Whose pistons are you using ?
Carbon burn deposits beneath rings ?
You can relieve the top ring land area on most pistons to give it a little wiggle room. Same goes for sanding down bottom portion of ring to give it a little more room in the ring land area. If snagged on exhaust port, re measure port width and see if somebody didn't just have a field day widening it. Measure piston taper and if using forged pistons, for example JE is known for lots of taper, you must request custom pistons depending on application. We had failures on 4 stroke Honda Sportbike stuff. I have not seen many issues with Wiseco pistons for pwc's with the exception of ring land location pin backing out.
Taper honing also helps a little no more than 1 - 1 1/2 thou top to bottom.
Probably just need to chamfer the ex port a little more, soften it up, all over not just top and bottom. Sounds like 40:1 mix is way lean on oil for recreational ride.
Engine dyno will actually show you'll make better power around 25:1- 30:1 ....
Pollute it and enjoy, I think a lot of people are fixed on throttle response, Oil flow rate when pre-mixed can still be dialed in.
Are you running a new pipe or an old one. Manifold spewing pin holed leaks or water injection not sealed properly ? (Steaming up exhaust side/ more oil would handle it)
Just not a fan of lean pre-mix ratio's old roadrace bikes performed well with rich mixtures and throttle response is more crucial there.
