Inliners International Forums General Discussion Engines carb circuits and gearing/tire sizing

This is how I see the relationship of gearing and performance/milage.

Carbs got "circuits" which take care of certain conditions that happen to engines.

Idle circuit
When the throttle is at idle it is barely open, and there is manifold vacumm sucking gas from tapered screw adjusted discharge ports just below the butterfly-VERY CRUDE metering system, very rich, very uneconomical. gas continues to be sucked out until throttle is open enough for manifold vacumm to lighten up and/or butterfly moves away from these ports less and less draw at the idle discharge ports 'til no gas is being drawn out. A very rich, wasteful circuit.

Main Metering circuit
When trottle is opened enough to get some air flowing down the barrel of the carb, through the venturi fast enough to draw gas up from the float bowl through the main jets and out through the main metering discharge ports. This is the carb's best working circuit, very accurate, (as long as the main jets are properly sized)very economical, though the idle circuit can still be functioning as the main metering circuit is comming on line (depends on how close the butterfly is still to the idle circuit discharge ports. This circuit will handle most of the carbs duties-its the best its got.

Power circuit
To get a load going,or maintain speed up a hill, we step deeper into the throttle (vacumm goes down until the carb goes into a richer mode to produce more power called a power stage) at this point additional fuel is drawn into the engine (just because the vacumm dropped)(bigger cams, bigger carbs do this too!) making the air fuel mixture richen (by additional jets or a richer potion of a metering rod) from a general 14:1 ratio to a powerful (but uneconomical) 12;1 ratio as long as your foot is into it. good for power- bad for economy. Back off the throttle and higher vacumm returns and you are still on the main metering, but not with the additional power stage, and back to economical cruising.

Choke circuit
Just that big top butterfly that limits how much air can flow down the barrel that causes the fuel being drawn out of the idle and main circuits to have very little to go with the drawn gas-thus real rich mixture and no economy and a no chance for much airflow down the barrel so no real power either.
Only necessary 'til motor warms up.

Low numerical ratio rear ends will keep a motor's rpms down (at any given speed)and in general, more throttle will be used to enable the car to perform acceptably, and with that, lower vacumm will be seen, causing the power stage to be be in use more often, and with its less economical air/fuel mixtures less mpg.

If a car with 30" tires and a 3.08 rear gets some 26" tires, it will think it now has 3.55 gears-this is how tire diameters affects gearing.

Any buyers?