GN125 Engine Rebuild (REALLY NEED ADVICE!)

[CasperAndRed]

Hey guys new member here and pretty damn new to bikes (that said I don't think ill ever be happy again without at least one..).

Basically about 2 and a half months ago I passed my CBT had a GN125 waiting in my back yard and two weeks later tried to take it to Ireland. Got about half way to the ferry and it went bang.

Turns out the spark plug exploded into the engine and now its full of shrapnel.

So I've already tried replacing spark plug and running it to see if the metal would blow through the exhaust, it wouldn't turn over, so now I have the engine out of the bike and am planning on rebuilding it.

I have no workshop, and need to buy tools etc for the job but does anyone fancy helping me out by telling me what I need? I don't want to go and blow next months pay on a bunch of useless tools that will break or be wrong for the job.

Any help would be great guys cause I cant ride until I either fix this bike or buy a new one and turn this into a project.

P.S. I doubt anyone on here will be heartless and say scrap the bike and buy a new one (heard it from all my friends before that don't ride) but im not willing to give up on my baby after getting it to ayr before my trip and then going back through to pick it up.

Just so you can all laugh at the newb biker here, it took me 16 hours to get from Edinburgh to Belfast and I arrived at like 3 in the morning and had to get picked up by my girlfriends mum at the ferry.
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If you want to live, don't touch my girlfriend, my dreads or my bike.

[RhynoCZ]

Hello and welcome,

If the source of the metal bits is the faulty spark plug, then I'd take off the head and clean the engine properly, before attempting starting it again. Once the head is down you can see if the valves, piston and other bits are still alright. I'd be also draining the oil, to see if the metal bits got also somewhere else.
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[CasperAndRed]

(noob question here)

Normal socket set and some spanners enough to do this? don't want to start if it get half way through and find something im not prepared for.
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If you want to live, don't touch my girlfriend, my dreads or my bike.

[RhynoCZ]

Yes, the tools you already own should be enough for this job.

EDIT: The head and oil is what I would do. Some would just change the spark plug and ride it, since the only way the metal bits can possibly go, is the exhaust. I'd check at least the piston for dents.
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[CasperAndRed]

Cheers mate, like I say I tried to run the bike but it wasn't turning over, not sure if something got jammed, my carbs exhaust manifold and air intake(?) (that's my best description of what was in the pic) are littered with the metal
____________________
If you want to live, don't touch my girlfriend, my dreads or my bike.

[RhynoCZ]

Drain the oil, check for metal bits in it. Then, it's the head off time. Check for any damage in the head and combustion chamber. This could also be a cracked cylinder sleeve. No need to worry, if the engine is past a reasonable repair, then there are tons of engines that will fit your frame.
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[CasperAndRed]

As for cleaning the metal out wipe down just do? should I use fluid for cleaning the carb?

Thanks btw for taking the time to reply
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If you want to live, don't touch my girlfriend, my dreads or my bike.

[RhynoCZ]

The carb won't need cleaning, but you could use a chain cleaner for instance. Anything that does not harm rubber and is not greasy should be alright.
The engine: if you use an cleaner (engine specific or once again something that doesn't eat rubber - DO NOT USE WD40), then I'd use clean engine oil to lube the moving parts and cylinder sleeve after you're done.

To take it apart and then back together, find the service manual to your bike. It's here on the BCF + there are tons of videos on youtube, where people work on the GN engines.
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[CasperAndRed]

how do you remove the tach cable housing? mines on solid?
____________________
If you want to live, don't touch my girlfriend, my dreads or my bike.

[RhynoCZ]

I'd just disconnect the cable and leave it on. By the way, loosen up no. 16 while it's till in the bike. It'll be easier than trying to loose it up in your bare hands.

https://www.motolights.cz/pd/picown2/417980.png
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[CasperAndRed]

[RhynoCZ]

I'd keep taking it apart and inspect the engine. If the piston is the only thing, that needs to be change, look for another piston. If there's more damage, just buy a new engine of ebay. The GN engine was put into many, many motorcycles and still is used in chinese bikes.

Your engine should be the same, or very similar to Suzuki GZ 125, the stuff Sinnis makes and others. Google around.
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[Robby]

Spark plugs do not typically explode. To turn a piston into little bits of metal you normally need to introduce something hard for the piston to bash into. This process will generally also wreck the cylinder head.

Those little bits of piston can then drop down into the (still spinning) engine to damage other things - such as by getting caught between gear in the gearbox. The saving grace is that the oil pickup on most engines has a strainer, to prevent these little bits of metal from destroying your oil pump and crank.

Either way, fixing it is an involved process which involves stripping the engine and examining all of the bits. If you don't know what you're doing, you can't do this.

A replacement engine is the easy way forward, but also try to work out what caused this one to fail in case you take out the next engine in the same way. It could be a dodgy plug, but I'm not sure where you would be able to buy such a thing these days. It could be a part of the carb that had somehow come loose and been sucked into the engine. If you aren't running an air filter, then you should be.

[Carvel]

The likely scenario is that it's dropped a valve.

As mentioned a replacement engine is the sensible answer. It will have scrapped most of motor.
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2002 Aprilia RSV1000R, Cbr600Fs, Mk1 B12, Vfr400 NC30...

Matt B: I can deal with crushed nuts to beat Ollie.

[Ariel Badger]

One of two things has happened, the piston has hit a solid object such as a valve or it has overheated from a weak fuel mix and has melted. Either way a new engine will not cost a lot more than a rebuild.
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