Why would the mixture change of its own accord on the hs4 on my 1275 engine.
After a run last week end the idle was rough and lumpy. Checked the timing which was correct and the plugs indicated slightly lean. On checking with EGA, it was lean. 1 to 2 flats rich brought it back to where it had been previously.
Anyone got any ideas.

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Cheers Dave.

There have been several SU problems on here since the site's return.
You probably know more about carbies than I do, but SUs seem very sensitive to fuel bowl level - for example, they don't idle well for long periods of time, unlike fixed jet carbs. The fuel level will rise and be richer as the engine idle slows.
And depending where you are located, a period of colder weather might alter the air/fuel ratio. I know SUs run so much better on colder, damp nights when the air is denser - so a bit leaner.

Thanks Bill
I have been thinking float level also colder weather. I hve also wondered about air leaks etc.
Now we are back in lockdown I can spend all weekend buggeri g around with it and hopefully find a reason.

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Cheers Dave.

Another idle thought.....
Minis don't like to be ignored. If you don't run them regularly they seem to develop all sorts of random issues they didn't have last time. And the SU design is a very mechanical set of systems that work best when used constantly. I am finding odd things like this now my minis aren't being used as daily runners.

The biggest problem with SUs is the throttle shafts wear out
Idle is the main thing affected

Spring loaded needle type Then needle and jet wear out
Fixed needle centered properly The brass throttle shaft and the body

As much as I agree regarding wear in SU carbs it does not happen rapidly.
I am going out on a limb here and thi king stale fuel is the problem.

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Cheers Dave.

My car been sitting forever Fuel will be crap . Hydraulics wont be good Time to get off my bum lol