Volvo D2-40 slow start

[dougweibel]

I have Volvo D2-40s which have always started within two revolutions unless very cold. I was pressed for time with family visits, etc. so hired a marine diesel service company to do some scheduled maintenance including fuel filters, belts, cleaning heat exchangers and adjusting valves. Since that service both engines start slowly. They have not failed to start but routinely take 4-8 seconds of cranking to start which is completely not like how they have behaved previously. If I have one running, shut it down, then immediately restart it then it will start quickly as expected. If they have sat for any length of time they are slow to start. They run normally once started.

What should I look at to diagnose the cause of this issue?

[Tin Tin]

Sounds like batteries are dying or battery terminals are loose. When they serviced it they may have greased the battery terminals on the joining side of the terminal, make sure it is a grease free connection then put grease on top.
Do you have an emergency cut over switch with the house bank? Try that and see if they start more easily when cold. After that it gets more complicated and you will need a multimeter.