Just following up on this, came across this old post on the US site:
http://forums.sohc4.net/index.php?topic=74546.0Looking down the thread there's a post from "TwoTired" which pretty much backs up my understanding of how the pilot circuit works (TwoTired's words not mine):
"I have to disagree with Mark (Hondaman) on this point. My understanding of physics refutes his explanation, as does my testing experience with an exhaust gas analyzer.
A given orifice size, flows a fluid based on the pressure differential across the orifice (assuming laminar flow of the fluid, which the standard jet operates within, in its normal regime).
The jets don't flow when the engine is not running because the pressure in the carb throats is the same as the pressure on the fuel in the carb bowls. No pressure differential across the jet orifice equals no flow.
When the piston falls on the intake stroke, less than atmospheric pressure is created, and this pressure loss (commonly referred to as vacuum pressure) then is presented to the carb throat. The exit port for the pilot system exists within the carb throat. Both the pilot jet and the air air jet feed this exit port. The air jet presents atmospheric pressure and an air source, while the pilot jet presents fuel pushed by the atmospheric pressure on the fuel in the float bowls.
The air bleed screw regulates the inrush of air and also lowers the pressure differential applied to the pilot jet orifice. With the air screw more open, both these factors reduce the fuel air ratio mixed in the emulsion tube (at the fuel jet exit) before it travels to the low pressure source in the carb throat, creating less fuel for the increased air volume, and reduced pressure differential applied to the fuel jet orifice.
The only way extra negative pressure in the pilot system could be created was if there was a venturi to increase the speed or flow of materials as there is in the carb throat, which it does not. The flow of materials, air and fuel, in the pilot system is governed by physical principles and the pressure differential between what exists in the carb throat and the outside atmospheric pressure. It really doesn't matter if the differentials are pulsed, the rates will be determined by the average over time.
I am sorry to disagree with Mark on this point. But, my actual testing does not seem to support his theory (whether it is printed in his book or not). My testing does exactly follow my understanding of the physics and science involved, and the hydrocarbons sniffed at the tailpipe show that the air bleed screw turned out makes the mixture leaner and turning it in makes the mixture richer. (For the non-PD carbs found on our SOHC4s, of course.)
I hope I didn't put more typos in the above.
Cheers,"
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