“Effect of altitude and climatic extremes on standard tuning
The standard tuning employs a jet needle broadly suitable for temperate climates from sea level up to 6000 feet. Above that altitude it may be necessary, depending on extremes of climatic heat and humidity, to use a weaker tuning. The factors of altitude, extreme climatic heat and humidity each tend to demand a weaker tuning and a combination of any of these factors would naturally emphasize this demand. Tills is a situation which cannot be met by hard and fast factory recommendation owing to the wide variations in the conditions existing, and in such cases the owner may have to do a little experimenting with alternative weaker needles until one which is satisfactory is found. To assist in this matter it is normal for a recommended weak needle to be listed.”
From this link.
https://sucarb.co.uk/technical-su-carburetters
I don’t know if this is what’s causing your problem. Hopefully others will chime in.