timing specs (help)

[720s]

does anyone know the right timing specs for a z24 ?

[distributorguy]

Try 8-12 BTDC at ilde, vac disconnected.  Experiment a little and set it where your truck is happiest - it changes with mileage and stretching timing chains.  

[datzenmike]

Z24 with idle at 700 is 3 degrees BTDC +- 2 degrees. Don't forget this is a dual spark plug head and the timing is very much less because of the shortened burn time.

 

This is a ported vacuum advance so at idle there should be none as the port is well above the throttle plate and reading atmospheric pressure. None of the factory service manuals mention disconnecting the vacuum line to set timing. Although there's no reason to (at proper idle) there is no harm in doing so either. Disconnecting the vacuum advance hose is a hold over from the 50/60s when intake vacuum was used. No, you don't need to plug the disconnected hose either.

[distributorguy]

Our modern fuel likes 14 BTDC at idle.  Due to how much timing is in that distributor, the engine will be MUCH happier if you jump it up to 8 at idle.  Factory settings that are more than 20 years old need to be modified to run better with all the additives in today's fuel.  Anti-foaming agents necessary in high fuel pressure engines generally mean you need more initial timing at lower rpms.  I've rebuilt a few of these.  Its perfectly safe.  

[datzenmike]

All engines like more timing at idle, however, adding static timing will add to the mechanical when revved up. If the total timing is 34 degrees you are now at 38. These engines are dual plug they don't need that much advance because the fuel burns in less time. In addition, they are almost hemispherical combustion chambers that are very detonation resistant to begin with. It may be pinging and you may not notice it.

[distributorguy]

If I'm not mistaken, its got a 12 degree advance curve - 24 plus 10 = 34.  

[datzenmike]

My '84 FSM says 9.5 (or 19) @ 2,100 mechanical, 14 @1,345 and 0 @ idle. So it would seem you could run more advance than recommended.

[Lockleaf]

I have run my z24 up in the 10 degree initial timing zone for quite some time. It was set there based on ear and butt dyno then checked with a light later on but left alone.

 

I'm not stating it's prefect, but it definitely woke up across the entire rpm range.

[distributorguy]

Never trust factory timing specs.  That distributor should have a 12.5 degree cam in it, so its "14" according to the book, but not in practice.  Double that for crank advance, then add your base timing.  There's easily room to go to 8-12, or even higher which is why I recommended it.  If you have the 9.5 degree cam, then go up to 14-15 BTDC at idle.  I don't give offer up numbers unless they're real and tested - there is liability involved since I do this for a living.  

 

Then add in CA fuel if you have to deal with that (is that what you have in HI?), and you can add a couple degrees of timing to make up for the extra additives.  Maybe a little less at sea level?