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Ignition timing for a 4plug head


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I've read somewhere that the 8plug ignition effectively advances the ignition too. Also read that spec is 3 degrees btdc with vacuum plug off at idle. 

Now, 2 things:

My truck uses 4plug head, almost all z24's where I live do. So I assume more advance is required.

I live in a 4000ft (1300meters) elevation which I've also read need higher advance.


I used to run it at 10-13 degrees btdc in here and it wouldn't ping, power was good but it failed emissions. The local 'expert' unplugged the vacuum advance hose and it passed emissions. flawlessly and I was sure the carb was adjusted. So I assume it's a bit too much of advance. I'm running it at 5 degrees now and the power is not bad but I feel like it could be better and I'm due for emission inspections. 


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Think of a candle that you light and it takes an hour to burn down. Now if you light both ends it takes about 30 minutes to burn completely. So you could light both ends a half hour later than the single flame and have both finish at the same time. Same with dual plugs the mixture burns faster with two ignition sources so you don't have to light it as soon a as single plug head. 


You should fill out your profile more completely. In North America only 8 plug heads were used. Vacuum advance shouldn't hurt emissions, it makes part throttle power more efficient.



Assuming you are somewhere else that did get 4 plug heads then yes it will need more advance. So where are you?

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Okay here goes

I'm in Iran, Tehran with 4500ft elevation.

My truck is actually a Nissan Patrol but for some reason they put Z24 on them and they're all 4plugs.

It was strange to me too about that vacuum advance. I literally just unplugged the hose and it passed but that's not really important.

What I'm concerned with is the amount of advance that I need. I'm looking for a good balance between power and fuel consumption. 

I have a stock Hitachi carb but fully rebuilt and ready to go (except the accelerator pump doesn't work for some reason, despite replacing it with what came in the rebuild kit) engine is in perfect condition and really the only thing I'm not sure of is the mechanical advance.

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Iran, Ok, that makes total sense.


There are 3 kinds of advance. Static which is adjustable by turning the distributor. Mechanical which is built into the distributor and advances with increased RPMs. And vacuum advance which is load dependent. Light throttle more advance.... heavy throttle almost no vacuum advance.


I have a 200sx manual for the Canadian Z20E which is a 4 plug head and the timing is 8o BTDC. There should be an emissions sticker on the underside of the hood and the timing printed on it. This would be a good starting point I think. Be sure your idle is around 750. If it gets above 900 the mechanical advance will start to come in and also the vacuum advance. There should be no vacuum advance at idle so be sure the hose to the distributor goes to the vacuum advance port on the base of the Hitachi.







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That's the thing, the tune up spec on that sticker said 3 degrees, just like the original 8 plug and that's why I was confused. 8 degrees seems adequate. I'd say 10 degrees total to compensate for altitude would be good. I always set my idle at 750-800 and when I unplug the vacuum hose the manually block it with my finger to prevent vacuum leak, the engine's behavior doesn't change so I'd say there's no vacuum advance at that RPM.


By the way, I've been a long time lurker and used many advises given here especially by you on other topics around here, but when I posted this topic I didn't actually expect you to answer so quickly so thank you for all your inputs 🙂


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Maybe the vacuum diaphragm in the distributor is bad...did you try sucking on the hose with the cap off to see if the base plate moves a little bit?

Often they are worn-out, shot or leaking...or possibly the plates are dirty or dragging, keeping it from advancing. I've seen this often on old Datsuns.


Vicdat 610

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