Erik, Here's why...

The short answer is; unmetered air entering the intake tract makes it impossible to tune the carburettors.

The longer answer is; You must understand how the SU carb works before you will understand the effect that worn throttle shaft bushings have on the mix.

SU carbs meter the amount of fuel dependent upon the volume of air moving across the floor of the carb under the piston. Metering is accomplished by retracting the needle from the jet, allowing more fuel to be pulled into the airstream. See the Swem web site for a more thorough explanation of how the SU carb works, some tuning guidance, and references. There are now also a series of videos made by University Motors on YouTube to help you visualize the operation of the SU carb.

At any rate, the piston is acted upon by a pressure differential (relative vacuum above the piston), thereby raising the needle, metering the fuel.

SU carbs are "constant depression" carburettors. In the USA, we say constant velocity, we mean the same thing. The principle is that the depression, or negative pressure relative to barometric pressure, remains constant above the metering orifice. That means that the area immediately above the jet orifice will always "see" the same pressure regardless of throttle opening. (Not entirely true; there are spikes and sags, but for the most part, pressure at that spot remains constant during operation. The spikes and sags just happen to occur when they are needed, acceleration and deceleration to richen or lean the A/F mix. This "glitch" in the operation of the carburettor is used to advantage to meter the fuel during periods of transition. To me, this is simple elegance of design; a correct application of theory to practice.) As the throttle plate angle increases, more engine vacuum reaches the top of the piston, drawing it up. Because the opening is larger (the piston is rising) the volume of air across the jet orifice increases. More volume at the same pressure means more air moving past the metering point and more fuel is "allowed" into the airstream. Beautiful, efficient, and utterly simple. But I digress. Back to the throttle shafts and air leaking around them.

The amount of fuel was metered into the air stream at the jet orifice. If the tuner chose the correct needle, spring, and dampening fluid, and if the car is operating in nearly the same conditions (temperature and barometric pressure both effect the amount of air entering the carburettor), the mix is as near perfect as mere mortals can make it with a mechancial fuel mix calculator across the operating RPM range.

Where is the throttle shaft located? Why it lives downstream of the jet orifice. So after the very short trip from the jet orifice to the throttle plate, the near perfect air/fuel mix is ruined by the addition of air being pulled into the intake tract past the worn bushings, leaning the mix. It is that simple.

What to do about it? Rebush the carburettors. You can have it done professionally or DIY. Joe Curto, Rhys Kent, or Tom Bryant all do good quality work at reasonable prices. If you want to try it yourself, see the Chicagoland MG Club article and this downloadable PDF.

I hope that helps some.

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Mr. Shannon DeWolfe -- (I've taken to using Mr. because my name tends to mislead folks on the WWW. I am a 52 year old fat man ;-) -- KD5QBL