Low compression problem

[Crofty1984]

What could cause low (but not no) compression? The Starfire had been a bit reluctant to start then just died after about 5 minutes of riding. I've got fuel getting there, sparks, I've checked the EI hasn't slipped on the tapered rotor, so spark timing is OK. I'm showing about 75psi.
I checked the pushrods are free to spin as per Rupert ratio so I don't think the valve clearances have closed up. Put a bit of oil down the bore and that's impeded the compression based on kickstart feel, I'd put the test away by then.
And then how do I fix it? Thanks in advance all!

[skippy]

Put a bit of oil down the bore and that's impeded the compression based on kickstart feel, I'd put the test away by then.

I take it that impeded is improved, if this is so your rings are worn. I had this problem with mine, it had been started and run without the bore being checked and the rust slowly ground ground thing away.

[mlb50]

Knackered piston rings take a while to wear down and start to smoke badly.
Sudden loss of compression can be due to a chip off a valve face or eroded seat. Doesn't take much to bleed off PSI. Pull the head and have a good look, best also to remove the valves and either replace or recut.

[kommando]

Even a small loose piece of carbon stick to a valve will do it, once it comes off normal service will be resumed. Had it on a C15 that stopped on way to work, 50 easy kicks later it restarted.