Thanks Brit - well, I cleaned the cylinder head around the combustion area, but I have to admit I left all the burnt carbon on top of the pistons
So, this morning (between rain showers) I removed my RH carb and replaced the accelerator pump crank roll pin (the old one sheared off a couple of years back and I had replaced it - not entirely satisfactorily - with a bent nail). A very satisfactory 5 minute job. Replaced both carbs and refitted the throttle balance bar, refitted all the coolant hoses and tightened the unions, poured fresh oil into the LH cam carrier and replaced the cam cover, fuel hoses, plug leads. Topped up the oil and refilled with coolant. Turned the engine over a few times to make sure all was well, before igniting the fuel!!!
It lives! The Green Machine lives again!!!
I sat on the floor with the canopy closed as the rain hammered down, revving the engine to try and get some heat into it. The factory manual says run it until the fan cuts in; that's an indicated 73°C for me, but try as I might, I could only get the temperature to 69-70°C. Hopefully that's close enough. I've let the engine cool down through the afternoon, helping the process along by giving the engine and engine bay a good degreasing, so by 4:00 pm I was ready to re-torque the cylinder head bolts.
Annoyingly, this meant I had to drain the coolant again, because my radiator coolant hose obstructs access to the top left cylinder head bolt; four are accessible from underneath (through the wheel arch), with the centre top and top left accessible from above. I slackened each bolt off by a quarter-turn, then re-torqued back to 64 lb/ft. Once that was done I was able to refit the coolant hose, refit the road wheel and then refill with coolant again, before restarting the engine.
As I type this the engine has had another 10 minutes running time and there are no signs of oil leakage from the cam carrier, or coolant or oil leaks from the cylinder head gasket. I am daring to hope that I might have finally fixed it!!!
Now guess what? I found the mangled remains of a self-tapping screw in the tread of my NSR tyre. What are the odds that the screw has gone all the way through? I was going the change both rear tyres anyway, but wouldn't you know it? My local tyre supplier hasn't got the size I need in stock
That's the problem with old-skool tyres...