Anyone any ideas what caused this?

[colint]

Anyone any ideas what caused this R100GS para roughly in line with exhaust port valves seats and guides OK bike has been stood for several years and started monthly and run for a few minutes by previous owner it was coked up and not obvious when i took the heads off.
Other barrel no wear or signs of wear

 

[Timolgra]

Could be valve adjustment too tight in an attempt to make it run quieter

 

[NeilDD]

Or it could be that the bike was not run long enough to burn any moisture off???

 

[MattW]

In two strokes, overheating is generally held to be the cause of cylinder plating flaking (apart from some 'lemon' models that had bad plating to start with) . How's the piston and plug?

 

[colint]

In two strokes, overheating is generally held to be the cause of cylinder plating flaking (apart from some 'lemon' models that had bad plating to start with) . How's the piston and plug?

Piston, plug and rings OK just really coked up no burning though.
When I bought the bike I started it and it ran for a few minutes but have not run it since (about 2 months) just stripping it down and re furbing I have bought a new (second hand) barrel but I've never seen this before just curious
I've de coked the heads and lapped the valves in but the seats weren't burnt.
When I rebuild engine, I'll clean and reset carbs check timing etc.
The other barrel is perfect no problems which would suggest as has been pointed out exhaust valve problems or maybe carb running too lean.
Thanks for all the replies

 

[Rob Farmer]

More likely damp getting in though an open exhaust valve when it was stood. The barrels don't look to good further down the bore. Is that a corrosion mark from the piston rings?

I'd hone it out and then pretend I never saw it

 

[cristhemuso]

My guess would be pre-ignition. The exhaust valve was probably running very hot due to lean mixture, most likely, (or uneven mixture due to coke build up) and the squish area at the edge of the piston would detonate early as it is ignited by the (glowing hot) exhaust valve. Another possible explanation is carbon build up sufficient to bridge the gap between piston crown and head causing a mechanical impact and damage to nikasil plating.

 

[(RIP) Kaister]

it was aliens from jupiter.....

 

[Rob Farmer]

Found this. I'm not suggesting this is the cause but I never realised fuel could damage the lining.

"High Sulphur Petrol sold in the UK market (until recently) was the cause of Nikasil liner breakdown. It has a cumulative effect, i.e. if the engine has ever been run on high Sulphur Petrol, damage may already have been done and premature bore wear may result."

 

[Steptoe]

Found this. I'm not suggesting this is the cause but I never realised fuel could damage the lining.

Do you remember shell "formula" petrol from the mid eighties. They had to stop producing it due to it causing engine problems = nikisal barrel damage .