1985 CB750F-D bad hot starting (lazy starter motor)

[ScottishBadger]

Hi folks, I'm noticing my CB750F-D is very slow to crank on the starter when the engine is hot. It spins over just fine when cold and starts easily (now that I have removed the stupid vacuum fuel tap!), but once hot it cranks very slowly. It always starts even when cranking slow, but it's worrying that one day it might not.....
So far I've checked the charging circuit, the battery capacity, the starter solenoid and all connections, everything checks out fine.
Is this a known issue and is there a recognised cure? Thanks.

 

[mrtwowheel]

Next, check the continuity from the battery negative cable lug to the bare metal on the starter motor, should be at 0 ohms. After that...........blame the starter motor. I have an '86 GoldWing that started showing it's starter motor problem in the same way.

 

[ScottishBadger]

Next, check the continuity from the battery negative cable lug to the bare metal on the starter motor, should be at 0 ohms. After that...........blame the starter motor. I have an '86 GoldWing that started showing it's starter motor problem in the same way.

Zero ohms confirmed, no voltage drop across leads when cranking either. Not sure what the cranking speeds ought to be, but my CBX1000 for example spins over at around twice the speed of the CB750 when cranking over. The 750 seems to spin quick enough for normal cold starting (although a little more speed wouldn't hurt), but it's almost like the battery is flat when hot - slow over compression then picks up speed and fires up. I also tried connecting jump leads from a fully charged spare battery in the workshop (a stonking great 95Ah one!) and it made absolutely no difference to the cranking speed.

 

[brettp]

Sounds like the starter is going out. Maybe when hot the metal has expanded, or maybe the lubrication is poor, etc. Rebuild it if you can.

 

[ScottishBadger]

Update. Stripped the starter, bit of brush wear (not excessive), bit of discoloration of the commutator segments. Checked all resistances etc, skimmed the commutator in the lathe, cleaned out everything, fitted new brushes, now works fine both hot and cold.

 

[brettp]

Nicely done.