though the air still has to be warm enough to vaporize the fuel properly.
that will happen inside the chamber by heat of compression, as long as the carb is doing a proper job of atomizing the fuel.
Since your original question has been answered, let me run on for a while.
Long-runner slantys often have trouble keeping the fuel in suspension when cold in winter.
The bigger problem with slantys, is the lack of idle-timing, and very slow rate of advance.
Another is that often, the vacuum advance does not start until just before the stall rpm, say 1500 to 1800 at very light throttle. IDK if you have a slanty, you didn't say. OOPs; I see you did say.
Any engine will make best power with peak cylinder pressure occurring around 25 to 28 degrees AFTER TDC; irrespective of rpm or load. All your timing controls are designed to help make that happen.
As the rpm rises, we need to start the fire ever earlier.... to get the magic number, but
as the load increases, sometimes simultaneously, we need to prevent a runaway pressure situation from smashing the bearings to bits by way of detonation; and so,the easiest and best way to do that, is to start the fire later.
At WOT; some place around 3600rpm, or a lil sooner, timing can more or less flatline, and so, the spark-timing is normally "set" to about 34*BEFORE TDC @3400 rpm, so that all the fuel will be finished burning by the magic 25 to 28degrees.
At Part-Throttle, the timing is targeted by the distributor system, in conjunction with the vacuum advance system, to again attempt to achieve the peak pressure at 25 to 28 ATDC.
But the idle timing CAN NEVER BE OPTIMUM, because to have it so, would mess up all the other timings. So most often, the Idle-Timing is whatever it gets, to best meet the off-idle needs of the engine.
And so, the factory puts all kinds of band-aid fixes on the Idle-System. And that's even before the pollution control efforts go on!
The point is, that at the factory settings, and in "perfect factory" tune, the settings are just barely adequate for the engine to start and remain running.
To see what I mean;
with the engine idling, just advance the Idle-Timing little by little until the rpm no longer rises. When it stops, peak cylinder pressure is occurring with the crank at the optimum position to transfer as much energy in the expanding gasses as is possible, to the flywheel, at that rpm and load setting. Were it not so, the engine would be running slower.
Now read your timing lite. On a SBMs (all I know) I'm never surprised to see a number in the high twenties/low thirties. But you cannot drive it like that, so put it back where you found it. You cannot drive it like that because, as soon as you put a load on the accelerating engine, it will want LESS timing; whereas the factory D will put in MORE timing; and so detonation is inevitable.
But that does not mean that the factory Idle-Timing is right for your engine, operating in your local environment, for your driving style.
The problem is, the factory carb is already set up for the factory tune; which includes the factory timing, and properly working vacuum controls, including pollution equipment (if any), etc. And so, when you buy a replacement carb, at the jobber, you have no idea about it's state of tune. The same basic model carb might bolt on to any number of different-year engines. But the factory tune over those years, may have drastically changed.
This is especially true from the early hi-compression 318s to the later low-compression 318s, and the following smoggerteens. At least three different basic engine tunes. (SBMs are all i know)
One thing they all have in common is very little Idle-Timing, with a variance of no more than 5* (IIRC) .. and a second thing is the wet fuel level. But that's about it.
When you buy a carb rebuild kit, for a certain model carb, you will get to sift thru the instructions and choose only those adjustments which pertain to your application.
Ok, enough of that.
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When searching for performance;
Everybody is quick to recommend more ignition timing. Which if you have a 4bbl is probably the thing to do. But what about a 2bbl? Or a Single?
Well, every carb has a set point in throttle-opening, which starts the spark-port. When yur under the set-point, no vacuum advance will be obtained. When you are near to or past it, vacuum advance will be doled out, in proportion to the manifold vacuum, and the V-can preload adjustment.
If your factory ignition system was designed to Idle at TDC; then lets say the vacuum advance might begin at about
2* at 1600rpm @15inches of vacuum, in Neutral
Now suppose that you advance the idle-timing to 10* a common recommendation. Now your engine is producing more idle torque, so the idle-rpm goes up. And dutifully, you slow it back down. In so doing, you have resynchronized the transfer slots to a smaller opening below the blades, and the engine is Idling lean. So you open up the mixture screws, and voila the engine is happy at idle.
Ok but, you decide to take her for a spin. You put her in gear, and the increased load causes the engine idle speed to drop. And the lower airflow/speed past the transfers, produces a momentary hiccup as the engine has gone lean. Maybe it recovers. Maybe it stalls. Maybe it just runs rough. But you soldier on.
As soon as you step on the gas even ever so lightly, the engine goes lean, because the transfers are so slow to respond, and you feel the momentary hesitation. But then immediately, the engine goes rich, as the transfers come on line together with the richly adjusted mixture screws. And the engine will remain rich at ALL throttle settings because of the richly adjusted mixture screws.
But hang on, remember the Vacuum advance?
With the advanced Idle-timing, like said, the engine now has more low-rpm torque. And the vacuum is up slightly, and so it requires LESS throttle to get moving, and less throttle to maintain a particular speed, AND so maybe your Vacuum advance
doesn't come in until a higher roadspeed than at the factory timing. Or maybe you enjoy the new torque and drive just a lil deeper; and the vacuum advance puts her into Part-Throttle detonation. That really sucks.
All that from 10* additional Idle-Timing?
IDK; I'm just saying what CAN happen if you start monkeying around with a factory system.
The single-most important part of the Idle-tune, is, IMO, to get the transfer slots the right length. I don't care what the idle-timing has to be to get it right. The engine doesn't much care either.
What the engine does care about is that she is not over-advanced at WOT,between the stall rpm and the lowest rpm at which she will take full-timing, usually found between 3200 and 3600.. ALL the rest of the engine timing systems have to be synchronized towards this one particular running mode. Cuz if you get this wrong, detonation usually breaks engine parts.
