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So I just purchased my first outboard boat motor. It’s a 9.9hp 1997 Mercury. I should have known better but didn’t realize I needed to run 2 cycle gas. So I ran it for about 15-20 minutes total. Mostly at a medium-low idle speed. I revved the engine a couple times during this process. It ran fine throughout and it didn’t stall/seize. Well, my neighbor stopped by and we were talking and I realized my error. So my plan at this point is this... drain the present non-mixed gas and fill with proper mix. I also purchased a valve pressure test tool to see if I’ve lost pressure. Any other recommendations? Thanks!

  • Welcome to Motor Vehicle Maintenance & Repair! – Pᴀᴜʟsᴛᴇʀ2 Apr 28 at 1:53
  • Engines that size are supposed to run double oil (break in mix) for so many hours before switching to its recommended oil ratio, consult the owners manual for instructions – Moab Apr 28 at 2:44
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I think you're doing what you need to do. You won't really know (and I cannot tell you) how much damage you've done ... I'd suggest at this point, your engine is good and broke in! The fact you didn't rev it too much is a good thing. As for going forward, you should put the 2-cycle mix in it and let it idle on that for a while without revving it at all. This is to get the oil into the engine and let it do it's thing. Once it has idled for a good five minutes, I'd think that's about the best you're going to be able to do. Then run it like you stole it.

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