SU Oddity?

[Charles Levien]

OK, will one of you clever chaps explain this to me:

Moderate tune Chummy. 1 1/4" SU. Semi downdraught. Pumped feed.
Tuning on the road with an AFR meter on a Lamda on the downpipe.
Steady, fairly steep hill. Holding 5000rpm flat out in 2nd gear. AFR steady at 12.3, happy! Holding same revs around  left hand corner and AFR drops to 12.0. This was repeated a couple of times. 
What's going on to make it go richer on corners?
What's the cure?

This paints an exciting picture — might be helpful to know if you were going up or down the hill. If the latter I would suggest changing into top. According to Bill Williams tyre constants that will give you 68 mph on 19 inch wheels. 
Can’t  think of any useful suggestions concerning cornering but in view of cars performance I should try to avoid them.