The front third of the upper sieve should be completely clean. The middle section should carry a mixture of grain and crop residue. The rear third should carry only trash. (See diagram)
If the sieve is totally covered, increase the fan speed. If the front half is clear decrease the fan speed. Heavy top sieve loadings can also be caused by over threshing in dry brittle crops, reduce the aggressiveness of the threshing system to help.
If there is some grain on the rear third of the upper sieve, open it slightly to allow the grain through to the lower sieve.
Sample quality can be impacted by closing the top sieve, try not to have the top sieve as wide as possible to stop grain loss as this could have the opposite effect with a dirty sample.
2. Crop flow
Top tips to boost combine harvester output