ccrobins wrote:
>
> Everybody,
> Buy new needles and jets. Stick them in there and try setting the
> mixture per the manual. If that doesn't work AND before you mess with
> the float heights, replace the throttle shafts. You'll be surprised how much
>this helps!
>
> Also, the shafts seem to be much softer than the bushings. So put in
> the shafts first and see if they aren't too loose. If you put a new
> shaft and you can feel the side play still, then you either need
> oversize shafts or you need new std bushings and shafts. In my case the new
>shafts did it. YA CAN'T TUNE THE CARB IF THE THING LEAKS AIR PAST THE
>THROTTLE SHAFTS!!
>
> Charley Robinson
> `699 B
Because of the air-leakage by the throttle shafts, SU developed the HD
series, fitted to posh cars like Rolls Royce, Jaguar, etc. BMC fitted a
smaller SU HD4 to the Farina MG Magnette, the only MG to ever get them.
They have a rubber diaphragm that raises and lowers the jet tube,
and.........special end fittings on each throttle shaft, to stop air
leaks. The 'fitting' is simply a CORK SEAL that is compressed into the
outer edge of the carb body around the shaft....not unlike the early
fuel sealing of the jet tube on the XPAG/ 1489cc 'B' series, and early
Midgets.
It proved too expensive, and LUCKILY for the MGB MG went back to the
jet-tube arrangement, but now using a flexible tube for the choke tube
from the float chamber, ( SU HS4,) and later the concentric float
chamber,( SU HIF4.). However, the air leakage on the shafts continues to
be a problem!!!! Some USA spec post '74 models had the Zenith 175 CD5T,
( called a Stromberg in the UK,) fitted.
Kits to repair the HS4 are about £25 each, kits to repair the HD4 are
£80 each, ( from Burlen Fuel Services, a dollar is $1.60 to a £1.)
Each time I remove the oil dipstick to check the engine oil level, I
'drain' it onto the carb shaft ends, lubricates them and they last
longer.
Neil
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