The emulsion tube (the main jet screws into it), in the Firestorm instance, has six holes (bleed holes) drilled into the side of the tube, up to approximately 3000 rpm the fuel level sits just above these bleed holes, the fuel height is higher than the actual fuel-bowl fuel level, held up the emulsion tube by the air speed in the venturi creating a slight negative pressure (vacuum).
As the rpm rise, between 3000 & 4000 (aprox), the fuel level in the tube drops to slowly uncover the six bleed holes as air is introduced to the fuel coming up the tube, atomizing it for release into the venturi, under the control of the needle & needle-jet relationship. This is the point at which the fuel mixture control changes from pilot-jet to needle, the height of these bleed holes, and the fuel bowl fuel level settings alter the timing of the switch-over, and the mixture at that point. The rpm range that the mixture is being affected is the time the level drops from uncovering the top bleed hole till the bottom bleed hole is uncovered, bigger bleed holes equals leaner mixture at that point (about 4000 rpm).
After 2001 the firestorm has a larger bottom bleed hole (up from .8mm to .9), therefor giving a touch leaner mixture at the end of the changeover period.
As I've discussed in another post recently, this is why the bleed holes in the rear cylinder are higher (2.5mm) than in the front cylinder, because the fuel level is different in each fuel bowl (in relation to the emulsion tube bleed holes), due to the carbs sitting on a different angle to each other. If the fuel level in the bowl is too low, or the bleed holes too high, air is introduced into the mix too soon, so for the two carbs to be matched (only for the Firestorm), the higher bleed holes in the rear cylinder, time the introduction of air at the same time as the front carb lower fuel level.
I hope this is understandable, I've studied many articles on the matter, plus worn-out many Firestorm carbs learning, I've tried to keep it all basic, someone is bound to trip me up on a point but hey, that's life
John Orchard
