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Totally agree with Kawboy here and hope its not pistons hitting valves..................
Does your bike have the STANDARD Cam Chain Tensioner fitted ? If it does.........GET RID OF IT NOW and swap for a ZRX1100/1200 type now !!!!

Pete F
UK

Hi Peter, yes changed this 2 years ago.

Hi Stan, UPDATE. Well tell me what you think of this ! 1 Removed the right hand cover and turned engine over off the nut, with a spanner no problem. But would not turn over off the starter. 2 removed the spark plugs, engine now turns over off the starter, put plugs back. So now under compression will not turn over ! 3 decided to eliminate all electrical systems, connected fully charged independent battery to the starter + and earthed the other lead. Plugs still in, connected power Will not turn over !! Plugs out engine turns over. While the plugs where out I put a bore scope into each plug hole to look inside, engine looked very clean with no signe of damage.
So I can't work it out !! before I did all this, I swapped the starter motor from my Z1R

Just for the record Phil - That's where I was "'trying to go " with all my stupid questions, before everyone went ape-shit with "Bent Valves"" .
So much for my "soft" approach. Regardless - glad it's not the worst case scenario.
So......continuing with my "limp-dick" approach -
With power direct to the starter:
Plugs out - Turn over
Plugs in - no turn over
What are the odds that both starters have an "issue"? Internally - Poor contact between brushes and commutators ( AGE ?) Poor Grounding somewhere in circuit ? As is with both starters - enough juice to spin with no plugs - Plugs in and - not enough current passing through to deal with the compression?

Just my luck ! but that's the way it could be. Strange thing is that the same starter was OK in the Z1R but as you say 40 + years old, May have made it worse by removing the starter lead. Wish I could have them checked out under load, do you know if any specialists exist to do this type of job ?

Phil - I'd say maybe your hand is stronger than the starter? haha The compression might be enough to stop it from cranking.

Another thing - perhaps there is a ground short in one of the plugs so the power drains right to the case?

I'd put all the spark plugs in the boots, and just place them against the case and crank the engine. I'd also do it in dark and see where the ignition discharge jumps.
If the engine turns the same as without plugs, I'd install the plugs one by one and keep testing cranking with each additional. Anything changes? Is the starter loosing the crank power? Maybe after installing a particular one? Could be very well that the starter needs to be checked if you see it loosing power when working against compression if all the spark plugs are good.

You can rebuilt the starter yourself. It's not difficult. I've posted photos of how it's done in my thread. You can order new brushes, no problem. It's 2 - 3 hours to rebuild a starter if you do it without rushing.

That's what I would do next.