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Johnny280

Vacuum line question 280Z SU Carbs *with pictures

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Hi, 

right now i am in the process of fixing all small issues with my Fairlady. Since you guys already helped me I want to try my luck a second time. I had a look at the vacuum lines and to me they seem to be set up in a weird way. The car is a '78 280Z with a 280ZX engine (L28E F54 block with P79 head) converted to Ztherapy SU carbs and original AC. The engine runs fine apart from a slight fluctuation in idle speed (every 3-5 second it goes up from 800rpm to maybe 840rpm and then back down again). I checked all the FSMs to use the correct terms in my drawing and I added three pictures of the current situation. Especially the wrong plumming of the servo diaphragm and control valve +the missing connection to the carbon canisters vacuum signal line seems odd to me. If i understand the original piping correctly then I would expect that the servo diaphragm is doing nothing in this setup (have to check if it moves during let go of the throttle) and that the carbon canister is only doing its job in 'idle mode' due to the bypass mentioned in the haynes manual (because the valve for the purge line is never opened by the vacuum signal line).
Another point is the missing of the EFI specific FCID. The AC is not working at the moment so i don't know if the car runs fine without raising the idle rpm. I wonder if it would make sense to pipe the FCID magnet valve to the control valve to use it as a 'carb FCID'?

 

My overall question to the experts is 'Do i need to be concered about this vacuum setup (do i need the servo diaphragm/carbon canister to do their work) or is this maybe a clever solution?'


Thanks in advance for your time and help. 

overview.jpg 

Picture 1: 
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Picture 2. 

2.JPG

Picture 3.
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Well I certainly won't claim to be an expert, but I have some input anyway.

5 hours ago, Johnny280 said:

the wrong plumming of the servo diaphragm and control valve +the missing connection to the carbon canisters vacuum signal line seems odd to me. If i understand the original piping correctly then I would expect that the servo diaphragm is doing nothing in this setup (have to check if it moves during let go of the throttle) and that the carbon canister is only doing its job in 'idle mode' due to the bypass mentioned in the haynes manual (because the valve for the purge line is never opened by the vacuum signal line).

Another point is the missing of the EFI specific FCID.

Yes, your throttle opener is plumbed incorrectly. You have the input and output switched on the control valve. I wouldn't know for sure if the throttle opener control valve would be bi-directional, but I doubt it. I (like you) suspect that with it connected the way you have it, it's not doing anything at all.

And same for the vapor recovery carbon canister system... With the control signal capped off over at the can like you have it, the purge function is disabled. Not mission critical, but sooner or later, the activated charcoal inside the can will eventually saturate and no longer be able to contain the fuel vapors.

Neither of the above have much (if any) performance impact or affect the engine running in any way.

So, other input about things you didn't ask about?

The thing that concerns me the most is that you have your vacuum advance for the distributer connected to the wrong source, and if you set the ignition timing like that, then it's set incorrectly. The vacuum advance should be connected to the small vacuum nipple right behind the round dome on the front carb. It's a ported source, not a constant source and there will be no vacuum present at idle.

Other than that... The thing that concerns me the least? It's not "FCID"... It's FICD. Stands for "Fast Idle Control Device"   LOL 

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Thanks for answer. I checked behind the front carb dome and there is a capped of nipple. I will connect the vacuum line to that source, cap of the one on the balance tube (or run a vacuum line to the carbon canister from here) and set the ignition timing. 

And for FCID=>FICD I will try better next time 😁  But will the engine run fine without raising the rpm if the AC is running?

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Yup, that capped off nipple just behind the dome is where the distributor should be connected. And, since you're in there.... That same nipple is also where the control line to the CARB canister is supposed to be connected. That ported vacuum source off the front carb is supposed to drive both the distributor and the carbon can. So if you're in there messing around in that area, you might as well plumb a new line over to control your canister at the same time?

As for the dyslexia and the fast idle control device...  The engine idle RPM will drop when you turn the A/C on because of the additional load, but I don't know if it's enough to cause a problem. The original designers thought it was an issue though, as indicated by the existence of the device. However, I don't know if they were just trying to keep the RPM constant, or if they were trying to bump the RPM's up to do something else like improve cooling by pulling more air through the condenser and radiator.

In any event, if connecting it up correctly is a PITA, then I would just try it the way it is and see what happens. If your engine stalls when you hit the A/C then you'll know you need to do something else?

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Thanks for the Info I will reroute both carbon canister and vacuum advance to that ported vacuum source. Regarding dyslexia I blame Television and maybe my foreign origin : /

Will keep your advice in mind about the AC when it is running again. 

