does anyone know where this vent attaches on the '84 model? Below is a picture of the canister, which has fewer hose ports and pieces than the one listed in the VLPT.


canister: https://picasaweb.google.com/1094029405 ... 9275067634


Second, on the right side of the carb photo, you'll notice another port that never had a line attached to it. Does any know where that's suposed to go?

Thanks for your help. I'll be posting pictures, questions and outcomes as I get this thing back on the road.

[dreadsiren]

do you have emissions testing where your at? the only vac line you realy need is the vacume advance to the dizzy, but if your fighting emissions then there is alot of very good faq's here on complete vacuum line redo's etc. *edit: sorry, didnt see you had already found said faq

post6080291.html

there is also a grip of info in the dyi sticky in this forum

it does connect to the canister via the same way the picture does (mentaly delete the non existant ones) all the nissan 720's I have ever owned (4 in all from 1982-1985) over half of the bulk was removed and capped off. so the other one your looking to see what it is has always been capped off on mine from its prev owner or factory... I believe factory.

[fastboatman29212]

Hi all,

I recently purchased a '84 720 with a high idling problem. I read through the vacuum placement forum and a couple others, but none of them seem to match my engine exactly. I'm hoping to use this thread to gather enough advice to get this thing running like new, so bear with me as i begin this exploration. I've attached an image of the front of the carb below. In the picture, there are two open ports that don't seem to match the advice on the 'vacuum line placement' thread (VLPT). So, here we go...

First, i'm confused by the carb vent line (the port on the left). My carb has one, and the VLPT suggests that it connects to the canister, but as you can see from the second image, my canister doesn't look the same. Because of this, it's not apparent where the carb vent should attach, if at all.



If that image doesn't attach, you can see the carb here: https://picasaweb.google.com/1094029405 ... 9087920034

Does anyone know where this vent attaches on the '84 model? Below is a picture of the canister, which has fewer hose ports and pieces than the one listed in the VLPT.


canister: https://picasaweb.google.com/1094029405 ... 9275067634


Second, on the right side of the carb photo, you'll notice another port that never had a line attached to it. Does any know where that's suposed to go?

Thanks for your help. I'll be posting pictures, questions and outcomes as I get this thing back on the road.

I have a FSM for 1984 at home. We'll figure this out once I get to look at it with book in hand. My guess is that the previous owner may have turned up the idle screw to compensate for a few leaking vacuum lines.

More later.

[braiden]

Hi!

Thanks for the info! Unfortunately, I do live in a state that requires emissions testing, so i' hoping to resurrect the vacuum system. I've replaced most, if not all of the vacuum hoses (except the two noted above), and it's still idling very high. Directly after starting the engine, it begins at high RPM (no tach, so i' not sure of the reading) and depressing the gas pedal only increases the idle a small amount. So, I'm thinking that it's idling close to it's maximum. I've messed around with the idle screw to no avail, so now i'm a little lost.

Thank you!

[dreadsiren]

have you adjusted the mixture screw at the bottom passenger side of the carb? (not the idle screw but one at the very base, recessed) also, did you check to make sure there are no leaks in the intake? not as common but have you checked your idle cam to make sure it isnt sticking (back side of carb) or missing a spring. these are all issues I have come across even without the emissions. I would also test to make sure your booster doesnt have a hole in it. you have capped off the mising line I am asuming?

[fastboatman29212]

have you adjusted the mixture screw at the bottom passenger side of the carb? (not the idle screw but one at the very base, recessed) also, did you check to make sure there are no leaks in the intake? not as common but have you checked your idle cam to make sure it isnt sticking (back side of carb) or missing a spring. these are all issues I have come across even without the emissions. I would also test to make sure your booster doesnt have a hole in it. you have capped off the mising line I am asuming?

I'd leave the mixing screw at the bottom of the carb alone if it is still plugged with the factory seal. It is the least likely culprit IMHO.

[dreadsiren]

there are two screws on the bottom of the carb, top one covered by brass (factory set) and one just recessed (user adjustable) most used nissans have had one too many people fiddle with them... and most will mess with the mix screw that is exposed. does choke still work on carb or has it been disabled? (ie butterfly pulled, linkeage disabled)

[fastboatman29212]

From looking at the charcoal cannister you have, I really can't say if you have a Calif emission system or non-Calif one. If you have the Boost Control Valve, then it is a non-Calif system. The Calif models didn't have the BCV. It really doesn't matter much except the diagram will make more sense if you have one.

So as far as the fuel bowl vent, you can leave it open or cap it. Either way, it has nothing to do with your fast idle.

For the other vent tube, cap it or run the tubing like in the diagram. Again, it probably doesn't have anything to do with your fast idle, but you can prove it to yourself by capping it off.

Here are some scans from my 1984 FSM.
This 1st one is just straight from the FSM.


I added stuff for the next 2 pics.

[Rev_D21]

Is the auto choke working? How about the fast idle cam? Is it free and working properly? Is the butterfly sticking?

[braiden]

Thanks for the advice and the diagrams!

I'm beginning to think either the canister or the carb was switched out on this truck. While I have some carb features for the high altitude version, I don't have the high altitude canister. Looking at the first diagram, i'm pretty sure the boost control valve is hooked into the BCDD on the driver's side of the carb. I've hooked that up properly, but there's still another port. I've plugged both the extra ports for now, and i'm headed out to test the other advice.

