CV/CD Carbs

By CV carbs I mean the constant velocity/constant depression carbs such as the SUs or Zenith/Strombergs as fitted to the XU1s and any number of pommy cars. And I may as well say right now that there is probably no performance advantage to be had by using one brand over the other. There may be a minor advantage to the SUs in that their piston design eliminates any potential diaphragm problems. I've used CV carbs on a variety of engines over the years, and they have never failed to give excellent results.

It's practically impossible to overcarb an engine with CVs, and they seem to tolerate being fitted to overcammed street engines very well. Inch and three quarter CV carbs are almost the defacto standard on a hot six, and they will certainly perform well on a very wide range of engines from near stock to quite high outputs. On a very well breathing 186/202 it will pay to go for 2" units to achieve the engines full potential, and unlike fixed venturi carbs there will be no penalty from using the bigger carbs as the throats only open as far as necessary.

Some people are wary of using these carbs because of their undeserved reputation for being hard to tune and/or keep in tune. The reality is that as long as the spindles and linkages are in good condition and well set up they require very little attention at all. There must be no play at all in the bushes or rod ends, and the main shaft must be set parallel to the throttle spindles. Also it pays to run individual return springs directly on each butterfly to minimise the effect of any play in the links. Once the carbs are synchronised and the mixture set they will run for a very long time without any further fiddling. While there is a huge variety of metering needles available it's very likely that the factory fitted needles will be very close to ideal, and it seems that the standard needle can cope with a wide variety of engine types.

Summary: CV carbs will give excellent results on nearly any engine.