Before tearing down the engine, do a compression test, its mandatory to do when checking for bad leaking valves, and then finalized with leak down test.
If you cannot find someone who can remove the manifold to test then find someone owning a fine scale DSO and with an amp clamp do a relative compression test with clamp on starter battery cable, with fuel pump disconnected. This can show you a bad cylinder too.
You can probe the crank sensor to see which cyl is causing the starter to pull lower amps on its compression cycle.
Do not just tear down stuff without exhaustive analysis otherwise you may waste a ton of money.