Honda VT 125 Shadow starter problem

A big ,,hello " to all the members on this forum. This is my very first post.

I have a problem starting my bike. If I press the starter button, it only clicks once , headlight dims, neutral indicator dims.

What have I tried so far:

Battery voltage 12.6(measured)

Bike starts with bump start

Bike won't start with jump start (car battery connected acros the bike battery.

Tried bridging solenoid terminals . Just a big spark, starter motor won't crank the engine

Removed starter motor, cleaned and lubricated. It works connecting it to the car battery. It jumps out of my hand.

Reinstalled starter motor on the bike, connected the car battery again straight to the starter. Just a big spark, no crank.

Before the reinstall of the starter motor, i have checked the visible pinion on in the engine and I can freely turn it by hand.

I don't know what else to do now. Please let me know if any of you came across this.

Thanks in advance.

That's an unusual problem. When you bump start it does it idle and ride ok?

The fact that you are getting big sparks from the starter motor is worrying! Poor grounding or terminals wrong way round?

Thanks for your reply. Once started, runs and idle without problem. Battery connection checked. Grounding checked.

See you've tried it on the bike direct to car battery with jump leads ..

then i would place an issue with the starter motor internals failing under load

Ie brushes worn , dodgy solder joints ...

Now I believe there are two posibilities:

1. As TimR said, the starter motor being ,,lazy" under load

2. Starter clutch assembly jammed.

What are the opinions here?

The starter motor is a Mitsuba SM 14. The prices are crazy for a new starter. About 170£. Scratching my head.

Problem solved. I had to remove the clutch cover to realise that the starter, once fastened down, its jammed. I removed it, and tightened it down, checking that it turns freely.