Steve, yur breaking my heart; That thing wants a ton more gas, a ton less air and a buncha more timing.
Just for reference;
What is the cylinder pressure? and
What are the cam specs?, and
What is your base timing?
First off let me say that I have never tuned a dual quad on a passenger car. But here's what I would imagine;
First off, at idle,
The PCV will have to be working normally, and
There cannot be any vacuum leaks.
> Your distributor needs to be reindexed. (see note-1)
> The metering rods need to stay down (see note-2)
> the choke on the primary needs to be working. (see note-3)
> I'm assuming the float levels are NOT LOW. (see note 4)
> All primary throttle blades are gonna have to be cracked just a little. (see note5)
> ALL secondary throttle blades, on the 273, are gonna have to be closed
up tight but not sticking.
> you are gonna set the idle speed with timing NOT with the curb-idle screws; and set the idle quality with: Idle-Air Bypass, (if required); or IABs, or fuel-level.
> this will get you into the ballpark.
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Note-1
This might be as easy as just pushing the V-can towards the firewall, and then rewiring the cap to put the #1 wire back to where it needs to be at TDC#1/compression. Yes this will work. Your mechanical timing should bring in about 20>24 degrees; to allow a base timing of 10>16 degrees...... to begin the tune. The all-in should occur in the range of 3000>3600. It only needs to be close at this point in the tune.
Note-2
If you have to, take all the springs out of all four wells, and reinstall the covers. This will make starting, without a choke, a lot more difficult; but they HAVE to stay down.
Note-3
The choke does a lot more than just limit the air intake. The closed choke, is supposed to crack the throttle open which then moves the intake "suction" from underneath the throttles to underneath the closed choke-blade. This causes a huge draw of fuel to be pulled from the float bowl thru every exit in the bowl. Both low-speed circuits will be contributing, possibly even the mains. The T-blades will be up the transfers quite a ways. So then;
The choke on the Primary carb should be set to the factory specs, as a starting place.The choke on Secondary carb does not need to function and can be tied open.
Note-4
All carb circuits are designed to work with a given WET fuel level. If the level is low, everything goes lean; if high, everything goes rich.
If it varies while running, for whatever reason, good luck tuning that sumbish.
Note-5
At this point in the tune;
The primary throttle-blades, all of them, need to be up on the transfers to a height of about square to prevent a tip-in sag (see note 8) or outright misfire/backfire/bog.. Since the engine will be drawing from FOUR venturies, AND all four transfers are gonna be active, the idle-speed will be high, and the mixture screws will need to be set a lil leaner than usual; as for me, I would start off with all 4 set the same. AFTER you get the engine warmed up and idling, you can start taking away transfer fuel by the adjustment of the Curb-idle screw(s), to help get the rpm down. If or when you get a tip-in sag, STOP, and put some fuel back into either the Transfers or the mixture screws, depending on where the mixture screws are already set to.. As the rpm comes down, this will become more critical.
In the beginning, of the tune, I would change the fuel adjustments equally on both carbs, simultaneously, just a lil atta time. As the rpm comes down, eventually you ARE GONNA find the tip-in sag, there is no getting around it. At that point, put some fuel back onto the Primary carb, (probably the Transfers), and leave it alone. If the rpm is still too high, AND the timing is down to 10* or less, AND all secondary throttles are closed, then you can try taking fuel/air out of the Secondary carb.
If you suspect the secondaries are pulling air, the simple test is to cover/seal the back-half of the air-horn, which should produce little or no rpm change. Sometimes the T-plates need to be reclocked to seal them up.
Note-6
your engine, when tuned right and broken in, will idle with the timing set anywhere from TDC to something like 25>30 degrees, so don't sweat the actual numbers it at this point. The more timing you give it, the faster the idle-rpm will be. So maybe it likes to crank at less than 10*, but it's sure gonna like 15>25 after it's warmed up, and especially during the warm-up period; I would give her whatever she wants without regard to the numbers.
