If you mean the needle get's stuck even though the engine keeps revving higher then there must be something wrong with the gauge. Carefully pull it apart and give it a clean, you should find if anything's broken. I'd recommend doing it in a clean place and on a nice big tray though. Nothing worse than having a spring fly off and land in the filth somewhere never to be seen again.....

I just installed a 1200 sedan datsun tacho into my 1979 datsun 1200 ute, it works but doesnt rev over 4000.. Would anyone know what the problem might be?
Cheers