I would recon the head...lap a new valve in clean up the piston/cylinder(if there's no damage)and go for it....its obviously put together properly so do it...

Neosophist wrote:0 psi?
only reason itll be 0 is if there is a hole in the piston or valves have broken off, and then that is bad news.

Ah it's as I suspected.. at least you know the compression testor was sealing.

Was the bike running lean? Could have burned out the exhaust valve this way? Especially as you mention old carb rubbers etc.

Maybe some of the pilot circuits got blocked up?

Could just be from a lot of heat cycles but i'd like to find out why it's failed first as there is no point rebuilding it to have the same thing happen.

Stock bikes, especially the 89 are quite lean from the factory.

Thanks Neo,
You asked about its jetting. Sometime ago I asked RO about jetting to uk spec. It was still running Standard jap jetting from new, no modifications, standard exhaust., just a lot of miles.plugs were always a nice tan/ grey colour, occassionally oily on front right pot. New flters all round every year. Fatigue quite likely. Yet to remove valve and check the bore but will give a running commentary, ask you guys for your opinions.....

Forgot to say it was on a H plate, which was registered in 90. Could still be an 89 model though. black and graphite grey,

if clearances are zero the valve could have been partly held open - burning it out

indeed.

coudl just be fatigue but some suspect issues id have that could have been contributairy factors.

tight valve clerances.. if the head is still on you can check these now.

slow jet / pilot circuit blocked in failed cylinder carb causing lean jetting on that plug.

look forward to the running commentary.

you shuold take this oportuninty when rebuildign it to service carbs and upjet too.

if its 89 spec bike it wont have the brass inserts on the front forks / gold calipers / rear shock no preload (these are giveaways but pletny of bikes have been in accidents / upgraded / downgraded / bitza)

stock jetting on an 89 is 110 mains.. 115/118 on later bikes.

looking at the bore I'm in luck. The picture I posted of the piston crown looked ominous, but Having removed the carbon it looks fine. The bore looks like polished glass,no scuffing . Spent today cleaning the gasket surfaces, measuring valves , shims and springs . I will definitely look at the carb on that pot as Neo said , would be a pointless exercise to have it happen again . Sure its 115 front 118 rear from memeory so likely a 90 bike. Where's that lapping paste...

another 100k in her i am sure. good perseverance skillz there, i doff my cap in your direction

RO and David silver to the rescue. Most of the bits have arrived .Aim to put some time in over the weekend. Once the top end is on I will be checking out the carbs, remeasuring valve clearances and balancing the carbs, providing she runs. No nasty surprises so far. Not doubting my ability but I'll be measuring twice , cut once.... if you know what I mean.

Good ya..nice to see you rebuild rather than chuck it for another!!!