stock 1973 318 V8 starts but won't idle without foot on pedal
At 4.5 to 5.0 turns out, your mixture screws are too far out, and have lost control.
The reason that your mixture screws are out so far is because the transfer slot is not providing the right amount of fuel. This can be because of; 1) incorrect curb-idle screw adjustment, or 3) incorrect 'Wet" fuel level, or 3) too tight a valve lash, or 4) because of a lack of understanding the role of Idle-Timing.
In a properly working system; The transfer and mixture screws, work together at idle, with the transfers doing the bulk of the work, and with the mixture screws just acting as trimmers. The star-player in this union is the "WET" fuel level. If the "wet" fuel level in the bowl is not what the carb designer calls for, nothing works right. You may be finding that out.
>Your idle-timing has nothing to do with making the engine run right. You can NEVER give the idling engine the amount of timing it actually wants, and still be able to drive it. It wants 25 to 30 or more degrees! at idle.
No; your idle-timing is "settled on", by the manufacturer, in order to create a balance of transfer to trimmer fuel delivery synchronization that allows the transfer slot to function properly at idle, with no tip-in sags, hesitations, nor requiring excessive pump-shot to overcome a bog.
>Your statement of;
I had to back out the screws 4 1/2 - 5 turns.
proves that your Transfers are not supplying enough fuel; normally the trimmers will be set to ~3 turns +/- .5 turn. You need to decrease the mixtures screw setting, and increase the transfer fuel, and if the idle speed is not right, change the timing until it is. If this requires excessive advance; something is wrong. Either your EFFECTIVE compression ratio is very low or your valve timing is out of whack. The principal player in these matters can be but is not limited to, the valve lash.
>To increase the transfer fuel, you have to increase the slot exposure with the curb-idle screw OR increase the height of the "wet" fuel level, or decrease the size of the IABs. If you fail to synchronize these, your ENTIRE low-speed mixture will be rich due to the fat trimmers. Your goal is to try and reduce the trimmers towards 2.5turns for best fuel economy.
>Let the idle-timing be whatever it wants to be, that makes the Power-Timing reasonably close but NOT so much as to cause detonation, which leads to broken pistons and such.
> if the engine continues to run rough, look to the cylinder pressure balance. In a good working engine, the numbers will be very similar for every hole.
