Lazy starter motor

[Crashy]

Real big engine here lol, 13.5HP Briggs and Stratton lawnmower.

Starter stopped working all of a sudden. Stripped it down, found the brushes were broken in bits so swapped them out with parts from a seized lawnmower I had. The bendix (I think that's the right term) was also tight where it comes through the top of the motor so I swapped it for the other one too as it turned freely by hand.

Anyway, the 'newly' built starter motor spins up well when connected to a battery and kicks the pinion up but when bolted to the engine, it hasn't got the power to spin the engine.

I can spin the engine by hand so I'm assuming the starter motor hasn't got the power to turn it.

Where do I go from here?

[Evoluzione]

Check the wiring to the starter, battery and the earth (if it has one), sometimes faults here lower the voltage and make it seem lazy.
Sure the battery is ok?

[Crashy]

Thanks for your reply.

The wiring is not an issue, I disconnected the complete engine from the lawnmower wiring and have it all wired up to a separate battery just to eliminate faults in the lawnmower loom more than anything. There are only 4 wires, one to a negative on the coil (engine shutoff, currently unattached or it would kill the spark), one to the top of the engine (charge to the battery), one permanent negative (currently attached to the top of the engine, Contact point is well sanded and there is a good ground), the other is a switched live to the starter.

Running from a fully charged 720 AH battery, more than enough power for a 13.5 HP engine.

The starter has the power to turn the engine 3/4 turn but once it comes onto compression, it dies. I can reach onto the top of the engine and push it by the compression stroke. It turns again freely for another 3/4 turn but again, once it reaches the top of the compression stroke, the starter motor hasn't the power to push it by it. If I pull the plug out, the starter can spin the engine full revolution but it struggles.

[anonymous-user]

The starter may have a shorted commutator. Not unusual if the brushes were knackered.

[Crashy]

Max_Torque said:

The starter may have a shorted commutator. Not unusual if the brushes were knackered.

What's a commutator?

[Krikkit]

[anonymous-user]

The brushes run on the commutator. They act to switch the DC current to the correct rotor coil at exactly the right moment, so that the magnetic field generated pushes and pulls the rotor around in a circle. If the damaged brushes have created a short across the copper segments of the comutator, this can act to seriously reduce the torque of the motor at a certain point in the rotational cycle. This isn't enough to prevent it free spinning, but can be enough to cause a stall under load.

You need to take out the rotor and inspect the commutator for damage

[Mikey G]

If the Commutator is fine, check your 'new' brushes are moving freely in the holder and the spring pushing them onto the 'com' is strong enough. Also new brushes need bedding into a com before full current can pass through them. Its the current that makes the power and if it cant pull the current through it then there wont be much power. If there is only a small contact patch between brush and com then they need to bed in.