Starter motors

[Dave W]

What actually causes a starter to go lazy or slow?
There's not a lot to them. Coil, magnets and brushes. Brushes are a consumable but what else wears?

 

[holwellcourtfarm]

First check is always voltage drop at the terminals in my experience....

 

[Hilly]

What actually causes a starter to go lazy or slow?
There's not a lot to them. Coil, magnets and brushes. Brushes are a consumable but what else wears?

Dirt/ carbon as much as anything , you can strip them and do them up few new bits n bobs , I know a man who makes a living doing it but often as cheap to put new one on just .

 

[Netherfield]

Poor contacts in the solenoid can be a problem, causing reduced voltage.
as @holwellcourtfarm says

With a good voltmeter, start at the battery, while cranking read the voltage.

Next the starter motor, check the voltage between the body and the terminal on the solenoid where the battery is connected, should read the same when cranking as the battery, if not there's a bad connection on one of the leads or the leads aren't big enough.

Now check voltage between the starter body and the other big terminal on the solenoid, this should also read the same, if it does not the suspect the contacts are bad.