# D21 Z24 vacuum advance trouble



## ludens (Mar 19, 2011)

Hello,

I have a problem with the vacuum advance of my 1992 Nissan D21 Terrano. This SUV has the Z24S engine, totally old fashioned, simple and pure. Carburetor, point contact ignition, and so on. It's the same engine that came in pickups, specially the 4WD versions, roughly 1986 to 1989. 

For setting the correct ignition timing, the Nissan shop manual for this model tells to disconnect and plug the vacuum line, rotate the distributor to get the ignition at 3 degrees BTDC at idle (less than 800 rpm), then reconnect the vacuum, re-adjust the idle speed to 800 rpm if necessary, and check the ignition to be at 15 degrees BTDC, plus/minus 5 degrees.

When I do this, I get no vacuum advance whatsoever at idle, so that the ignition at idle stays at 3 degrees BTDC. This results in uneven idle, and requires setting the idle mixture pretty rich. That way the exhaust smells quite awfully!

As soon as I throttle up a little bit, the vacuum advance immediately kicks in, and the engine runs great. Only at idle there is no vacuum advance, so that the ignition is far too retarded.

I checked and measured everything I can imagine, and the results are these:

- The vacuum advance bulb, mechanism, and hoses are all fine. I applied controlled vacuum to the bulb and measured the resulting advance. The manual says that the advance should start acting at 133 millibars of vacum, and top out at 307 millibars, advancing a total of 8.5 degrees at the distributor (17 degrees at the crankshaft). I measured 125 and 270 millibars respectively, which seems close enough to me, and the angle is correct too. So the distributor and its advance system is fine.

- The vacuum is taken from the carburetor, at the throttle level. At idle there is almost no vacuum, and when throttling up a little, the vacuum builds up. When the throttle is pretty far open, I get the full manifold vacuum, which in turn depends mainly on engine load. This behavior seems perfectly logical, and fully explains why I get no vacuum advance at idle, but the shop manual tells that I should be getting a very significant amount of vacuum advance at idle, and the engine clearly needs it!!!

- If I rotate the distributor to get about 10 degrees BTDC at idle, without any help from vacuum advance, then I get a great, nice, smooth idle with pretty clean exhaust, but then the engine pings severely when working under moderate load, due to excessive total advance. That's of course not acceptable.

- Just in case it's important, the manifold vacuum at idle is about 600 millibars, and the atmospheric pressure at my home (in the hills) is roughly 900 millibars. The centrifugal advance is working well, agreeing with the data printed in the service manual. 

- I tried readjusting the idle with a more open throttle, closing down the mixture setting screw all the way, but to get any significant vacuum advance that way at idle, I need to set the idle to about 1200 rpm, which is of course far too high. 

This looks like the carburetor is simply taking the vacuum sample from too high up in the barrel, but I suppose these little port holes can't move up on their own, and I definitely haven't moved them! ;-)

Can anybody advice what's wrong, what I can check, how I can fix this and get a good idle along with good overall engine performance? 

Maybe somebody knows if the vacuum is sampled through only one port hole in this carburetor, or if there are several, some of which might be dirty and plugged? That's my main suspicion, but I hesitate to take apart the carburetor to check this, without knowing clearly what I'm looking for! The shop manual gives no details about the vacuum porting.

Or maybe somebody has a different shop manual (Haynes perhaps?) and can check whether it gives the same procedure for setting the advance?

Any help is welcome!

Manfred.


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## rogoman (Dec 16, 2004)

Your intake manifold is low. It should be around 700 millibars (20.6 InHg). A possible vacuum leak somewhere in the intake system.


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## ludens (Mar 19, 2011)

Thanks for the reply. But I don't think the manifold vacuum I measured actually indicates a problem. As I wrote in my post, the atmospheric pressure at my home is rouhly 900 millibars, that's about 100 lower than at sea level. So it makes a lot of sense that the manifold vacuum is also about 100mB lower than normal, at this altitude. 600 mB of vacuum at my place would result in the same absolute intake pressure (about 300 mB) as 700 mB of vacuum at sea level.

When I drive down to near sea level, idling becomes very slightly faster, which is normal too, but the problem with the vacuum advance totally dropping out at idle remains.

There is still another factor making my intake vacuum lower than normal: It's precisely because the ignition timing is off! With the timing so far off the optimum, I need to set the throttle limit screw a little higher than normal to get the correct idling speed. This of course results in lower manifold vacuum.

If I set that screw lower, and richen up the mixture using the needle valve, then indeed I get a higher vacuum, at a somewhat lower idling speed, but then the vacuum at the carburetor port going to the ignition advance bulb gets even lower! 

So my question basically boils down to this: Does the carburetor of the Z24S engine take the vacuum sample ONLY from the little hole next to the throttle butterfly, or should it also have another, separate port further down , maybe at the mounting surface of the carb? I ask this because it's so hard to tell what internal passages a carburetor has! I can take it off and look at it, but I have no idea which of the many openings and holes and passages should be internally connected! If some should be connected but are blocked by dirt, I won't ever find out!

If the vacuum is taken only from that hole near the throttle, then there is just no way to get significant vacuum at the connection to the distributor, when at idle. In that case I wonder why the shop manual tells that the ignition should advance by about 12 degrees when connecting the vacuum line. But I can't see any other hole further down that might connect to this line, so I wonder if there is some internal passage doing this, which might be blocked by dirt.

Any help is welcome!

Manfred.


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## rogoman (Dec 16, 2004)

If you don't get any positive reply's from other members, consider rebuilding the carb. There may indeed be some plugged internal passages. It's worth a shot.


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## ludens (Mar 19, 2011)

Actually I rebuilt it several months ago - complete disassembly, washing, blowing compressed air through all passages I could find, then reassembling using new parts from a rebuild kit: jets, gaskets, O-rings, springs, acceleration pump plunger, and so on. This rebuild cured several minor problems that affected certain speed and power ranges. Also it noticeably improved fuel efficiency. But it did nothing at all to the lack of vacuum when idling. 

I have to say, though, that I wasn't specifically looking for the vacuum problem when I did that work, and so I might have missed cleaning some hidden vacuum passage. I just don't remember if there was any hole that could have been an additional vacuum port opening below the butterfly.

Seems that I have no choice other than taking off the carb again, and have a look. 

It would still be very helpful if somebody could tell me whether or not there should be such a vacuum port below the butterfly, in addition to the one that opens right at the butterfly when it rests roughly in idling position.

Manfred.


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