Hi,
I'm working on diagnosing what caused cylinder 1 to lose all compression on a Mercury Optimax 115. Serial #: 0T920655. 2004 115ELPTO
The outboard starts great, idles fine and can even get the boat up on a plane. It cannot reach full throttle (climb to higher RPM is very slow). At full throttle position, after reaching a higher RPM (boat planed out), the engine dies.
I bought this boat and outboard with full disclosure that the outboard was 'not good'. I went on one test run on a nearby lake. Before my test run cold compression numbers (I didn't want to do more damage than I had to, I get it, these numbers are not good for much) 1: 30psi, 2: 90psi, 3: 90psi. After the test run #1 (engine still warm) had dropped to 0psi. By #1 I mean the top cylinder as described in the service manual.
I have the Mercury service manual and I'm reading through it, "hatching my plan". I also bought the Diacomm cable.
Here are a couple photos I took with a borescope through the sparkplug holes. The first two are of #1, I couldn't see any cross hatching, where the last two images of 2 and 3 have quite a bit of crosshatching still visible. There's also more carbon that looks more wet in 2 and 3. Seems #1 was hot.
https://postimg.cc/gallery/2y0p2fhca/
I'm hoping to find root cause before I tear everything apart, gauge the bores, order parts, and start rebuilding. Current theories involve running lean with a bad or dirty fuel injector or leaking direct injector. And/or low flow from the oil pump, bad oil check valve, plugged hose, etc. I did verify with the diacomm cable that all the injectors fire (sound/vibration from each) but have not removed anything to verify actual performance.
Any theories, ideas, tips, or tricks?