What determines a total carburetors flow is the venturi size(s), the annular boosters, the air bleeds, the discharge slots and the jetting.
Together these work in conjunction to give your carb a balanced flow through-out the complete rpm range and load for your vehicle application and engine size and load requirements.
Along with that is some hidden or obscure parameters such as engine modifications, engine volemetric efficiency, Stoich air conditions (how dense the air is at a given moment), elevation.
So lets take the carburetor parameters only (since tuning to the others could be covered in a several books).
If all those things are set up for a 480CFM flow for a 289 engine with around 170HP (stock 289) in a mustang. Everything is running nice and smooth. Now someone places a 600CFM from a galaxie on this 289, right away you can see the engine that was happy and being well fed is now being STUFFED with fuel, this is OK at high rpm, in fact it is now running harder and further than ever before, but at idle and especially at that critical trasition period when the idle circuit drops off and the cruise takes over it is glutted with fuel.
So to correct this a person was told to change jets, which they do by only a "minuscule" two sizes, now you have changed the way the entire carb processes fuel, EXCEPT at idle. So it still is flowing lots of fuel (Remeber the carb thinks its still feeding a 390 galaxie)at idle, at transition the jets now start to take over, and they now are flowing less fuel but the air bleeds are still suppling the SAME AMOUNT OF AIR for the stock jetting, so your air to fuel ratio now went very lean even though you still have MORE AMOUNT OF THIS air/fuel mixture than your 289 can use. SO you still bog but now you also backfire (thats extreme most times you might hear a burble if your listening for it) from larger amounts of air mixture.
Now you go through the RPM ranges still lots more fuel than you need but its more air enriched and than fuel, till you get to the top end. Now your not running as good as you were but you are also flowing lots more air which now can be a dangerous lean (read burnt valves, piston holes).
The california emission carbs had this very problem when installed on to the 352/390/428 engines.
SO can it be fixed, of course, but FORD in its infinite wisdom does not have any information on Annular booster specs, air bleed specs, discharge specs or rates, they do not even list rates for their autolite jets.
So without that information one would just be guessing (and in the process ruining carbs) by drilling out and experimenting.
In this way the holleys and all their specs are much better for racing since they can be adjusted in ways the Autolites can only be guessed at.
Hope that helps,
Bill White
White Automotive
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