Try BPR6ES-11 . Wider gap . You will love your ZT carbs.

Did you measure cylinder pressure correctly?  Do the SU's require a person to prop the pistons open?  Seems like they'd block flow otherwise. 

That is a lot of variance, you have some weak cylinders, if you used the proper procedure.  With the popping and backfiring you're seeing, I'd guess that you have some leaky valve seats.  A leakdown test will tell you a lot.  Some people simplify the leakdown test and just push air in to each cylinder, and listen for leaks.  Crankcase, exhaust pipe, or intake manifold.  I'm going to guess that your engine is damaged or just worn.

Your plug gap looks right for your Pertronix.  You're using the Ignitor I which is not the high energy system.  You can't run a big gap with that one.

The SU's are known to leak at the throttle blade shafts.  @Captain Obvious and others have written about it.

2 hours ago, Jughead said:

Cylinder         PSI

1                152

2                149

3                154

4                179

5                152

6                179

179 x .8 = 143.  You are right on the edge.  Most engine builders today use 10%, not 20% (actually 80% of highest by Nissan's description).

Your backfiring and popping could be caused by "missing".  Raw fuel in the exhaust pipe.

I need to redo test - forgot to open throttle.  What does "choke open" mean?  Full choke? I wonder why that would make a difference... I'll update compresion test results tonight.

@Zed Head's is a good question: Should the carb pistons be lifted/roved for the compression test? 

 

 

Edited March 30, 20214 yr by Jughead
clarification

 I don't lift the carb pistons. I remove all the spark plugs, adjust the valves, turn the engine over with the starter until the gauge shows oil pressure,  install the c gauge,  open the throttle all the way and turn the engine over until the c gauge needle won't go higher. I always start with #1 and the #2-#6, then I repeat #1. Frequently the reading on #1 will improve slightly because of increased oil circulation after repeated engine revolutions.

Get a vacuum gauge and plug it in somewhere it sees full vacuum . You should be 15-20 hg at idle . 
Closed throttle will give funky compression readings 

20 hours ago, Jughead said:

I need to redo test - forgot to open throttle.  What does "choke open" mean?  Full choke? I wonder why that would make a difference...

Throttle open simply allows air into the cylinders.  I haven't lifted the pistons either, but it's possible doing so could change the results.  As long as you are consistent, it shouldn't matter.  With compression testing, it's more about cylinder to cylinder variation than actual numbers.  Most gauges will vary a bit.

I can't see where opening the choke would make any difference as all it does is lower the needle to enrichen the fuel mixture.  That shouldn't matter at all.

Gentlemen, Here's the latest update:

1)      I redid the compression test using with better tester and throttles open.  The readings look good (to me):

1=165   2=160  3= 174   4=170   5= 160   6=165

 

2)      I removed valve cover again to recheck cam sprocket position @ TDC on compression stroke.  Please see new pics below, and give me a sanity check on the following:

a.       The notch on the sprocket is directly below the groove, on the camshaft locating plate.  (This is factory setting, per Haynes).

b.       The distributor/oil pump shaft “tongue” seems to be positioned correctly. (I did confirm that distributor shaft was NOT installed 108 degrees off!)

c.       The #1 cylinder cam lobes are in the “bunny ear" position 

d.        I used a zip tie to verify TDC, and checked that the rotor was in #1 position to verify I was on the compression stroke. 

e.        Question: Should I be concerned that the #1 mark on the cam sprocket is not on the “bright” light. (It looks to be on the adjacent one.)  I have no idea whether the other bright link is correctly positioned on the Crank sprocket.)  Everything else seems to be spot on...

 

3) Hooked up vacuum gauge and, after adjusting timing and mixture,  I got between 19-20 psi of vacuum.  The engine "purred" at these settings (at idle).  And yet, when I test drove, I got backfiring & sooty plugs (again).  And (again), when I adjusted to lean out, I got backfiring (i.e. "front-fire") out of the carbs. I've sprayed starter fluid on the manifolds and didn;t notice any reaction.   

So now I'm focusing on the distributor.  I'm looking for a new/rebuilt dizzy to swap in & see what happens.  Anyone have a solid distributor they'd like to sell?    I also need a cam oil spray bar ("oil distribution tube").

Thanks all, for hanging in there with me!!!  Jughead

 

I would do this first.  Sounds like you have the lean AFM problem.  Don't adjust the AFM, adjust the temperature circuit.  It's easy and cheap.

https://www.atlanticz.ca/zclub/techtips/tempsensorpot/index.html

Zedhead:  Does this apply to a '72 240Z?  Do I even have an AFM?

No you do not.

Zed Head helps so many people in different threads it gets confusing sometimes. I do it all the time.

I don’t know guys - looks like that dizzy drive shaft is off a tooth if that’s TDC .