求翻译：　　2、喷油响声检查。用手压泵作喷油试验时，针阀的跳动会发出清脆的响声。响声的不同，在一定程度上标志着座面密封和喷雾的好坏。针阀跳动的响声，很难通过两个偶件之间的研磨得到改善，如果座面密封良好，就不必研磨。正确的接触印痕应在针阀锥体大端处。是什么意思？

2, fuel injection sound check. Hand pump for fuel injection test the beating of the needle valve will be issued a clear sound. Sound different, to some extent, marks the seat surface to seal and spray is good or bad. Needle valve to beat the sound, it is difficult to be improved through the grinding