Combines Black 438 corn head

This is a common problem with wear and stretching of the chains. Replace both chains on a row at the same time and make sure that when installing that both are exactly the same length(n of links). This fixed the problem on my A438. The sprockets if worn will also change the timing_length.

Down under the drive sprocket is a key joint. Make sure key is not sheared and driveshaft slipping inside drive sprocket. That can be a time consuming job to replace so check it before heading to fields. We have sheared a few keys over the years.