SOHC.co.uk Forums > CB500/550

Is this right?

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K2-K6:
With that description, a good place for a link to that 250/6 demo run https://youtu.be/CFbLXSoklI8 you can move the camera view round to look forward and include the rev counter too !

Stalled it more than i thought, note description in comments of "no idle jets"  ;D

JamesH:
Just following up on this, came across this old post on the US site:

http://forums.sohc4.net/index.php?topic=74546.0

Looking 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,"

K2-K6:
The original description (that observers think is incorrect) looks remarkable in iits similarity to another type of carburettor.

The Webber DCOE behaves in this way, having a directly adjustable fuel passage to meter at idle as described. They are butterfly type throttle which when closed (open slightly for idle rpm) has a port that's already mixed for a/f ratio coming in on engine side of butterfly into that vacuum space. It has a taper screw directly in that entrance. It only adjusts FLOW at one point wuth the butterfly closed, with transient fuelling demand through additional holes (supplied by same circuit ) exposed along the carb bore as the butterfly arcs open to give transitional mixture in rising rpm, as each successive hole is passed by that butterfly edge when throttle is used so they are exposed to engine vacuum drop. On this system, screw out gives more fuel and visa-versa.

These SOHC carb have a fully transitioning idle circuit, balancing fuel air mass according to relationship of each bore component, air to fuel bore and their relationship in size and flow to the venturi volume.
It's effectively an air proportional valve that gives accumulating "slope" in idle and transition fuelling to bring the engine up into competent range of main jet running.

The two are fundamentally different.

K2-K6:
Interesting view of intake here (and one for Ted  :) )  as it shows very clearly just how much mixture is ejected in "standoff" (the fuel mist outside intake with engine at full power) and usually contained within a good airbox system for it to be ingested by another cylinder https://youtu.be/hHjf9zhQIRk starts at about 1 min when he gets the throttle to full open.

Johnwebley:
Oh the howling straight cut gears!


Remember them well

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