William Hartwell Woodruff wrote:
>>>My 1980 MGB with a ZS carb doesn't want to idle at a sensible speed and I'm
>>>hoping someone can give me some pointers.
>
> There is an air bypass on the later ZS carbs. I THINK it is on the
>side of the carb and looks sort of pointed with a screw in the center. The
>purpose is to reduce manifold vacuum and the resulting rich mixture when the
>throttle is snapped shut. If this thing is sticking open, it would give you
Actually, no. The device is there to control the speed at which the engine
returns to idle, by sensing overrun via hi manifold vacuum, and opening a
bypass around the throttle plate to compensate.It is there to keep the
engine,when returning to idle, from overshooting the target rpm, and cutting
off, or stumbling. Engines that run artificially retarded ignition timing
at idle are particularly prone to this problem. Other makes of cars, such as
Saab, Honda, and most American, use a mechanical "dampener" on the throttle
shaft to effect the same result.
MGB's, partly due to more sane ignition settings, and partly to the use
of a gulp valve as the air injection divertor, do not need, and so do not
have, this device.
And yes, Roger, Scott, and other technoids, the real reason for the decel
valve is to mantain cylinder pressures in the conbustable range. But this
does determine that rate at which the engine will slow down.
Randy
randy@taylor.wyvern.com
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