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! Commented Apr 28, 2020 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
    Commented Apr 28, 2020 at 2:44

1 Answer 1


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.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .