just for giggles....I would expect a 'pop' from the booster if you removed the elbow fitting/check valve FROM the booster canister. I would expect no noise/pop/whoosh/ anything if removing the hose from the check valve at the engine side?
Can you start shooting flammable around the base of the carb and other manifold vac joints to sniff for leaks?

And, remind us....when was everything last running good? with the holley 2bbl before the accel pump leakage?
anything else change in the interim?

 

Got it to hold idle, though very rough and lumpy by turning the idle way up. Last ran good with 4412 Holley. Went offroadung in mountains, and over the next few weeks ran worse everyday until it started stalling under any acceleration and became undrivable. I did remove the whole elbow at booster, not just hose.
anyway, after stalling started i checked for vacuum leaks to find none, so checked fuel pump- pumped at 7.5/4.7 after regulator but had a flow back issue over night+draining lines, and filter) new pump installed, replaced all rubber lines, and all steel lines. That's when I noticed fuel on intake under accelerator pump. Can't find a 50cc diaphragm local so ordered one, and proceeded to put a backup carb in place to get me where I need to be

 

Gonna get out the spark tester and check for a cylinder without fire now..
while doing that I'm gonna run a unlit propane torch all over the intake

 

Ok spark is nice snappy blue, swapped wires for new, regapped plugs at .44, they were black sooty before this we're tan perfect. Gaps shrank to .30 since early summer tune up. Got it to idle better with mixture screws 4 turns out, but still very rough. Vac was normal 15 dropping to 13 or so every few seconds randomly. 15 is my norm at 8000 ft elevation. Even with mixture screws 4 turns out, I still didn't smell gas in exhaust

 

Troubleshooting can be very tedious, I know.
I can't quite tell if you idled fine with the holley, and it was just pump squirt (and fuel leaking) that was the problem?
If it did idle fine, then one approach would be to try that holley again, just to see if you have a good idle.
if yes, then it's something with your MC carbs.
if no, then it's something else (vac leak, spark, etc.
try to change one thing at a time as much as practical, so it will be clear when you hit a proximate cause, eventually leading to the root cause.
I will come back to the thick gasket again, that thing looks like it could be a leaker due to the impressions not perfectly aligning with the carb/spacer body?

 

I find the unlit propane bottle to not be the best fuel source (yes, it *should* work--I just find carb cleaner (flammable kind) to work better for leaks.
Is your spark tester a test light from the distributor HT lead to ground while running (RPM drop test)?

 

are you dura spark or points?
4 turns out (should be quite rich, especially at altitude) is making me think sneak air source (vac leak).

 

The spacer to manifold gasket is new, and the thick one was used a week with the same carb on it now. The dents are the same. Definitely helped somewhat, and convinced it's a vac leak somewhere, but it's invisible. The propane torch sucked. I even tried some down the carbs throat with no change at all. Ran fantastic with the Holley. Changed jets to 68 was all it needed. I suspect intake leak, as the normal spots produced nothing, even the trans modulator. Booster, vac advance, pcv.i did a head job a year ago, and remember that manifold was a pain to line up with gaskets moving/weight. Checked manifold bolts, and we're loose even looser than the low torque spec it already has. Is there a check I can do before tearing it apart for mabey nothing?

 

Horsetheif, I'm following along as I have a sudden no run issue as well. Mine is a 79 with a 460. Good luck and keep us posted

 

Update timeSo messed with truck right now after work. Was thinking I started mix screws at 2.5 out and went up from there, but since it suddenly showed Rick plugs let try going in on them.
didn't change.
So I messed with timing on dizzy. Advanced it clockwise a little, and made it worse. Retarded it counter clockwise a bit, and got worse .....until I got pretty far retarded ...... Suddenly it smoothed out nice. The vac adv can is always pointing about straight to front, but now is like more towards the driver's side tire. Timing gun decided to break, so not sure where it is now, but had been around 30° to the right of the zero mark for the past 6 years. Did I skip a tooth or something? It seems to run better than ever at the moment. Starts with a key bump, burns the tires like a *****. I don't get it, but something must have happened internally. Is it compensating for something with the timing change fix? Is it safe to drive to work not knowing where it's timed? The fleet mechanic at work can check timing, and stuff for me at work. What gives!?!?

 

Even gained almost an inch of vacuum!

 

So didn't fix a dang thing. But I got some more symptoms since I've been driving it the 10 blocks to work every day.
if I try to just start and go with a minute of warm up it will die as soon as I put it in drive, or any gear. Like just turn off instantly. It will fire right up, but if I dont warm it up it will just doo the same. I can give it half throttle and slam it in gear and go. Mabey get a few stutters out of it, but will go. When taking off normally from a stop it will stutter a time or two and shut down, but if I gas it the first stutter I feel it will launch off the line nicely. Might have to stab the pedal a few times fast and it will hold on. But normal everyday acceleration it will shut down. At the very least under normal acceleration it will have a bad flat spot until about 1500-2000 rpm, then catch itself and take off like a banshee.
In the driveway it runs top friggin notch. Can't find a damn thing wrong with the engine. Starts with a very small bump of the key, sounds great, throttle response fantastic, vacuum great, and steady, timing isn't bouncy, no vac leaks anywhere- I spayed 2 damn cans of starting fluid, propane, carb cleaner, water, can't find anything. At all! Tranny has fluid, and shifts fine. I'm gonna burn it down soon!!

 

Get your timing sorted out .
initial (base timing), mechanical advance, and ported advance from carb, not manifold.
Engine will behave much different under load vs. revving up in neutral.

Then you can tune your carb or replace it.

 

Timing is currently at 12 initial, 28 total. Was at 18-32 for the past 6 years. That where it was this morning, and only died once on way,. At lunch ran it with mechanic at work in park and changed it to 12-28 to mabey see how it was on way home. It was worse in all catagories other than parked. Carb is set to highest vacuum, and just slightly smells gassy out of the exhaust. Only at low throttle load is it having the most problems. That's all with 3 known good carbs, all acting the same. Anybody hearing the truck start up and idle would think that's one fine running fe!

 

12 initial 28 full mech sounds like it won't be your issue----unless it's not ramping in smoothly. Did you/friend confirm linear progression of mech advance? at what rpm does it start coming in, and what is full in rpm? There are a lot of advantages in driveability to ALSO have the vacuum advance connected to ported on the carburetor (above the throttle plates).
You mentioned fuel pump earlier. Where are you measuring fuel pressure, and what is it if you do a say 1200-1400 rpm power brake (how much does it dip under load)?
It sounds like your carburetor pump shot is inadequate and you are getting lean-bog at part throttle tip in. But that is a guess without hard data, and the way to know for sure is with a wideband O2. Else you can try trial and error to increase the accel pump discharge (does your MC carb have the different holes in the pump lever to increase diaphragm travel)? Work methodically, make incremental changes, and consider keeping a written log.
Perhaps your Holley 50cc pump will be in soon? the holley system has a lot more adjustability to fine tune. with squirter orifice sizing and the accel cam. Don't forget on your holley to keep a little wiggle on the lever arm PAST WOT. don't mash and over-stretch that diaphragm (perhaps why it started leaking to begin with?). there is an adjustment screw to help with that.