Help me somebody...spark knock 123Ignition dizzy. I’m

[ZBuzzard]

I have a ‘72 240Z w/2.8L engine with a high compression head(10.5:1), triple Weber’s(40 side drafts), and a 123Ignition dizzy.  I’m pretty sure I have the carbs dialed in, it gets a bit lean.  between. 2500-3000, so I think I’m going to go up one size in the mains, but that’s the least of my problems.  I get spark knock at low RPM’s under load.  I have tried a dozen or so different ignition curves and no matter how low my total advance is, spark knock.  I’m a newb when it comes this programmable dizzy.  In addition to the ignition curve, I can edit the MAP curve as well.  Trying to figure out how to get this thing to retard the timing when I put my foot in it at low rpm’s.  Is that possible by using a specific MAP curve, or is that unrelated.  Replaced 1 head gasket already, pistons checked out ok,  but don t want to risk damaging them.  Any advice would be appreciated.  Oh yeah, anyone have a few good ignition curves for a 2.8L engine(N47 block, E88 head, Schneider 275 cam, triple Weber’s).  Cheers!

[240260280]

What is air/fuel at cruise on flat highway at 50rpm?

What is air fuel ratio at idle?

What is fuel level?

You may be just running lean.

 

The trick is to get the fuel levels right then find the right idle jet to give a cruise between 14 and 16 (depending on fuel efficiency you want). Then you set the idle (13 to 15 depending on how stinky you want it.... idle is smoother at 13 but it is rich). Repeat the cruise test as the idle adjustment can make ~ 0.5 a/f difference at cruise.

 

Typical fuel level range is 27mm down to 29mm down.

 

What size chokes/Venturis do you have? 32-33mm are nice for 2.8l 40DCOE on the street.

 

 

Edited April 21, 2019 by 240260280

[LeonV]

Did you check your actual timing after you installed the 123? First thing I did after install was make a "setup map" with timing at 10° everywhere and put a timing gun on the pulley. Timing was substantially off so I clocked the distributor until the gun showed 10°. I can post my timing curve when I'm on my tuning computer.

Edited April 21, 2019 by LeonV

[Zed Head]

48 minutes ago, ZBuzzard said:

 In addition to the ignition curve, I can edit the MAP curve as well.  Trying to figure out how to get this thing to retard the timing when I put my foot in it at low rpm’s.  Is that possible by using a specific MAP curve, or is that unrelated. 

Yes, MAP is what you would change to get timing to retard at low intake vacuum, when you open the throttle at low RPM.  MAP is intake manifold pressure, and centrifugal, or RPM, is engine speed.  Two different curves, typically shown in table form.  They interact.

[240260280]

Webers like ~ 20degrees advance at idle but this is too much to cranks at start. 10 degrees is nice for start cranking. 

With 10.5:1 compression max advance may be 30 to 34 degrees.

You will also need 93 octane gas to help reduce ping.

Edited April 21, 2019 by 240260280

[Zed Head]

I found the link to the manuals for anyone that wants to browse.  The 123USA web site is in bad shape.

http://www.123ignition.nl/files/manuals/123manual_TUNE.pdf

https://123ignitionusa.com/pages.php?pageid=10

[240260280]

Stock L24 High Compression no emissions advance is 17 degrees at 650rpm.

 

Here is vacuum advance map for L24 high compression engine:

5.5 degrees in by 245mm HG

 

Here is mechanical  advance map for L24 high compression engine:

6 degrees in by 1000 rpm

 

 

Edited April 21, 2019 by 240260280