Dellorto pilot jets for 19-21mm PHGB carburettors. The pilot jets are responsible for the mixture at low revs. Problems can occur if the item ordered is in any way wrong. If the pilot jets are too small, the engine will cut out at lower revs. In some cases, adjusting the idle can be problematic and the engine can be prone to cutting out. If the jets are too large, the engine will stutter and have poor pickup and run too rich. Different pilot jets are available and the size is generally stamped on the jet. You will receive the size indicated by the product description.