I have an `85 Evinrude, 25 horse, that was supposed to be a 100/1 mix, after the first two tanks (50/1). I was on my 3rd or 4th tank at 100/1 and it was knocking some. I asked my mechanic about it and he said to go back to 50/1 and stay there. Around ` `88 or later, OMC came out and said "OOPS, we shouldn't have gone so lean. Never mind. Go to 50/1." No new manual, no help if you had messed up your motor, just a letter. Luckily, I must have caught it on time. That may be the case with your yammy, too.
Call or email the company. They may tell you to go stronger, or maybe not. Sometimes a company will NEVER admit an error, so I'd call an outboard mechanic and ask them, too. If you don't know a good outboard mechanic, check with several. Why more than one? Well, in over 30 years of using and beating up outboards on a daily basis, I've found that there are a few individuals that call themselves outboard mechanics that couldn't find their butts with both hands.
Semper Fi, Marines
Casey 1997-2011 RIP