RE: o s 91 4-stroke
Hi!
If you haven't tampered with the cam timing before why think it has changed suddenly? -It cannot! The tappet gap is easy to check!
It should be between 0,04 -0,1mm or more precisely, a 0,04mm steel shim should pass under but a 0,1mm should not.
Are you sure the plug works? Have you checked that it glows? If the engine gets fuel,air and glow...it cannot say no to work!