I'm about to look in the butterfly (think it's free), and make sure the fast idle cam is hooked up, but i'm not sure how to determine if the auto-choke is working properly. Any advice?

I'll take more pictures of the set up and post them to make sure everything looks right to the panel. Thanks again for your help! I'm so glad this exists.

[Rev_D21]

Checking the auto-choke is a visual process. Start by stepping on the gas pedal while the engine is cold to close the choke. As the engine warms up the choke should open on it's own. The auto-choke on my old 720 didn't work and caused high idle all the time so I bypassed it by using a clothes hanger pin to hold the choke open once the truck got started, I'm saying that's the correct fix by any means...mine was a hack job.

[braiden]

On a cold engine, directly after starting it, the choke was seemingly open all the way (was closed before starting engine). Does that mean that the auto-choke isn't working?

Thank you!

[Rev_D21]

It should open gradually but as long as it is opening that means your problem lies elsewhere.

[braiden]

Well, I suppose I'll adjust the choke after the idling issue has been solved. It seems like the fast idle cam is moving freely, though I haven't made any adjustments to it, and I can't really see it with the carb on on the engine. Before I remove it and begin messing with that, I think i'll try to adjust the mixture crew. In the image linked below, is the open hole on the bottom the screw you were referencing?

https://picasaweb.google.com/1094029405 ... 2613521170

Thanks!

[fastboatman29212]

If you have an automatic choke, then there is nothing to adjust. It either works or it doesn't (like broken).

The hole in the photo on your last post is for the idle mixing (idle adjusting screw in the photo) and usually has a factory seal over it. I would not adjust that screw, but I WOULD stick a screwdriver in there to see if the screw it actually there or not. Or you could use a flashlight and mirror to look inside maybe. If the screw is missing, then that could be your idle problem. The screw tip is used to narrow a passage that fuel flows through so the motor gets the right amount to idle. If the screw is missing, then the passageway is wide open and the motor would idle higher than it should.

If the screw is missing, don't worry, I'll send you a new one. I kinda doubt that is the problem though. Have you tried adjusting the throttle screw? (Not the idle mixing screw, this is a different one.) See pic.

[braiden]

Hi!

I have attempted to adjust the throttle screw anywhere from a half turn out to 2.5 turns out, and it doesn't seem to make a difference. When I looked in the hole earlier, it did have a screw, and I remember when I purchased the truck, the seller mentioned that he had to raise the idle to keep it from stalling at rest. Since the factory seal is missing from the mixture screw, do you think an adjustment he may have made could cause the fast idle?

[fastboatman29212]

Absolutely! He may have it backed out too far. Here is what you should do -
1. With the motor off, screw the idle set screw IN while counting the turns until it is lightly seated.
2. Write down on paper how many turns it took. That way, you can always get back to where it is now.
3. Back the screw out 1/2 turn from seated and try to start the motor.
4. If it doesn't start, back it out another 1/2 turn and try again.
5. Repeat this until you are at the same number of turns OUT that you counted in step 1 and 2.

The Factory Service Manual says that the idle mixing screw is set properly when the carbon monoxide is between 0.5% and 2.5% but folks like us have no way of measuring this. You will just have to wing it or find someone with the necessary tools/meter. I imagine that if your state has emission laws, you will need to get this screw set at the optimum point so you will pass emissions testing.

[dreadsiren]

yup! thats what I was thinking, people (prev owners ) try to mask symptoms instead of fixing... could be the timing on the dizzy was a bit retarded or crack in vac lines so he cranked'er open... fastboatman covered the steps on that adjustment quite well
have you properly timed it? rotated dizzy & used light or top dead center kind of timed? (or even red neck version, advance/retard till it sounds good, lol) wanted to make sure you got it timed before adjusting everything just to have to re-adjust it after timing the beast correct (you tuned it up I am asuming?)

[fastboatman29212]

1. With the motor off, screw the idle set screw IN while counting the turns until it is lightly seated.
2. Write down on paper how many turns it took. That way, you can always get back to where it is now.
3. Back the screw out 1/2 turn from seated and try to start the motor.
4. If it doesn't start, back it out another 1/2 turn and try again.
5. Repeat this until you are at the same number of turns OUT that you counted in step 1 and 2.

To clarify what I meant on step 5 above, I should have said - Repeat step 4 until it idles properly and then you might need to tweak it a 1/4 turn either way for a final adjustment. Hopefully, you will find the right setting before it gets backed out to where it was before.

[braiden]

Hi!

So, I tried to back the mixture screw all the way in and pull it out a half step, but it didn't change the idle. Then, deciding that this thing would idle high even if it didn't have an engine in it, I decided to screw the throttle screw all the way in and turn it on. Now, I've turned both screws in all the way and it still idles pretty rough, though a little slower than it did before.

I haven't timed the thing, but I did replace/gap the plugs and change the wires. I'm at a loss. Oh, and another hose question. In the image linked below, there's hose that's attached to the driver's side of the firewall, and is long enough to reach the carb. As far as I remember, it wasn't attached to anything when I pulled it apart to begin the line replacement. Does anyone know where this goes?

https://picasaweb.google.com/1094029405 ... 7692097874

Thanks

[braiden]

Also, sorry for the delay between posts. I'm currently trying to get my new apartment together as well, and it's been monopolizing my time.

Thanks again for all your help!