[Tigers] "Speaking of S.U.'s - "

[Dave Munroe]

Steve;

You said in your very interesting review of the history of the S.U carb:


" ..........is operated by a vacuum operated diaphragm or the later large 
piston.  Good mixture
 control, but NO accelerator pump for instant  enrichment."

This is true of course. But in the newer S.U. Carbs there actually is a 
built-in mixture enrichment feature that is adjustable to an extent. It is 
the "dash pot"
filled with oil. When you hammer the throttle, the oil in the dash pot 
inside the diaphragm-controlled piston slows the rise of the piston,
allowing fuel to be drawn in to the venturi without a balancing hit of air 
due to the piston's slightly delayed rise in its barrel.

I have an S.U. equipped supercharged '74 MGB. There is an O2 sensor in the 
exhaust manifold that drives an air/ fuel/meter on the dash, and the effects 
of changing the weight of the oil in the damper can readily be seen. The 
heavier the oil, the slower the piston rise, and the richer the mixture 
following a suddenly opened throttle. With a blower on full throttle you 
want the mixture on acceleration at the "very rich" end of the scale at 11.5 
or 12:1, along with some good gas in the tank and a retarded ignition
around 25 -26 deg BTC. I am currently using 90W diff. oil in the dash pot.

Don't you just love the techincal stuff??

Dave