Two things. If you haven't already done it then stop right where you are and do not attempt to dismantle, i.e. separate, the two halves of the metering head.
Before condemning the head check the connections to the warm up regulator, if there is no currnet flow then the mixture will be permanently enriched. Check the earth too.
The mixture can be adjusted. Remove the red plug from the body and use a 3mm allen key to turn the adjuster anticlockwise to make the mixture leaner.
I am working from memory here, and as I recall, the only O ring is between the head and the plate housing, and provides an air-tight seal between the body and the casting. I don't recall seeing one in the bore for the piston/plunger.
The plunger and bore are honed to extremely tight tolerances and it has always been my understanding that the components seal due to their good fit. They do wear and let fuel past, and if this is the case there will be fuel residue in the bottom of the air-filter plenum chamber below the metering head. The system is so sensitve to friction that I think any O ring used to seal, would, over time, swell, and inhibit the movement of the plunger, making the metering of the fuel hoplessly inaccurate.
Am I right in thinking that you beleive that there should be an O ring or seal in the groove in the piston, or plunger? If so I am afraid that is not the case.
The position of the groove, relative to the outlet slots in the bore within which the plunger slides, are what controls the volume of fuel flow, and, as I understand it, the position of the plunger, relative to the slots, is proportional to, and controlled by, the angle of deflection of the plate. Simply stated, and this is a simplified version of the operation, when the regulator is cold, and is in it's bypassing state, fuel pressure, which is present both above and below the diaphragms and plunger is reduced in the area above the plunger, because the bypass orifice is larger than the inlet to the top of the metering chamber, so cannot stabilise to system pressure. This unbalances the system slightly and allows the plunger, which damps the movement of the plate, to move further up the bore thereby exposing more of the metering slots to the void created by the groove in the plunger, allowing more fuel to flow as the plate finds a new stable position in the airflow for the same flow of air mass balanced against the increase in fuel flow, which offsets the imbalance caused by the bypassing at the warm up regulator. SIMPLY STATED.
Hope this helps.
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