If you get stuck, here is the not-so simple way to diagnose this.
but first, some assumptions;
1) this is not a LeanBurn engine
2) you have done your best to find a vacuum leak, including checking the crankcase, and not found one.
3) compression is about 135psi and fairly even. see Note-2
4) you have eliminated a possible reverse polarity situation at the magnetic pick-up; see note-6
5) the exhaust system is NOT severely restricted
6) the transmission and/or Convertor is/are NOT faulty, and nearly locked up in Park. see note-5
7) that's all I can think of
Ok so lets get started;
1) Make sure the brake booster is not sucking air; pinch the booster line shut and wait a second. If the idlespeed changes and stays changed, the booster is bad, so disconnect the hose at the booster and plug the line. Make sure the PCV circuit is correctly plumbed, working, and not sucking air from a cracked hose.
2) prove the gas is CLEAR, fresh, and not contaminated.
3) remove the carb, make sure the choke is off and stays off, flip it upside down and using the curb-idle screw, adjust the Transfer Port exposure to a lil taller than wide.
4) remove the mixture screws, and give the ports a couple of short bursts of low-pressure compressed air or any aerosol-solvent, then re-install to 2.5 turns out.
5) after this; DO NOT adjust the curb-idle screw.
6) reinstall the carb, fill it with fresh gas, start it up, warm the engine up.
7) if nothing has changed, you may have to clear out the Idle-Air Bleeds. With the engine idling, look down thru the airhorn and find them. Then put one finger on each one to plug them, which should cause the rpm to noticeably slow or stall. If this happens, then the slow-speed circuits are working, but if not, see note-1.
8) with good working IABs, a blown-out low speed circuit and no air-leak found, you can now rule out the carb and fuel.
9) Defeat the Vacuum Advance, and with the engine running, grab the distributor, and pull in some timing; leave the timing light in the tool box. If the rpm goes up, you are on the right track. Stop advancing when the rpm increases to more than 750rpm. But if the rpm decreases, then retard the timing to find the highest rpm , less than 750.
10) but if there is no way that the engine will come to 750, then you will have to check the cam timing. see Note-3
Note-1
To clear the IABs; I do this by reving the engine up to about 2000, keeping it there, and slamming the choke shut until the rpm drops to 1000 or so, then still holding the throttle in the same place, I let the engine rev back up. As soon as the engine has recovered, I let it return to idle. The high vacuum created underneath the choke acts like a big vacuum cleaner sucking fuel frome everywhere, mostly the IABs tho,lol. Sometimes I have to repeat as necessary.
Note-2
As to compression; I have never seen a Smoggerteen318 with less than 120 psi on all cylinders. That doesn't mean they don't exist. But If I had one, I would yank it out for a rebuild.
So then, if you get some sadder than 120 psi, on all cylinders, you can pretty much assume that something is wrong. Most likely it would be a valve-timing issue.
Note-3
The best way to check the cam/valve timing, is take the front of the engine off and degreeing it. But there is a short-cut that you can do, to get an idea of where it's at. It's called checking split overlap.
Split overlap occurs at or near TDC but NOT on compression stroke, rather 360degrees past TDC compression. This is the time when the intake valve is just beginning to open, and the exhaust valve is almost closed. The factory 318 cam has only about 20 degrees of overlap, so finding the split is pretty easy, theoretically.
I use #6 cylinder because it's easy to get to. So put it on TDC compression and then roll it around , one full turn, accuracy is not critical. Pull the valve-cover off. The best place to do this test is right off the lifter bodies. But you can get a pretty good idea from the push rods or retainers, because there is only the 20* of overlap. That means that the intake valve begins opening just 20 degrees before the exhaust is closed. You can watch the valve-retainers, and rock the engine back and forth and find a point that you think both valves are equally open.
Now go look for the index mark on the balancer, to see where it is in relation to the TDC mark on the timing tab. Ideally it will be slightly advanced, something like 4degrees or less. . Realistically with a stretched chain, it could be 2 to 4 degrees retarded. The engine will run reasonably well, with split overlap set anywhere from 8* advanced to 4 or even 6* retarded.
If split overlap is outside this range AND the compression is very low (or very high) on all cylinders, then the first thing you will have to do is prove that the index on the balancer has not slipped, (see Note 4), and if the mark is accurate, THEN
you will have to actually dig into it.
Note-4
You prove the TDC mark by using a piston stop. Basically you just screw the stop into the sparkplug hole then rotate the crank until the piston hits it, make a fine mark on the damper, then roll the engine over the other way until the piston hits it again and make another fine mark. Halfway in between your marks is TDC.
Note-5
If you suspect the trans, just jack the car up, put it on axle-stands, start up the engine and let it idle in Neutral. If the wheels are spinning, and the engine is happy, apply the brakes and note the change in rpm as the wheels come to a stop. If it stalls something is wrong inside the trans or in the convertor. In either case, it will have to come down.
Note-6
When the polarity of the magnetic pick-up is reversed, it will drive your timing-lite crazy. It may idle fine, but with wildly fluctuating strobes. As soon as you increase the idle speed, the strobing will go crazy with missed strobes, extra strobes, and the timing mark could appear to vary wildly; and of course, the engine will run stoopid.
Note-7
If your valve seats are rusted up keeping the intake valves open, your manifold vacuum will be low. With the loss of vacuum, the engine will be down on power. and you might need to keep it running with your foot. But if this is so, then it will also be very hard to start, sluggish to rev, and probably back-firing thru the carb. Since you didn't mention this, neither did I, lol.
Ok so have fun this weekend, lol.
