Right hand crankshaft (stator side), spinning freely on Apache Tomahawk 50

[MikeyC85]

Hi guys - 

Working on my daughters Apache Tomahawk 50cc (GY6 engine I believe).

Wouldn’t start, noticed that the stator Magneto wasn’t turning with the engine.

Stripped it down fully expecting the woodruff key to be sheered/moved however this wasn’t  the case.

It appears instead that the shaft itself isn’t engaging and/or is engaging intermittently as the engine turns over which is effecting spark timing.

Any ideas on what could be the problem - I’m at a loss!!

Edited March 8, 2023 by MikeyC85
Clarity

[Mech]

The crank will be broken.. 

[Gwbarm]

Thats interesting, I mean bad, are the valves working properly, does it have compression.

[MikeyC85]

Hey - yes engine turns perfectly, valves all functioning and has good compression. Right hand crankshaft just doesnt seem attached 😆

[Gwbarm]

 It Does sound like broken crank, can you rotate rotor freely by hand

[MikeyC85]

It was 😂

Is it common for one to sheer in this way?

[Gwbarm]

You have been busy, Wow it snapped right off, what a clean break, I would think something else must have happened to cause that, is the bearing seized, 2 stroke or 4 stroke.

[MikeyC85]

😂 

It’s a 4 stroke, from what I can see everything else is exactly as it should be … engine turns smoothly and easily, bearings all spinning freely and everything in its place.

absolutely puzzled. @Mechmentioned there may have been an existing fault in the steel which may explain it but other than that …. Very surprised.

It was running absolutely fine as well then whilst sat idling just slowly died … I presume from the flywheel stopping.

[Gwbarm]

I thought the GY6 was 4 stroke, but the larger engine, wasn't sure about the 50cc, i have a GY6 style engine on a Chinese scooter that's 50cc and its 2 stroke. Good thing I think parts are fairly cheap for those. I think Mech is right just plain metal fatigue I have had it happen on other parts but not a crank. Sorry for your loss, but got to keep the daughter happy, I know I have 3.

[Mech]

The cheaply made bikes seem a bit prone to snapping things, like gearbox shafts, and kick shafts, and it seems, crankshafts too..

In that particular case I'd suspect the radius in the corner between shaft and throw is a bit to small, and perhaps a badly balanced flywheel and it could happen.  And that's ignoring the fact it's probably poor grade steel..

[kp3004x4]

I've never seen such a clean break as that has to be manufacturing default,seen breaks before but cracks actually and have pieces missing here and there. Could be rebuilt though just make sure to get all metal dust out of case.  Hope the best.