Lawrie Alexander touched on one of the reasons for too-rich operation of
an SU HIFcarb, (enrichment device) but there are other possibilities:
Unlike earlier SUs, HIF carbs do not have a choke linkage that operates
a sliding jet tube. The jet is movable in an HIF, but only by the
mixture screw, and the mixture screw operates by tilting a bimetallic
jet support. The mixture screw operates BACKWARDS from a typical
('murrican or riceburner) carb mixture screw. Turning the screw in
(clockwise) richens the mixture by moving the jet downwards, turning it
anti-clockwise leans out the mixture. A lot of folks mix up this
relationship and get their carbs out of tune as a result.
The jet can stick in its bore, or the bimetallic support can
malfunction, causing too rich of a mixture. Also, the HIFs seem very
susceptible to float leaks, causing the float to sink and a very rich
mixture as a result. If the float sinks, no amount of mixture
adjustment will correct the problem. I think our rotgut gasoline we are
getting in California with its magic MTBE additive may be aggravating
float leak problems. I plan to carry a spare set of floats for the HIF
carbs in my GT V-8 when I get it on the road.
Finally, the enrichment device that Lawrie mentioned can cause a rich
mixture in an HIF. There is a small O-ring and an equally small round
paper gasket associated with this device that can leak and allow too
much gasoline to enter the air stream even with the choke off.
Hope this helps,
Paul Kile
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