Note-7
as to the Transfer-slot exposure
Since only the Primary carb is active at Drive-away, this carb will likely need a hair more T-slot exposure than the secondary-carb. Do not make it any more than it needs to be to prevent a tip-in sag or stumble upon drive-away. Ima guessing a good starting point for the mixture screws might be 1.5 turns out
The thing to remember is this;
At idle, with the choke fully open, your idle fuel mixture is supplied by BOTH the Transfer slots AND the mixture screws. You can play one against the other, until you get it right, and it is NOT right until the engine does not suffer a tip-in sag. ......... and the more timing you give it, the more idle-speed you get, which artificially removes the tip-in sag.
By empirical evidence, on a single 4-bbl, it has been shown that a good starting place to set this balance, is to set the mixture screws in the center of their working range, then set the T-Slot exposure to, from square to slightly taller than square. Then you use the mixture screws to help you fine tune the Transfer slot exposure.
If from a normal idle-speed, set by the timing, you get a tip in sag, then the T-slot exposure is likely not enough. To prove, you richen up the Mixture screws (They are just Idle-trimmers), and if the sag goes away, you then increase the transfer fuel, just a tiny bit, then return the mixture screws to the base setting. and retest.
The Mixture screws NEVER quit flowing. So if they are too rich or too lean, then the entire low speed circuit will be too rich or too lean.... with attending symptoms.
Now
Like I said; I have never tuned a dual carb on a Passenger car, but
have tuned hundreds and hundreds of 1 carb per cylinder motorcycles and snowmobiles.
Once you get the engine idling at whatever speed, start taking timing out at the rate of say 100>200rpm atta crack, retuning the tip-in sag as may be necessary, until you get the rpm down where you want it. If this ends up at less than 8 degrees, STOP!, something is not right
Note-8
Oh, I better explain the tip-in sag. This happens at tiny and slow throttle-tip-ins. It is horribly annoying, and often leads to a hick-up or carb backfire. It's like a flat-spot as the engine goes lean, just for a fraction of a second, then recovers as the transfers start flowing.
Yes you can, use the accelerator pump-shot to cover this, but you don't really want to and sometimes you can't. And the reasons for that are twofold; 1) sometimes there is just no way to get the pump to respond fast enough if at all, and 2) if you do get it to work, then it works all the time/every time, costing you additional fuel.
I try to get that out, as much as possible, by keeping the transfers alive, even if that means a less than optimal mixture screw setting.
Note-9,
When yur all done with the idle-system, it should be in the range of; 650/750 in N/P and about 100>150 less in gear (depending on the stall), it will not bang the trans on the P/N to in gear engagement (depending on the shift-kit), and the tip-in sag will be eliminated. The actual idle-speed will vary with the cam. The timing should finish in the range of 5>15 degrees, depending on the cam and stall-rpm. I wouldn't worry about the idle-timing too much; it's just the idle-timing........
Other notes
>If the engine is fresh and tight, it may take more rpm or more timing, to stay running. Tight rings can be a PITA.
>As to idle-timing; More idle-timing produces more idle-power. You only need enough Idle-power to prevent the engine from stalling.
But, the critical point in the rpm-band off-idle, is at the stall speed. Too much and the engine may detonate. Not enough and she will be lazy. But getting to that stall-speed at the correct timing, from a very low idle-timing number, can be a bit of a battle. So, many guys with automatics, just crank it, whereas the correct thing to do, IMO, is fix the doggone timing curve.
> Obviously, tuning the idle is done with the car stopped , the air cleaner(s) off, and the hood up. Once everything is put back together, the hood is down, the catb is hot, and the engine is sucking HOT underhood air, things can change. Be prepared for that, accept it, and don't sweat it. It doesn't usually take much to get it back on track, unless the gas is boiling.............
Ok, I gotta go to work now; take two and call me in the morning ,lol
Happy Hot-Rodding
