You are getting fuel, the dizzy is turning and you said you saw spark and had a backfire that may have been from the dizzy being 180 out. Besides, the car was running fine for a while...

I would get your friend over to swap the CDI box if after confirming you are on TDC for #1 the car doesn't start, before tearing more stuff apart.
CDI boxes are known to overheat and fail intermittently.

Broken cam chain(s)..??

Found this method posted by Gunter 9 years ago. Going to do this first, and then start down the line of possible issues.
http://forums.pelicanparts.com/porsche-911-technical-forum/278971-distributor-question.html#16

Turns out she was totally lined up already. I also put a spark plug directly on the coil output and am getting a nice bright blue regular spark while cranking. I'm moving to treating this like a normal no-history no-start at this point, as it seems the distributor is working just fine.

Next step is fuel pressure, right? I'm going to pull the fuel line before the pump and see if I get proper fuel coming out.

After you confirm fuel flow,
Look for a crack or cracks in the airbox or some other indicator of a large vacuum leak after having a backfire, pop off valve seated correctly? Vacuum grease on the o-ring is a good thing to apply to the pop off.

I pulled the fuel line to the fuel filter and it has good flow coming from the front. I pulled injector 4 and it has a nice even spray. I made it run in both cases by raising the pressure plate thing inside the airbox. Is this different than if I crank it over from the key?

She died while idling at a light - the backfire came later during various testing. But maybe a leak opened up somewhere. If I have blue spark and injectors are spraying... I don't know. I'll go back to checking hose connections for air leaks.

My shop vac doubles as a blower, so I hooked it up blowing into the exhaust. Sprayed soapy water around everywhere I could think of, but didn't see any air bubbles. I first did it with the airbox lid on. Then I took it off and checked again. Even with the airbox top off, the vacuum seemed to be under a very heavy load and kept wanting to pop out of the exhaust.

Found if I turned the engine a bit, I got air to expel through the popoff valve (when I raised it).

This also allowed me to get some leakage at the injector seals. Not a ton, but definitely some at each injector. I have a pretty high power vac blower (I use it to dry the car after washes).

EDIT: Is leakage at the injectors enough to keep it from starting at all? Somewhere else in particular I should spray? I tried to get around all the cloth hoses, and the giant rubber ones below the airbox.

I think some leakage at the injector seals is normal, particularly if they are old. I wouldn't expect that to be enough of a leak to cause a no-start.

I'm going to suggest trying to swap the CDI next. It might do nothing but allow is to eliminate the CDI as a cause. It wouldn't hurt to test fuel pressures either. If neither of those turns up anything I would start thinking about a compression test.

EDIT: I shouldn't have said "normal" -- I should've said "to be expected".

Forgive my ignorance but isn't pressuring the intake very different from pulling a vacuum.

I can see where pulling a vacuum (normal operating conditions) will further pull the injectors towards their seats thereby compressing the O-rings for a good seal while pressurizing the intake will cause the injectors to move ever so slightly and thereby causing the leaks you observed. In other words your test is improper to diagnose a potential injector seal leak.

Ingo

The fuel pump has 2 "run" modes. First, it's powered with the ignition in the start/cranking position. Second, once the fuel metering plate is unseated due to intake airflow there is a microswitch circuit that powers the fuel pump.

Can you have someone lift the fuel metering plate just SLIGHTLY as the starter begins turning the engine over?

Is it possible that the engine starting injector is fouled or inop?

He already did that.

Bad CSV might be an issue at cold start but wouldn't explain the shutoff at the traffic light.

While cranking the engine?

Pressuring the intake was just intended to help find possible leaks. The only spots that seemed to be letting any amount of air out were the injector seals, which as you note are more effective at holding a vacuum than at preventing escape. I was hoping this would reveal a real leak somewhere else.

I'll try giving the fuel metering plate a little push while cranking. I presume you mean raise it enough that it starts to open, but not enough to reach that 2nd part of the motion where I am moving 2 things.

I can also get some new plugs pretty easily. The poor combustion during my previous arcing issue (resolved with distributor cleanup) was leading to some buildup on them. I did drive over 100 miles at good speed after the cleanup, before she died at the light.

I'm hoping my friend with the CDI can come by sometime this week. Although it is looking less electrical, I'm happy to try anything at this point.

With such a healthy spark as you described it doesn't seem probable that you have CDI problem....

Just died at the light while idling or was there maybe a backfire involved... ? Purely a shot in the dark since you have a pop-off valve installed and backfires at idle are not generally very EXPLOSIVE.

In the alternative you could unplug the microswitch connection in which case the fuel pump will run anytime the ignition is on.

Throttle plate safety switch...


http://forums.pelicanparts.com/porsche-911-technical-forum/642924-1979-porsche-911-sc-throttle-body-help.html

I have new plugs in now, no difference in behavior. I cranked it for a few seconds before I started changing them out. They all smelled like gas and had some dampness when I pulled them out.

I pulled a fuel injector (with the airbox and all together), and ran it into my glass bottle. It left gas behind there! I was the one doing the cranking so I couldn't see the spray pattern. If the injector is spraying, does this mean the fuel metering plate is moving?

Battery is now back on the charger inside the house. All this cranking the starter has resulted in it notably having less power.

The old plugs were a little dirty but not that bad. Number 3 (driver's side forward?) had some heat discoloration on it. I'll keep poking and looking for vacuum issues, but I'm not sure exactly what else to do. No cracks in sight, all hoses I've found seem snug.

Next maybe I can check the spark on each spark plug wire. Previously I had checked spark at cylinder 4, and later at the coil. Although that will result in a fair bit of gas getting into each cylinder unburnt.

It does seem more and more like you have cracked airbox... Thrust me, it doesn't take much of a crack, opening, to negate the starting vacuum. Personal experience, my '78 once backfired during cranking, it never started again until the airbox was replaced.

I had to pull upward on the airbox in order to see the crack seam.

Strange as it may seem I have seen pop-off valves installed to atmosphere.

How hard is to remove the airbox for a more thorough examination?