Engaging the choke = leaner mixture??

[MOWOG1275]

Greetings:

I must be missing something here…

I borrowed a wideband air fuel ratio meter and set about tuning a close to stock MG 1100 engine with a single HS4 SU. The car had a problem with a stumble at tip-in that could be helped if the choke (enrichment) was engaged while driving. I had assumed that the carb needed a to be adjusted for a richer mixture. I was told that a Lambda reading of approximately 1.0 was good for idle and a 0.7-0.8 slightly lean reading was a good for a acceleration and normal cruising speed.

The system was set up and calibrated with the O2 sensor clamped to the tail pipe and approximately 1.0 Lambda was the idle reading when warmed-up. Full throttle and a steady throttle would generate an initial stumble and a rich reading from 1.2 up to 1.6 depending on the throttle position.

After pulling the choke knob the stumble disappears, but the Lambda goes leaner to the 0.6-0.8 range. This is counterintuitive! How am I getting a leaner mixture with the choke engaged? What am I missing?

Thanks for your help.

Kirk

[Smiffy]

Your getting confused, don't forget you measuring the O2 content, so if it's reading less than 1 it's actually rich not lean.

[timmy201]

Stoichiometric mixtures for petrol is 14.7:1 -this converts to a lambda of 1.0

As mentioned above, a lower lambda means a richer mixture

Probably the lowest you’d want to go would be 0.8 for a naturally aspirated road spec A series

[MOWOG1275]

Aha! Of course.

Thank you.

Kirk

[BAD942B]

you have the readings the wrong way round but lambda sensors are easily fooled by certain conditions, see https://www.austincc.edu/wkibbe/lambda.htm
for a full diagnosis you need more info

[BAD942B]

not really be fooled but quite tricky to read problems with it without further information