Starter motor testing?

[Jimbo550]

I've had a problem with the starter motor struggling to turn the bike over. Fitted a brand new battery today and was hoping this would help but it doesn't seem to make a difference. I've stripped and cleaned the starter motor and the brushes are within tolerance as stated in the book. After reassembly it still doesn't appear any better so I've measured the voltage drop when the starter is operated and I,m getting a drop of about 3volts.  Does this seem to high a voltage drop or would you expect this value? Any other advice or suggestions would be great fully received.

[Lobo]

Jimbo....are your earth straps / positive leads all in good order, clean etc, and the earthing points clean and 'solid'?  Often the ends of these leads are factory crimped to the copper core, and in time dirt & vibration can penetrate / destroy the continuity needed for these high currents.
A 3V drop appears, if anything, a bit low, implying a lesser current drain than might be expected for cranking.
If all connections / leads pukka then perhaps a dirty starter solenoid - easily checked by using a car jumper lead from the starter motor live direct to battery +ve. But... take care, biggum sparks.... don't do this anywhere near petrol etc.
If still sluggish try the same jumper lead trick, but now starter motor casing direct to -ve of the battery, but this time use the starter button.
Apologies if all the above 'obvious' & already tried.
Cheers.

[Jimbo550]

Thanks or the info lobo, all advise is gratefully  received. I've cleaned all the contacts this morning and checked the condition of the positive and earth cables which seem pretty good. I also taken the starter motor apart and spun up the centre part (not sure of the technical term) in the lathe giving it some emery cloth to clean the surfaces.  When all back together it seems to be better but I will see how it fairs over time.

[Jimbo550]

Thanks oddjob I will give the bushes a close inspection.

[Bryanj]

OK anything and everything CAN go wrong after 30+ years but the NORMAL cause of slow turnover was people painting the RH rear engine mount plate. From new there was bare metal both sides at the engine mount bolt to give a good electrical contact with the wire on the outside and the bolt done up TIGHT.

Try putting your meter cables on the end of the earth cable and the engine casing and see if it reads voltage when you try and start

[Jimbo550]

That's a very good thought as I have painted the engine.  The other problem I'm having is an intermittent fault with the starter button. I don't think it's a faulty switch/button  but could be a loose connection somewhere.