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MG MGB Technical - HIF Piston question

I have two sets of HIF carbs for a 73 B. The carb pistons on one set have the small tube/shaft flush with the piston face and two have the shaft/tube flush with the bottom of the groove. Which is correct or are they both? Thanks, Tom
Thomas McNamara

Tom -
Shoulder of the needle flush with the piston face. Book says the adapter is flush but I've found variance with this, so I set them as they always were before the spring needles.

FRM
Fletcher R Millmore

Hey FRM, I am trying to solve the riddle of the carb pistons raising at different starting points. The only difference was one adapter was recessed .009 and tha other is flush with the piston face.

When I checked on the "spare" pair, they are both flush with the bottom of the groove. This would allow air flow when the piston is at rest, even though the needle "collet" is flush with the face of the piston?

regards, Tom
Thomas McNamara

Tom -
The book says the "needle guide" aka collet, or adapter, is flush with the bottom of the piston groove. I've found that the needles shoulders sometimes are not flush with the actual piston bottom face when set like this, so I set the needles flush to the piston and forget about the guide/collet/adapter. The needle is what the engine works from, so follow suit. They shouldn't be far off; if so then somebody has moved the needles in the little flanges they are pressed into at the top - rare.

One of these adapters fell out, so which one was it? Can't go by that position! The flow through the piston groove is not enough to really matter; when the engine starts the piston will rise to a suitable point. You will note that there is a small plastic (brass on very early carbs) protrusion on the piston bottom face, which is there expressly so that the piston does not ever seal off the airflow completely. I had one (early HS, without the groove)that had had a small fire in the carb, which melted the plastic gubbin. On trying to start, the engine vacuum sucked the piston tight and jammed it in place - most puzzling!

The pistons rise according to actual airflow through the carbs. Assuming that the springs are the same, and the vac chambers are correctly mated with the right pistons, then any difference in airflow is a result of the throttle openings being different, or gross differences in flow between the two pairs of cylinders, ie a bad valve, or valve clearances way out, or flat cam lobes.

FRM
Fletcher R Millmore

One more HIF4 question.
I am actually attempting to extract cold start assy on 71 HIF4 pair.I Dismantled locking nut, fast idle cam, return spring and cold start retaining bracket. Does it sound logical to really pull out that hard this device from carb body? I see no retaining screw or lock, every part is free to rotate.
Rgds.
Guy RENOU

Job done on second carb. It's just necessary to overpass the O ring friction onto carb body housing.
Sorry for the question, but I hope that may help.
Guy RENOU

This thread was discussed between 21/02/2010 and 22/02/2010

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