Crank wobble CG125

Mikey7094 · Two Stroke Sniffer Joined: 11 Jul 2012 Karma :

Hi

I have stripped my CG engine because of terrible piston slap, and I know the bottom end is at fault because at idle the crank would wobble slightly front to back. I assumed it must be the crankshaft that was faulty, but someone suggested today that it could be the crankcase itself which is worn/warped/damaged somehow.

What confuses me is that the crank would wobble at idle speed, which you could see with the generator cover off, but then when the engine was revved the wobble would stop. What does this mean? Is it more likely to be the crankshaft or the crankcase that's at fault?

The piston and gudgeon pin are new, there's no play between the small end and gudgeon pin, and the cylinder was rebored (with appropriately sized new piston), so there's nothing wrong with the top end. The crankshaft was a spare from a Brazilian engine (mine is Japanese) which I put in during a previous rebuild (which obviously didn't do any good).

Sorry this is quite an obscure issue - I welcome any thoughts on the subject.

Cheers

Mike

moonzoomer · World Chat Champion Joined: 01 Jul 2012 Karma :

Sounds like your main bearings have had it which in turn means your crank has had it along with the big end and bore/piston etc, lack of the slippery stuff no doubt. Start looking for a second hand engine and forget about a rebuild which would no doubt cost more than a good S/H engine.

Mikey7094 · Two Stroke Sniffer Joined: 11 Jul 2012 Karma :

Thanks for the reply, but I already have all the spare parts! I have two spare engines, and it's a case of picking all the usable parts to make one good engine. I just don't want to waste time and gasket sets replacing the wrong thing.

Does anyone know why the wobble should go away when the engine is revved?

moonzoomer · World Chat Champion Joined: 01 Jul 2012 Karma :

Mikey7094 · Two Stroke Sniffer Joined: 11 Jul 2012 Karma :

The rest of the engine components are fine - I've rebuilt the engine before, which is when I put this crank in, which isn't the original crank. I have two spare crankshafts, but I'd like to know whether this is actually the problem before swapping them.

And I can't feel any play in the crank when it's not spinning. It was only when the engine was running that you could see and feel the generator wobbling, but when the engine was revved it stopped (if you put your finger on the generator you couldn't feel any movement).

unitynotsocri... · Banned Joined: 29 Jun 2011 Karma :

mixing chinese cg type bits,or early/later cg bits is a bit dodgy....i.e. 6 v and 12 v models......maybe wrong alernator flywheel or wrong crank match?

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probably fckld crank though
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jjdugen · World Chat Champion Joined: 03 Jun 2011 Karma :

I think there would be a bit more than a 'wobble' going on if the mains were shot. I would expect a loud rumbling noise and plenty vibrations increasing with revs.
If the engine is 'quiet' then I would say that the alternator rotor is unbalanced or incorrectly fitted.
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