RE: OS 55AX
I suspect bearing/cylinder damage after the thinner cleaning and before the engine got well oiled again. After such an intense cleaning you need to immediately thoroughly oil the engine. I'd suggest filling the case with after-run oil and removing the plug and adding oil above the piston and slowly turning over the engine by hand to fully lube ALL surfaces. Then dump out the excess oil and start with good fuel. I rinse all of my engine parts with clean fuel (oil in fuel) before reassembling a dis-assembled engine. Then oil lightly as above before starting.