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sorry about my delayed response to this, been a little remiss on checking posts...

i re-built a set of SU's for my conversion from EFI-carbs, and in the process i got rid of just about everything non-essential in the engine bay. no carbon canister, re-routed fuel lines and a host of other mods to reduce the spaghetti factor in there. you could probably find the old post (4-5 years ago, i think) i believe it was titled "out with the new, in with the old" or something to that effect. bunch of pics and pretty good amount of info.

i never did put in the AC - thought about it, picked up some of the parts, but lost momentum on the project and couldn't bring myself to put more stuff in the engine bay...

funny thing is, i have always had the exact same idle waver: about a 200-300 rpm up and down fluctuation and i've never been able to figure it out. the car runs great, the carbs have been rock-solid for many years now and it starts quicker on the choke than the EFI ever did. i've just learned to live w/the idle hunting, but would love to hear thoughts from others on what could be causing it.

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 Hi Geoff. Glad to hear the SUs are doing well. A fluctuation in RPMs, at steady throttle, is a sign of a lean mixture. Based on the lean condition theory, I'm assuming one could see the pistons rising and falling a bit during the fluctuation. Have you ever taken a look at the pistons when it's idling? What weight oil are you using?

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Two days of wrenching later, the Z now has a full Zstory exhaust plus Header (a little bit tricky with the orginal heatshield) and the vacuum lines were rerouted. The Vacuum advance is now at the right nipple as is the carbon canister signal line. They both seem to work fine - vacuum advance is advancing and signal line is signaling. As expected the  servo diaphragm and control valve are really doing nothing in this setup and could be completely removed. After setting the timing to 10 BDC (timing was pretty off) and checking if the mechanical advance and vacuum advance are fine i readjusted the carbs using a colortune and a syncrometer. I had to lean it out half a turn to get it out of the yellow into a rich blue (the 'raise the piston by 2mm' and 'get the highest idle' methods never worked for me so i tried that colortune) - i guess this is normal after timing and new headers? It runs great now, the throttle is very responsive and apart from the fact that it is still super loud and a little 'drowny' at 60-65mph, which i wanted to fix with changing the old manifold and 'mystery' middle secition to Zstory complete (Muffler was already the 'classic' version). Anyway the sound is really impressive and maybe i am just getting old and the noise level is normal : /

@Geoff and Mark about this idle waver:  It is still there and i really listened to it more closely and i was wrong with the 3-5 seconds frequency it is more like 1-2 seconds. And i had the feeling that it even persists during steady cruise. Like the car wants to accelerate a tiny amount and then drops back but ever so slightly. I will try to have a look at the pistons if they are moving. Maybe this could also be a tiny vacuum leak, because i experienced the same issue much more pronounced after i destroyed the first vacuum advance hose on the hot exhaust 😀. It felt the same but tenfold in magnitude. 

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 It's also possible the throttle shafts are worn and pulling in a bit of air but worn shafts would affect the idle. Let us know what the pistons are doing, if anything. 

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I’m still using the little bottle of oil I got from ZTherapy with my rebuild kit. No idea what weight it is. My rpm fluctuation is only at idle, about 1-2 seconds, pretty constant. More noticeable when the engine is hot. 

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21 hours ago, rossiz said:

I’m still using the little bottle of oil I got from ZTherapy with my rebuild kit. No idea what weight it is. My rpm fluctuation is only at idle, about 1-2 seconds, pretty constant. More noticeable when the engine is hot. 

If you add a very small amount of choke does the idle stabilize?

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I forgot: my carb oil is 3ml of 20W50.

Small amount of choke would be equal to adding a little bit of fast idle screw right? Since the choke is first moving the butterflies and later enriching the mixture. 

 

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The choke test...it is a quick way to determine whether the mixture may be too lean at idle. I was thinking that Rossiz's situation might be a lean mixture at idle. These cars, in my experience, like to idle a bit rich. When you add choke, you are dropping the nozzles...which is the same thing you do when you turn the mixtures down, away from the carb body, making the mixture more rich. So, if he adds a bit of choke and the idle stabilizes, it probably means he can solve his problem by adjusting the mixture a bit....if he is okay with the trade-off of potentially running a bit more rich throughout the power band.

Regarding the dampening oil in the carbs...I have never paid too much attention to the quantity of oil. I start with one carb and add tiny amounts of oil until I the piston drops steady, all the way to the bottom, makes a slight thump, etc, but doesn't free fall and go clank. It is kind of a feel thing that you get used to seeing and hearing when you have it right. Important too is to have the domes aligned as you tighten down the dome screws so that the piston falls freely. Then, I make the other carb piston drop the same. Having the dampening set correctly ensures that the piston doesn't rise too fast (but rises and drops fully) during wide open throttle.....which would cause a sudden lean condition. Someone correct me please if I am describing this inaccurately. Regarding Oil weight...I think 20w50 would be fine. I have always used the oil ZTherapy provides but I don't remember the weight they use. Probably 20 weight but....

 

 

Edited by jonathanrussell

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But at least for my carbs giving a little choke first moves the butterflies and much later in the lever travel drops the nozzles? I don't know if there are different throttle linkages out there?

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 The choke should move the nozzles first. The butterflies open a bit later in the choke actuation to increase the cold idle a bit. 

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