as it's the L side, check its not something to do with the speedo drive - it is sitting correctly ie raised part of the speedo casting below and behind the raised part on the fork leg casting to prevent it rotating forward too far? Or are the drive ears on the tubular plate correctly engaged with th...

sounds like a selector fork ( sorry) if it was one of the pawl springs or return spring it might affect all gears

Just a thought. Right at the beginning you wondered about ' incomplete burn" . As I remember, the zephyr 11 has 2 spark plugs per cylinder , and i remember reading in a magazine article that having an extra spark plug in the head was to compensate for poor head design. But if 1 of the spark plu...

Agreed - bit dodgy to oval out the holes to fit, you'll create a weak spot and then one day , on heavy braking ,the calliper will come adrift just when you need it to be rock solid

if it revved ok before the work, its not likely to be the advance/ retard unit

What are the compressions like now? Is the airbox inlet free of obstructions? Is the exhaust coked up? Are 1 of the cams 1 tooth out?

or Peter Bond in London

sounds like something stifling the engine - sock in the airbox, needles set too low, carbon buildup in the exhaust, or ignition not advancing.

chain lube especially if you've got a chain oiler will fling itself off the gearbox sprocket and run down the rear LHS of engine and then sump, causing these symptoms too.

Buy one it'll pay for itself the first time you use it. Any method that holds a valve open and then you rotate the crank risks another valve opening and hitting the open one - potentially bending both. The Kawasaki tool , once in place, will not let the cam ( and hence the crank) from going round to...

I think that's a round section O ring ( but I'm sure I will be corrected if wrong)

don't be tempted to use an o ring that's the right size. the seal has flat surfaces top and bottom where it presses against tank flange and tap face flange

Nice journey, thanks for the pics.
Gute Reise!

make sure its resistant to ethanol in petrol - I have a notion that it should have a "J" as part of the code number on the side. (If I've got that letter wrong feel free to correct me)

I remember seeing a comment on the website ( a long long time ago - decades ) that it didn't matter unless the crack extended all the way up the spark plug threads , from combustion chamber to outside world