You can't do anything with either timing or idle until the car is 100% up to operating temperature. Since you've got an electric choke thermostat, the choke must be fully opened by its thermostat--not your thumb--before you can get the idle nailed.
This is carved in stone at the top of the mountain, so anything you do whilst the car isn't at thermostat temp will simply frustrate you.
The best thing to do here is start from zero.
You say you can't get down to 5, much less 0° on the timing. Why? Turning the distributor clockwise
retards timing, while turning it counter-clockwise
advances it. The easy way to remember this is that the vacuum-advance nipple points in the direction of advance. Is the distributor hitting something? You said you got it to 10°, which is perfectly acceptable and actually a bit less than what I suggested.
Ignore the factory timing spec. It's nonsense
unless you plan to tow a 28' Airstream up Pike's Peak in mid-August with three passengers and a week's worth of luggage for all. If it's just going to be you and the wife most of the time with no trailer or trunk full of lead, 15° is perfectly safe. I wouldn't even consider running less than 10° initial timing. If you can get it to 10°, leave it there, tighten the clamp and don't worry about it for the moment. Leave the vacuum hose disconnected and plugged for now.
OK, on to preliminary idle settings, both mixture and speed.
Turn the idle mixture screws in until you feel them seat. Don't crank on them; you'll deform the carb passages. When you feel firm resistance to tightening, they're seated. Back them out
1/2 turn and leave them alone until later.
Now do a preliminary idle-speed setting. Open the choke fully by hand, and block it so it stays fully open (I usually just put a screwdriver down the bore). Rotate the fast-idle cam, highlighted in green below, clockwise until the fast-idle screw no longer touches it. You may need to open the throttle very slighty to do this, but don't open it very far at all--just enough so the cam can move without dragging itself along the idle screw.
With the throttle released, you can now do a preliminary curb (hot) idle setting with the curb-idle screw, shown below. Tighten until it just touches the throttle arm, then
another 2-3 turns.
OK, once that's done you'll want to remove whatever was holding the choke open and open the throttle fully (WFO) to set the choke. The choke should be
fully closed, and the fast-idle screw should once again be on the cam, situated about where it is in the first picture. Back it off its cam, then re-tighten until it just touches the cam, then
another 1-2 turns.
This can be a bit of a sonofabitch, because it faces
down--kind of a stupid design, really. Removing the throttle-lever extension might make this easier. I'd suggest using the appropriate 1/4"-drive screwdriver bit, the shorty ones they sell by the handful in "fishbowls" at the hardware store for fifty cents each. Use a 1/4" drive ratchet with a 1/4" socket to hold the screwdriver bit. Alternatively, you could remove the carb and do it that way, but you'll need new gaskets.
Now you're set to actually start tuning. You really want your tach for this job. Have whatever you used to adjust the fast-idle speed handy, as you'll want to fine-tune it right off the bat.
If you made the adjustments then walked away for awhile/overnight/whatever, open the throttle fully to make sure the choke is set. Start the engine. It will probably run at fairly high RPM, which it should on fast idle. Fast idle should be
1,900RPM, per the 1974 service manual. Once the fast idle is set, allow the engine to gain some temperature, cracking the throttle now and again
after the needle on the dash gauge has started to move. The idle may or may not slow during this process since the curb-idle screw is still in its preliminary setting (which might be too high) so
watch the fast-idle cam and screw. They should fully disengage one another once the engine's up to temperature and the choke is fully open. You may need to crack the throttle once or twice with a bit of authority to get them to disengage. If they do not disengage, figure out why and fix it. All those little linkage bits need to move freely as the choke coil on the other side of the engine is what moves them.
You cannot set the curb idle until the fast-idle screw and cam are no longer contacting one another. It's possible the engine will stall, but it should not. It should still idle fairly high with the curb idle on your preliminary setting.
Now set your curb idle to ~900-1,000RPM. This isn't your final setting, but you want it to idle steadily while you set the mixture screws.
Connect your vacuum gauge to manifold vacuum, preferably at the vacuum tree behind the carb. You must have full manifold vacuum, not ported, so straight off the manifold is better than off the carb. Turn one of the idle mixture screws in whichever direction increases vacuum. The idle speed may change; that's OK--leave it for now--but only go maybe half a turn. Now adjust the
other idle mixture screw in the same direction as the first, approximately the same distance. The vacuum should increase. If the idle speed has changed significantly, adjust it back down to ~900RPM. Turn the first mixture screw again, until you get the highest vacuum reading you can. If you go past it, back the screw off until the vacuum just starts to dip. Do the same with the other screw, then adjust the idle back down to 850RPM. You should end up with about the same number of turns on each screw, and you may have to back down the first to match the second when you get your highest reading. The goal here is the highest reading you can get with the mixture screws at
approximately the same number of turns, and but no more than whatever your peak reading was (it will start to drop if you go too far). It sounds tedious but actually goes by pretty quickly. Once you've gotten the mixture screws set, verify or adjust your curb idle speed to
850RPM in neutral, again using the 1974 service manual recommendation.
If turning the idle-mixture screws seems to have very little or no effect, you've got a vacuum leak which you need to find and cure before trying again. You should not need to futz with the fast-idle screw much if at all, but you may find your curb idle is way off the mark after fixing the vacuum leak.
If your timing is still where you left it, you should not need to adjust it further. If you back the timing off, you will need to re-adjust both your idle mixture and your curb idle setting. Timing well beyond factory is not unusual and you don't have enough compression for detonation to be an issue, even on pump 87, unless you have a massive vacuum leak. The additional timing will give you both better performance
and fuel economy.
Reconnect your vacuum-advance hose and you're done.
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Gasket shellac doesn't get along well with fuel. The only readily-available proper gasket sealant that is fuel-resistant of which I'm aware is Permatex High Tack. It's very thin (I use the spray stuff on every intake gasket I install) so if your gasket's not compressing fully it probably won't help. Seal-All is fuel-proof, but it can be a bit messy (it's rather watery). That being said, it
does work. The problem is that it can only be applied wet--you're basically gluing the fitting and gasket into the carb--and it dries as hard as rock. You must wait until it fully cures--hours--before you can check to see if it worked. If not, godspeed getting it off anything to which it adhered and starting again. The stuff can't be reasoned with once it's dry. In all honesty, I'm not a fan of the sealant idea a'tall.
One idea is to find a mom-and-pop type auto supply and see if they have an appropriate oil drain-plug gasket of the correct size that's a bit thicker than the steel one usually supplied with the carb. We have a slew of both fiber and petroleum-resistant vinyl ones at my store. Another option would be a PTFE (Teflon) faucet gasket, if one's available in the right diameter. It needs to be an exact fit, though. Sloppy won't cut it with straight threads.
Trying to use an O-ring is just asking for trouble. Put any such thought out of your mind. They are not designed to be compressed. It will leak, without fail. I learned that before I was old enough to vote: Holley 600, July of '88.
If your inlet fitting is not genuine Edelbrock, it may be too deep and not fully compressing the gasket before it seats. It's butchery at its finest, but you could possibly grind the "inside-the-carb" end away a bit, being very careful to clean it and file the threads back to perfection after doing so. If those threads aren't perfect, they
will wreck the carb threads--aluminum's pretty soft.
I'm not a fan of stacking gaskets, but it might work.