Here's what I have found in this situation; Not saying this is your problem.
If the exhaust valve is not sealing;
Be it for a broken spring, a burned valve, or just carbon on the seat, Then it may still run fine at rpm. But at idle, the piston on the intake stroke, finds it easier to pull air in from the exhaust system, than to pull air in from the intake where the throttles are almost completely closed.. And so, a large portion of the AFR in the chamber on the following compression stroke, is inert exhaust gas, which lowers the heat of combustion, and you can read that on the IR gun.
If your exhaust system is tight, you can prove this very simply;
Just go to the back of the car and put your gloved hands over the tailpipes without closing them right off.
If you have a Crossover tube, you can close one side. In your case, you already know it's on the Driver's side, so without a Crossover, you can work on just the Driver's tailpipe.
So then with the engine idling, and your palm set to seal the pipe, if you feel 3 pressure pulses and one missing, it will feel like the missing cylinder is sucking your hand onto the pipe. Obviously it is not, but, because you are pushing towards the pipe and fighting the pressure, when the pulse is missing, your energy, assisted by the atmosphere will push your hand onto the pipe. If you feel that, that is your proof that the exhaust valve is not sealing.
This does not show up on a Leakdown test, at 100psi, which forces the valve to seat. At that point the Valvespring doesn't even need to be there.
If you had tried it at say 40 psi, it might have been another story.
In any case, this test takes about 2 minutes, and is proof positive that one exhaust valve is not sealing. If this has only just started and if it's carbon on the seat, the fix may be as easy as taking her out for a blast. Or you can pour some carbon cleaner down the carb at high idle.
But, if this situation is allowed to continue, a burned valve will be the end result. But on the way there, the carbon can cause pre-ignition, leading to detonation, leading to broken engine parts.
Jus saying
BTW-1
IMO 4% LD @a test pressure of 100psi, is already quite a bit, and
if I didn't know that Camino CA was at around 3100>3500 ft elevation, I would have said that your cylinder pressure of 130-ish, was quite low.
If you have iron heads it might be fine for a DD;
But if it was my engine, at 900 ft, I would be tearing it apart; especially if said engine had alloy heads, which will easily support 190 psi on 87E10, here in Manitoba Canada.
At 3000/3500 ft, your performance engine needs a lot more than 130psi to be considered a performance engine.
BTW-2
I think the floorjets on the floor of the intake are a non issue.
I once took an intake off, looking for a missing one, cuz I was thinking; OMG if that thing rolls into a chamber this engine will be toast!
It turns out that the one floorjet was not even threaded, doggone-it. So then I cleaned out the heated-plenum crossover, since the intake was off anyway.
On my 2bbl intake, IIRC, those two floorjets were connected together, and to the EGR valve, which at idle is closed, and so they are supposed to be dead at idle. Furthermore, the EGR is supposed to stay closed when the engine is cold and at WOT.
The EGR valve is really only to be open when the engine is fully warmed up and you are cruising.
But there were several different methods used in those years, and I don't remember them all.
BTW-3
16 inches of vacuum seems a bit low, but, at your elevation may be ok, IDK. If you suspect a leak, have you considered a leak into the Valley? The way I prove that is to flip the PCV out of the cover and seal both the valvecovers, then put the vacuum gauge on the dipstick tube, then start her up. In this set-up, there should NOT be any vacuum to be read. Instead at idle, pressure should slowly build up; how fast will depend on how well your rings seal. Try not to let it get over 3>3.5 psi, cuz by 4psi, something is gonna want to blow, and if the rear cam plug pops out, yur gonna be real mad at me. Don't panic, the pressure does not rise that quickly.
But;
1) if you find vacuum, with the PCV flipped out, yur gonna have to pull the intake off and fix the dang thing. and/or
2) if you don't find pressure, then the CC is leaking somewhere which also will have to fixed.
3) With the PCV out, and the engine idling, you gotta have pressure; no store-bought rings in the world will seal 100%. Well, a Plasma-Moly in a straight round bore, comes pretty close, which is why I installed them in my engine.
Yeah so; best of luck to you
