GN250 top end

[MikeyG]

So the mighty gn has taken up smoking and using oil. Does anyone know how these usually wear? Can i get away with a hone and new rings or am i looking at a oversize piston?

 

[Big R]

Are you sure it's the piston and not the valves? Could just be valve seals.

It could actually be a small number of things.

 

[TwinPlug]

If it's valve guides/seals the usual symptoms are smoking on the overrun or a puff of smoke after opening the throttle following an overrun situation.

If it's rings is usually smokes the whole time, but there is only one way to find out for sure - open her up and check everything.

Another thing that can cause smoking is borewash, when the fuel mixture is so rich it washes the oil of the bore and then burns it as part of the combustion process, left unchecked it will cause premture wear.

 

[MikeyG]

Smokes when cold and when unded load. Seems ok on the overrun.

I'd replace the valve seals and lap or recut the valve seats when it is open.

The big issue is if the bore is worn or not. I can get a 2nd hand engine for less than a piston plus shipping. While cost of rings and gaskets is about the same as 6 months oil.

 

[TwinPlug]

If the bike is anywhere near original I'd be tempted to fix it, but if not then get the other engine presuming you know it's a good one, you could maybe keep the original motor until the bits 'turn up' for the right money and repair it.

I'd lap the valves / do the seals regardless while the top is off.