Hi and thank you for your question.
There are a couple of symptoms going on here. For me to narrow it down in my mind, and to provide you with valid information, I need some questions answered.
Chronologically, how do these symptoms correspond to the rebuilt engines?
On the rebuilt engines, are they the original engines rebuilt with OEM Yamaha parts, or are they aftermarket engines?
Also, where chronologically does the change to pre-mix correspond with the symptoms you are now experiencing?
What brand of oil are you using for your pre-mix?
Why did you chose to eliminate the oil injection? I am curious if it failed on you causing the rebuild.
Since I don't know geographically where you are located, is this early in your boating season this year that this happened, or have you been using it for awhile this year already?
Has anyone, especially recently, gone into the carbs to clean or adjust them?
Any non stock modifications to the boat or engines?
If you could fill me in on that info, plus anything else that may have come to your mind relative to any of that, I would appreciate it.